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Indium Gallium Oxide Alloys: Electronic Structure, Optical Gap, Surface Space Charge, and Chemical Trends within Common-Cation Semiconductors

Cite this: ACS Appl. Mater. Interfaces 2021, 13, 2, 2807–2819
Publication Date (Web):January 11, 2021
https://doi.org/10.1021/acsami.0c16021

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Abstract

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The electronic and optical properties of (InxGa1–x)2O3 alloys are highly tunable, giving rise to a myriad of applications including transparent conductors, transparent electronics, and solar-blind ultraviolet photodetectors. Here, we investigate these properties for a high quality pulsed laser deposited film which possesses a lateral cation composition gradient (0.01 ≤ x ≤ 0.82) and three crystallographic phases (monoclinic, hexagonal, and bixbyite). The optical gaps over this composition range are determined, and only a weak optical gap bowing is found (b = 0.36 eV). The valence band edge evolution along with the change in the fundamental band gap over the composition gradient enables the surface space-charge properties to be probed. This is an important property when considering metal contact formation and heterojunctions for devices. A transition from surface electron accumulation to depletion occurs at x ∼ 0.35 as the film goes from the bixbyite In2O3 phase to the monoclinic β-Ga2O3 phase. The electronic structure of the different phases is investigated by using density functional theory calculations and compared to the valence band X-ray photoemission spectra. Finally, the properties of these alloys, such as the n-type dopability of In2O3 and use of Ga2O3 as a solar-blind UV detector, are understood with respect to other common-cation compound semiconductors in terms of simple chemical trends of the band edge positions and the hydrostatic volume deformation potential.

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1. Introduction

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Indium oxide (In2O3) is the material of choice (most often doped with Sn) for applications requiring transparent conductive electrodes because of its propensity for n-type dopability, providing great electrical performance, and its large energy gap (∼2.9 eV fundamental band gap and ∼3.7 eV for the first dipole-allowed transition (1)), giving high optical transparency. (2) In contrast, β-gallium oxide (β-Ga2O3) is a rapidly emerging semiconductor material, and its fundamental properties have been relatively less explored in comparison. With a high breakdown voltage and very large optical gap (∼4.8 eV), β-Ga2O3 is a promising candidate for applications in high-power electronic devices and solar-blind UV detectors. (3) Indium gallium oxide alloys have generated much research interest in the past, often in the amorphous state used for thin film transistors. (4−7)
Favorable energy alignment when forming heterojunctions and alloys is imperative for optimal device performance. Hence, careful engineering of the optical gap and surface electronic properties of In2O3 and β-Ga2O3 would bring a multitude of new potential applications. Fortunately, alloys formed from In2O3 and β-Ga2O3 possess the potential for such desirable properties and tunability due to the large band gap disparity and different surface space-charge behavior. The potential for band gap tailoring as well as control of the surface space-charge is therefore a very exciting prospect for indium gallium oxide ((InxGa1–x)2O3) alloys. However, In2O3 and β-Ga2O3 possess very different crystal structures as presented in Figure 1. β-Ga2O3 has a monoclinic structure with two inequivalent Ga sites: a distorted tetrahedral site and a distorted octahedral site (Figure 1a). In2O3 has a cubic (bixbyite) unit cell also with two inequivalent cation sites: a distorted and a symmetric octahedron (Figure 1c). In addition, an intermediate hexagonal InGaO3 phase has been reported, which has a symmetric In site and a trigonal-bipyramidal Ga site (Figure 1b). (8−10) It is not yet well explored how these different structures affect the surface electronic properties of these materials. This fundamental understanding is of great importance for the development of new electronic devices and heterostructures. Recent developments in deposition methods and combinatorial material science (materials possessing a composition gradient) (8) give us the ability to quickly and rigorously determine the properties of many new material compositions using one film.

Figure 1

Figure 1. Unit cells of (a) monoclinic β-Ga2O3, (b) hexagonal phase InGaO3, and (c) bixbyite In2O3. Ga atoms are shown in green, In atoms in purple, and O atoms in red. For (a) and (c), the inequivalent cation sites are shown with different color octahedra.

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In this study, we employ the recent advances in high quality combinatorial growth methods with the rapid development in combinatorial measurement to fully investigate the electronic structure and behavior of (InxGa1–x)2O3 alloys. We use high-resolution scanning X-ray photoemission spectroscopy (XPS) as well as state-of-the-art first-principles calculations to determine the electronic structure of (InxGa1–x)2O3 combinatorial films over nearly the whole composition range (0.01 < x < 0.82). This combined with optical measurements helps us to understand the compositional dependence of the Fermi level with respect to the band edges at the surface of (InxGa1–x)2O3. We explore the surface space-charge transition from electron accumulation in In2O3 to depletion in β-Ga2O3, a property that has implications for the incorporation of these materials in devices in the future. We compare density functional theory (DFT) calculated density-of-states (DOS) for the different phases looking at the constituent orbital contributions and also compare this to the XPS valence band spectra and occupied semicore d levels. From this we observe indicators of p–d repulsion which affects the position of the VBM as well as s–d hybridization in the semicore levels which is associated with materials with similar energy s and d levels and occupied cation d levels. Looking at these orbital contributions, as well as knowledge of the volume deformation potential (how the material’s band gap varies with changing volume), allows us to fully explore the chemical trends of In2O3 and β-Ga2O3. This explains why In2O3 is extremely n-type dopable and why β-Ga2O3 has an extremely large band gap, even for an oxide semiconductor.

2. Methods

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2.1. Experimental Details

A continuous composition spread (CCS) (InxGa1–x)2O3 film was grown on a 51 mm diameter c-plane sapphire wafer via pulsed laser deposition (PLD) by using a 2-fold azimuthally segmented target for ablation. SiO2 (0.1 wt %) was mixed with the In2O3 and Ga2O3 source powders prior to target fabrication to achieve sufficient conductivity via Si n-type doping to avoid charging when performing XPS measurements. The growth temperature and oxygen background pressure were 650 °C and 3 × 10–4 mbar, respectively. Further information about the deposition process is given by von Wenckstern et al. (8) A 4 mm wide, 51 mm long strip of the wafer with the full composition gradient across it was cut, and all subsequent measurements were performed on this strip. The films’ surface roughness is not assessed here but has been determined previously for this system deposited in the same way. (11) Despite an increase in surface roughness when the cubic bixbyite phase is reached, the films should have sufficiently low surface roughness required for device applications including heterostructure-based devices.
We determined the spatial distribution of In, Ga, and Si content throughout the film by energy-dispersive X-ray spectroscopy (EDX) using an FEI NovaLab 200 equipped with an Ametek EDX detector. The spatially resolved X-ray diffraction patterns (XRD) from the film were recorded by using a PANalytical X’pert PRO MRD X-ray diffractometer equipped with a PIXcel3d detector operating in one-dimensional scanning mode with a monochromated Cu Kα source. The XPS measurements were performed by using a Thermo Scientific K-Alpha instrument with a monochromated microfocused Al Kα X-ray source (1486.6 eV). These measurements were performed under ultrahigh vacuum (UHV) conditions (<10–9 mbar). The scans were measured by focusing the beam width to 400 μm spot size on areas of the film with the film being translated by 1 mm between each measurement. An analyzer pass energy of 50 eV was used, giving a spectrometer resolution of 0.65 eV, enabling peak positions to be determined to within ±0.1 eV. Sample charging was corrected by a dual beam charge compensation system. Optical transmittance determination was performed by using a Shimadzu UV–vis–IR 3700 spectrophotometer with an integrating sphere detector which employs a photomultiplier detector to reach energies up to 6.5 eV. All measurements were performed at room temperature.

2.2. Computational Details

The theoretical calculations were performed by using the HSE06 screened hybrid functional (12) and projector augmented wave (PAW) approach (13) as implemented in the VASP code. (14) We include semicore 3d electrons of Ga and 4d electrons of In as explicit valence states and set the fraction of screened Hartree–Fock exchange to 32% for β-Ga2O3 and InGaO3 and to 28% for In2O3 (to better match the experimental band gaps). All unit cell parameters were optimized by using a plane-wave cutoff of 520 eV, and integrations over the Brillouin zone were performed by using a 6 × 6 × 6 Γ-centered grid of Monkhorst–Pack special k-points for Ga2O3, a 10 × 10 × 4 grid for InGaO3, and a 4 × 4 × 4 grid for In2O3. Densities of states were evaluated by using the relaxed geometries and the tetrahedron method with Blöchl corrections (15) and included scalar-relativistic effects for the In-containing compounds.
For β-Ga2O3, this choice yields a direct band gap of 4.85 eV and optimal lattice constants of 12.21, 3.03, and 5.79 Å for the a, b, and c cell parameters, respectively, in excellent agreement with experimental values. (16−18) Our optimized lattice constants for the hexagonal InGaO3 phase were 3.31 and 12.03 Å for the a and c values, with a calculated direct band gap of 4.13 eV at the Γ-point. The indirect band gap was found to be lower, at 3.91 eV, with the valence band maximum falling nearly midway between the Γ- and M-points in the Brillouin zone. For cubic bixbyite In2O3, our optimized lattice constant was 10.12 Å, with a calculated direct band gap of 2.86 eV at the Γ-point.

3. Results and Discussion

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3.1. Structure and Composition

Figure 2a shows a total of 45 single 2θ–ω XRD patterns that have been tessellated to form a false-color plot, with lateral distance across the film on the y-axis and 2θ on the x-axis. Solid black lines have been superimposed on the pattern at around ∼6, ∼9, and ∼40 mm to indicate the region of mainly the bixbyite phase (z ≤ 7 mm), the onset of the InGaO3 hexagonal (mixed) phase (∼7–12 mm), and the region of mostly monoclinic phase up until the end of the measurement region (∼50 mm). Figures 2b–d show single patterns corresponding to the top, middle, and bottom solid lines. Figure 2b shows that the β-Ga2O3 phase is (201) oriented. (8,9,19) This holds true for the majority of the film. Figure 2d shows that the In2O3 dominated region of the film grows with (111) orientation, although a small peak just to the left of the 222 peak (∼29°) is also visible which originates from the 0004 reflection from the hexagonal InGaO3 phase, consistent with Figure 2c. (8−10,19) In both plots we also see the 006 and 00 12 peaks due to the sapphire substrate, which have been left unlabeled. It is clear the (InxGa1–x)2O3 material is crystalline across the whole film (and so the whole composition range).

Figure 2

Figure 2. (a) False color representation of wide-angle 2θ–ω XRD patterns, taken at ∼1 mm intervals (55 in total) across the compositionally graded strip in the direction highlighted by a black arrow in Figure 3. Black horizontal lines indicate the point in the film corresponding to the monoclinic phase (top line), the onset of the hexagonal phase (middle line), and the onset of the bixbyite phase (bottom line). (b), (c), and (d) show individual diffraction patterns taken at the horizontal lines at 40, 9, and 6 mm, showing primarily (b) the monoclinic β-Ga2O3 phase, (c) the hexagonal InGaO3 (II), and (d) the bixbyite In2O3 phase, respectively. A small peak corresponding to the 0004 peak of the hexagonal (InxGa1–x)2O3 phase can be seen at ∼29° in (d). Note the intensity scale in (b)–(d) is logarithmic.

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We can directly quantify the spatial distribution of Ga, In, and Si content of the film using EDX and correlate these data with the XRD results by taking measurements at the same positions in the film, demonstrating the power of the combinatorial approach. Figure 3a displays an EDX line scan from 3 to 49 mm across the film. The Si content in the film varies minimally, and it is ∼0.6 at. % across the whole composition range.

Figure 3

Figure 3. (a) EDX line scan of Ga and In content along the film. Black dashed lines represent positions of different crystallographic phases (bixbyite to mixed bixbyite–hexagonal–monoclinic to monoclinic). (b) EDX false color map of In content in the as deposited (InxGa1–x)2O3 film (51 mm diameter), the black arrow displaying the position where the line scan was taken, and a 4 mm wide strip was cut for further experiments.

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There exists a mirror relationship between Ga and In content from z = 0 to z = 46 mm; that is, as the Ga content increases, the In content decreases by the same amount. This reflects the substitution of Ga for In atoms in the unit cell. Dashed black lines are used to indicate the position in the film where a phase transition occurs in the XRD pattern presented in Figure 2. Between z = 0 and z = 5 mm, the In (Ga) content slowly decreases (increases). Past z = 5 mm, there is a rapid reduction (increase) in In (Ga) content. This is a common feature for combinatorial (In,Ga)2O3 thin films (9,19,20) deposited under oxygen-poor conditions, as detailed in ref (21). The hexagonal InGaO3 phase is seen in the XRD regardless of the substrate chosen. Between z = 5 mm and z = 10 mm, there is a steep decrease (increase) in In (Ga) content which coincides with the phase change from bixbyite to hexagonal (mixed) phase. The atomic concentration of the mixed hexagonal phase is around ∼40–60 at. %, which correlates well with the expected ratios of In to Ga in the hexagonal InGaO3 structure (roughly one-to-one In-to-Ga ratio). Beyond z = 10 mm, the rate of change of In/Ga content across the film lessens, while the film takes on the monoclinic β-Ga2O3 structure. The false-color plot of In content seen in Figure 3b shows this well with a very gradual growth of In content from right to left, until around a third of the distance along the film, when the In content increases rapidly. We also see the great control of In content over the film from the CCS PLD deposition method. The In content varies horizontally across the film, but there is negligible variation vertically in the narrow strip of material investigated by using photoemission spectroscopy.

3.2. Valence Band X-ray Photoelectron Spectroscopy (XPS)

XPS is an extremely powerful tool for probing the electronic structure of materials. It is made even more informative when used as a combinatorial characterization method. Figure 4 shows the XPS valence band (VB) spectra of the material, with the leading edge enlarged in the right-hand panel. The topmost data set, which is colored in red, is taken from the Ga-rich end of the film, with subsequent spectra fading to black as more In content is incorporated. This is supported by the XPS peak intensities in Figure S1 where the Ga 2p peaks slowly diminish in intensity, while the In 3d intensity grows relatively moving down the plot. A very clear change in shape of the valence band can be seen in Figure 4, going from the broad β-Ga2O3 valence spectra to the narrower In2O3 spectra. This directly reflects the changes in structure seen in Figure 2. There is also a larger valence band maximum (VBM) to Fermi level (EF) separation for β-Ga2O3 than for In2O3 (ref (22)) as is clearly seen in Figure 4, as the VBM slowly moves from left to right as the In content increases.

Figure 4

Figure 4. High-resolution XPS plots of the valence band of (InxGa1–x)2O3 film. A total of 46 spectra were recorded across the film at 1 mm intervals, matching the EDX measurement in Figure 3. The spectra recorded from the Ga-rich end of the film are at the top, with increasing In content down the plot.

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The VBM position from each spectrum could be estimated by a linear extrapolation method to the leading edge of the valence spectra. However, because of the effects of instrumental broadening of the spectra, this is not an accurate method for determining the VBM position, especially for systems with a very high DOS at the VBM such as the flat bands of In2O3 and β-Ga2O3. Instead, it is best practice to fit DFT-calculated valence spectra to the data and determine the energy distance from zero. This particular case is complicated further by the fact that we have many phases contributing to our spectra, making true calculated representation of the spectra very difficult (many calculations would be required). Instead, we acknowledge an ∼0.6 eV shift determined here and elsewhere from XPS of single-phase In2O3 (23) and β-Ga2O3 (24) and perform a rigid 0.6 eV shift to the linearly extrapolated edges to higher binding energy.
Figure 5 shows the VBM to EF separation as a function of EDX In content. Also shown are dashed lines corresponding to the In contents where phase transitions are seen. There is a slow decrease in the VBM to EF separation from ∼4.3 eV on the low In(x) content end to around ∼4 eV at In(x) ∼ 0.5, followed by a large decrease to ∼3.5 eV when the bixbyite phase is reached. This reflects that β-Ga2O3 has a much larger band gap than In2O3. This would indicate a valence band offset of around 0.8 eV between Ga2O3 and In2O3 with respect to EF. Interestingly, the hexagonal (mixed) phase is much closer in energy to the monoclinic β-Ga2O3, possibly because its atomic coordination is closer to that of β-Ga2O3 than In2O3.

Figure 5

Figure 5. Energy positions of the VBM from XPS referenced to EF as a function of EDX indium content. Black dashed lines separate the region of different crystallographic phases.

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A similar qualitative trend in the VBM to Fermi level separation as a function of composition has very recently been reported for combinatorial (InxGa1–x)2O3 films grown on both yttria-stabilized cubic zirconia and c-plane sapphire. (25) However, the values differ due to the use of raw values from linear extrapolation of the leading edge of the valence band.

3.3. Surface Space Charge

The surface of In2O3 has previously been shown to display electron accumulation (23,26) while β-Ga2O3 displays electron depletion. (24,27,28) This is vital information for surface and interface sensitive devices such as chemical sensors and heterostructures. Naturally then we ask how the surface space charge evolves over the alloy composition range, with the vision of tuning the surface electronic properties to specific device needs. To determine this, we find the barrier height ΦB (analogous to the Schottky barrier height of a metal/semiconductor contact), which is given as the separation between the conduction band minimum (CBM) and EF. Optical transmission measurements and derived absorption spectra (see Figure S4) were used to determine the bulk VBM to EF separation (once the dipole-forbidden transition from the topmost valence band in In2O3-like alloys is accounted for) at different composition points of the material. Figure 6 displays the optical gaps extracted from transmission measurements. The intervals between measurements across the film were greater than for the XPS results, but the positions were again translated into indium content via the EDX results. Optical gaps for end-point compositions (In(x) = 0 and In(x) = 1) were also added to the plot from the literature. (1,24,29) An optical gap bowing curve is fitted to the data and shown in Figure 6.

Figure 6

Figure 6. Optical gap vs indium content as determined via transmission spectroscopy. A fit to the data points was performed by using the bowing equation, (30,31) with the fitted parameters displayed in the equation. Importantly, for x > 0.6, because of transitions from the topmost valence bands being dipole forbidden, these values do not correspond to the fundamental band gap.

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The optical gap bowing is described by
(1)
where b is the bowing parameter. (30,31)Equation 1 was used to fit the data, resulting in a bowing parameter of b = 0.36 eV. The optical gaps of the end-point binary compounds determined from the fit are found to be 3.62 eV for In2O3 and 4.71 eV for Ga2O3. Equation 1 is normally used to describe fundamental band gap bowing in ternary alloys where no phase change occurs, but we apply it here to the optical gaps that exhibit a smooth variation across the composition range. There is quite a widespread of reported band gaps for Ga2O3 in the literature, often ranging from ∼4.6 to 4.9 eV3 or even higher. We believe that our results are consistent with other extensive studies of the optical properties of Ga2O3; see for example ref (20). The fundamental band gap of In2O3 can be estimated from Figure 6 by accounting for the dipole-forbidden transitions from the topmost valence bands on the In-rich side of the data set (x > 0.6). The bulk VBM to EF separation is very close to the band gap of the material (see for example ref (20) and references therein), and so we make this approximation here. This, coupled with our XPS determined surface VBM positions relative to EF (ξ), allows us to determine ΦB = Eg – ξ. When ΦB is positive (negative), this corresponds to the EF lying below (above) the CBM at the surface.
Figure 7a shows Eg and ΦB as a function of In(x) alloy composition, where Eg is accompanied by data from the literature to better visualize the trend. (32−36) The highest VB to lowest CB transition in In2O3 is known to have minimal dipole intensity, (1) and so to achieve the real Eg value, an energy of 0.8 eV was subtracted from the measured absorption onsets (as this is the energy difference between the first allowed transition and dipole-forbidden transitions (1)). Here, we set an In content of x = 0.6 for the phase transition from bixbyite to hexagonal (mixed) phase, as no data points exist in this region. This estimated value is informed by previous literature (31) as well as the data presented in Figures 2 and 3. The band gap smoothly decreases from ∼4.7 eV at low In content and abruptly alters when the phase changes to bixbyite before decreasing further to ∼2.9 eV at high In content. ΦB is plotted below Eg in Figure 5a and displays a very similar trend to the band gap. For the In-poor compositions, a positive value of ΦB is observed corresponding to upward band bending (schematically shown in Figure 7b) and surface electron depletion. ΦB then gradually decreases and is nearly zero at x = 0.35, indicating a flat bands regime with the VBM to Fermi level separation being constant as a function of depth. This then switches to downward band bending and electron accumulation in the In-rich composition range (Figure 7c). A guide to the eye is plotted in green through ΦB, with a discontinuity at the composition where the phase changes abruptly. In the context of electronic devices, the electron accumulation layer in bixybyite (InxGa1–x)2O3 helps explain the relative difficulty in fabricating devices such as normally off transistors and Schottky diodes (37) compared to monoclinic (InxGa1–x)2O3, which displays upward band bending. Conversely, it is rather challenging to obtain Ohmic contacts with low specific contact resistance with monoclinic (InxGa1–x)2O3 unlike for bixbiyte (InxGa1–x)2O3. Additionally, the surface electron accumulation layer of bixbiyte (InxGa1–x)2O3 may also be useful for gas sensing applications, (38) whereas the surface depletion layer of monoclinic (InxGa1–x)2O3 is beneficial for photodetector applications.

Figure 7

Figure 7. (a) Variation of band gap (Eg) and barrier height (ΦB) at the (InxGa1–x)2O3 surface with respect to varying indium content (x). Energy gaps were determined in this study by transmission measurements (○) as well as values being taken from the literature (□, Oshima; (32) ⬡, Regoutz; (33) △, Wenckstern; (34) ▽, Yang; (35) ◇, Zhang; (36) ▷, Swallow (24)). Barrier heights (ΦB) are derived by subtracting the surface Fermi level to VBM (ξ) from the band gap (ΦB = Eg – ξ). The surface space-charge variation is schematically represented in (b) and (c) with positive ΦB indicating electron depletion and upward band bending (b), while negative ΦB indicated electron accumulation and downward band bending (c).

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von Wenckstern et al. recently investigated the Schottky barrier characteristics of PLD combinatorial (InxGa1–x)2O3 films with PtOδ contacts via current–voltage measurements. (8) Their results are in excellent agreement with our findings, showing that the β-Ga2O3 phase has a higher Schottky barrier height which decreases as In content is increased. They find the barrier height difference between the two end phases to be ΔΦSB ∼ 0.8 eV, reasonably close to the difference we see here of ΔΦB ∼ 1.2 eV. They also find that the Ga-rich phase has the best Schottky diode response (ideality factor) while the In-rich has the lowest series resistance. The band bending diagrams for the two binary materials are shown in Figure 7b,c. Interestingly, both InxGa1–xN (ref (39)) and InxGa1–xAs (refs (40and41)) also display an electron accumulation to depletion transition at the surface with increasing Ga content. Those cases are simpler as no phase transition occurs across the composition range. However, the same trend is observed with Ga compounds displaying a larger band gap and higher, positive ΦB and surface electron depletion, while In compounds have a smaller band gap with a negative ΦB and surface electron accumulation. This will be discussed further in section 4.

3.4. Electronic Structure

To further explore the effects of the changing cation composition in (InxGa1–x)2O3 alloys, we focus on the electronic structure of the stoichiometric materials In2O3, InGaO3, and Ga2O3, utilizing DFT calculated DOS for the three materials. The partial and total density of states are displayed for Ga2O3, InGaO3, and In2O3 in Figure 8a–f. These plots show the orbital contributions (without any cross-section corrections) and include a small amount of Gaussian broadening (0.1 eV full width at half-maximum) to assist in visualizing the data. For the calculated spectra, the zero of the binding energy scale is set to the VBM.

Figure 8

Figure 8. Uncorrected partial (colored lines) and total (black lines) DOS for (a) β-Ga2O3, (b) hexagonal InGaO3, and (c) bixbyite In2O3, with the insets showing an expanded region of the CBM. The valence band regions are expanded in (d)–(f). For InGaO3 in (e), orbitals with strong overlap are shaded to aid in visualization.

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The VBs of β-Ga2O3 (Figure 8a,d) and In2O3 (Figure 8c,f) are primarily composed of three features. The bottom of the VB, in each case, consists of O 2p mixed with Ga 4s/In 5s, while the middle of the VB is dominated by O 2p, with some contribution from Ga 4p/In 5p states. Finally, the top of the VB is heavily dominated by O 2p, with a small contribution from occupied Ga 3d/In 4d orbitals, indicative of p–d repulsion in these materials. (42) The p–d repulsion is symmetry-forbidden at Γ in octahedral symmetry complexes, which forces the VBM to occur away from Γ. This effect accounts for In2O3 having an indirect fundamental band gap with the VBM away from the Γ-point being ∼50 meV above the highest valence band at the Γ-point. (23) Similarly, β-Ga2O3 has an indirect fundamental band gap of ∼40 meV below the direct one. (43) These differences between direct and indirect band gaps are too small to be significant in most optical experiments. (Note that ZnO does not have octahedral symmetry, and so p–d repulsion is symmetry-allowed; ZnO therefore has a direct fundamental band gap at Γ.) The CBM for both β-Ga2O3 and In2O3 is dominated by Ga 4s/In 5s character, with some contribution from O 2s (see the inset in Figure 8a–c) as well as some from O 2p. These are in agreement with previous calculations for In2O3 (refs (1,44,and45)) and Ga2O3 (refs (46−48)).
The InGaO3 structure (Figure 8b,e) has a more complex DOS than the two binary phases because it has more than two cation elemental contributions. However, many similarities can be identified. The O 2p component is again dominant across the whole VB. The other orbitals tend to heavily hybridize throughout the valence band structure, likely because of the close proximity of In and Ga orbital energies. The bottom of the VB has predominantly O 2p orbital character (∼6.5 eV). The next distinguishable feature around ∼5 eV is again O 2p-dominated with strong contributions from In and Ga s states. The middle section of the DOS has additional intensity coming from Ga 4p and In 5p components, which have their maximum intensity around ∼3.5 and ∼2 eV, respectively. These states have significant intensity over the whole VB and do not obviously mix as readily as the s and d orbitals. The VBM is again O 2p-dominated, with contributions from In and Ga d levels. The CBM is dominated by O 2s, In 5s, and Ga 4s contributions with some O 2p character.
Figure 9 shows the cross-section-corrected DOS (using interpolated values for 1486.6 eV from the photoionization cross sections tabulated in ref (49)), including broadening by convolution with a Voigt function with ∼0.4 eV Gaussian and ∼0.3 eV Lorentzian component, compared to the experimentally determined VB spectra. The calculated and experimental VB regions are shown in Figures 9a and 9b, respectively, while the calculated and experimental semicore levels (Ga 3d and In 4d) are shown in Figures 9c and 9d, respectively. First, comparing Figures 9a and 9b, we see the general features match very well throughout all the spectra. Figure 9a displays the calculated β-Ga2O3 spectra in red, followed by the InGaO3 phase in the middle in brown, and finally the In2O3 in black at the bottom, with the PDOS contributions labeled. Figure 9b shows the experimental data where the phase transition is demonstrated by utilizing phase pure single crystalline data for the top and bottom spectra. (23,24) The brown spectrum in the center belongs to the hexagonal (mixed) phase InGaO3, while the spectra either side of this correspond to an In content of approximately 25% and 75%, respectively. All experimental spectra have been shifted to locate their VBM at 0 eV for comparison with the calculated DOS. Upon comparison of the spectra in Figure 9b, the highest binding energy feature of the β-Ga2O3 (Ga 4s dominated) is prominent around 6.5 eV but diminishes in size and shifts to lower binding energy as In content is increased. In the InGaO3 spectra, this feature is extended in energy but much less intense, matching the calculations in Figure 9a extremely well albeit with slightly less features resolved than calculated. This feature shifts and diminishes further, being located at ∼5.5 eV in pure In2O3. The leading edge at the VBM looks similar in the two end-point spectra, but the InGaO3 phase has additional intensity due to the contribution from two cation d levels.

Figure 9

Figure 9. Valence and semicore levels. (a) Cross-section-corrected and broadened DFT partial and total VB-DOS of β-Ga2O3 (top), InGaO3 (middle), and In2O3 (bottom). (b) Measured XPS VB spectra aligned to the VBM. The top and bottom spectra were taken from refs (24) and (23) for single crystalline β-Ga2O3 (x = 0) and In2O3 (x = 1), respectively. The other spectra represent intermediate composition steps, working from top to bottom In(x) is x = 0.25, x = 0.5, and x = 0.75, respectively. (c) DFT calculated semicore levels for β-Ga2O3 (top), InGaO3 (middle), and In2O3 (bottom). The semicore levels are dominated by the d-orbital contributions from Ga and In. Note that SOC has been included for the In2O3 which splits the d band, better accounting for the broadness of the semicore level. (d) Measured XPS semicore level spectra of β-Ga2O3 (ref (24)) (top), InGaO3 (middle), and In2O3 (ref (23)) (bottom). Two Gaussians are overlaid in the InGaO3 spectra to aid comparison with the In 4d and Ga 3d hybrid semicore level in the theory. The high binding energy region in each case is shown magnified ×15.

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Turning attention now to the calculated semicore levels in Figure 9c, we see both In2O3 and β-Ga2O3 have a large peak comprised nearly entirely of Ga 3d/In 4d and a higher binding energy peak of mostly O 2s character (with some cation d mixing). The shoulder on the low binding energy side of the large peaks has a small O s contribution also, although this is swamped after cross-section corrections are applied. This s–d hybridization is associated with materials for which the cation-d and anion-s levels have similar energy. (50) Comparing the energy position of the calculated and experimental data (Figures 9c and 9d), we see the characteristic underbinding of these levels by the calculations, (51) especially the lower binding energy semicore levels which are around ∼1–2 eV below the experimental ones. (23) This is a deficiency of the DFT approach used in these calculations; (52) despite the HSE06 functional generally giving very accurate band gaps, quasi-particle effects are neglected which have been shown to redistribute spectral weight at higher binding energies. (46,48,51) We also see that the experimentally measured peaks are much wider than those calculated (which are broadened in the same manner as the VB in Figure 9a). This may be attributed to an increase in lifetime broadening for the localized semicore levels compared with the much less localized valence states, (29) although we cannot rule out final-state relaxation effects working to narrow the valence band spectra, which are not included in the calculation. (53) Despite this, the measured regions share similar features to the calculated ones, such as the asymmetry seen in the low-energy peaks. It is clear in Figure 9d that the In 4d-derived peak is much wider than the Ga 3d peak. This is due to a higher degree of spin–orbit interaction which splits the In 4d band. Spin–orbit coupling (SOC) has been included in the DOS for In2O3 in Figure 9c, explaining its irregular shape relative to the Ga 3d level in agreement with previous studies. (54) See Figure S6 for a comparison of the In 4d level calculated with and without SOC included.
The higher binding energy peaks seen around ∼19 eV in Figure 9c,d, which comprise of mostly O 2s character, are vastly overestimated in intensity in the calculation compared to experiment, where the intensity had to be multiplied by a factor of ×15 before it was visible. This again may be attributed to the issues discussed above. The theory predicts a fairly strong contribution from the metal d level to this peak, giving further evidence that s–d hybridization occurs in all three materials. It is also worth noting that for the mixed phase InGaO3 spectra there are two distinct peaks around 13 and 15 eV in the theory (15 and 17 eV in the experiment). The peak intensities vary heavily with indium content (x), which is a good indicator of the extent of alloying and the stoichiometry of the system (acknowledging that the photoionization cross section is roughly twice as large for In 4d compared to Ga 3d, explaining why the In 4d peak is twice as intense as the Ga 3d peak in the stoichiometric InGaO3 material).

4. Chemical Trends in Common-Cation Materials

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The natural question that follows the determination of the electronic structure of a material is, how does this fit with the chemical trends of related materials, such as those with a common cation (or indeed common anion)? Here we compare the properties of In2O3, InGaO3, and β-Ga2O3 with each other and with other indium- and gallium-containing compounds in the form of the zinc-blende (ZB) and wurtzite III–V semiconductors, which have seen a lot of research in previous years. (55−57) The understanding gained from these arguably simpler systems can be used to further our understanding of the metal oxides investigated here.
Figure 10a shows the calculated band-edge positions for common Ga- and In-cation compounds, including In2O3, β-Ga2O3, InGaO3, In–V, and Ga–V semiconductor materials with respect to the charge neutrality level (ECNL). The calculated direct band gaps from the Γ-point were tuned by varying the fraction of Hartree–Fock exact exchange incorporated into HSE06 range-separated hybrid functional to reproduce experimental values (see for examples refs (1and61−63)). Focusing on the VBM for the Ga compounds in Figure 10a, we see a general trend emerge. First, the VBM are seen to increase in energy from Ga2O3 < GaN < GaP < GaAs < GaSb with respect to ECNL. As shown in Figure 8, the VBM of β-Ga2O3 consists of primarily O p character mixed with Ga d, while the VBM of the ZB Ga–V semiconductors are mostly of mixed anion and cation p-orbital character. (56) Therefore, both sets of materials have mostly anion p-orbital VBM character in common, so this is the major influence on the VBM position and not the cation energy level. The fact that the In compounds roughly share this VBM evolution supports this. Indeed, from Figure 10b of the individual elemental orbital energy levels, it is apparent that the anion p levels share exactly the same trend, increasing in energy going from O through to Sb. Also, as might be expected, InGaO3 has a VBM in between that of β-Ga2O3 and In2O3. Note that the indirect transition from the VBM between Γ and M to the CBM at Γ is highlighted for InGaO3, which is still between the VBM positions of β-Ga2O3 and In2O3. The only other significant indirect transition in Figure 10a is from GaP, where the CBM lies between Γ and X (ref (61)). (In2O3 and β-Ga2O3 may have a smaller indirect transition also, but these may be too close to the direct band gaps to definitively prove experimentally. (46,51,64)) Note also that the differences between the VBM position of the common-anion materials (see for example β-Ga2O3 and In2O3, where the VBM of β-Ga2O3 is lower relative to ECNL) have been explained for ZB III–V and II–VI semiconductors to be due to the p–d interaction, (42,65,66) a relatively small effect in these materials compared to the orbital positioning, but explaining why the VBM of Ga–V tends to have a lower energy than In–V.

Figure 10

Figure 10. (a) Theoretically calculated band-edge positions for Ga and In cation compounds relative to the charge neutrality level ECNL. Γ–Γ transitions are shown, but in cases where the indirect transition is significant this is indicated. (b) Atomic orbital energy levels for the constituent elements taken experimentally (58) and theoretically (59) where experiment was not available. These results corroborate the trends seen in other works. (55,60) (c) Absolute volume deformation potential at the Γ-point aVΓ, CBM deformation potential aVΓ,CBM, and VBM deformation potential aVΓ,VBM for compounds. (d) Calculated equilibrium unit cell volume and volume per atom (unit cell volume normalized by the number of atoms it contains) for compounds.

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The CBM of these systems all derive from cation–anion s–s hybridization. However, it is clear from Figure 10a,b that there is very little correlation between the trends of the position of the CBM and that of the individual anion/cation s levels. A useful parameter that has been used to describe the shift in band extrema with respect to an absolute energy reference (often the CNL) for crystals under volume deformation is known as the hydrostatic volume deformation potential (aV). Hydrostatic volume deformation potentials were evaluated over a series of volumes to identify the change in the direct band gap with volume according to the formula
(2)
where the direct band gap EgΓ was evaluated at the Γ-point from the conduction and valence band eigenvalues. While deformation potentials for relative energies, such as the band gap, are robust from first-principles calculations, changes in individual states, such as the conduction and valence band extrema, require knowledge of a suitable reference level such as a core level, vacuum level, or another state, whose dependence on volume deformations is known. (56,67−69) Cardona and Christensen (68) used Brillouin-zone averaging for deformation potentials referenced to the dielectric midgap energy, which was previously shown to be an equivalent representation of the branch-point energy (EBP) and the CNL for zinc-blende semiconductors. (70) Following this approach, we evaluated the branch-point energies for the oxides and III–V materials with the scheme of Schleife et al. (71,72) using 12 valence bands and 6 conduction bands for the 10-atom monoclinic and hexagonal unit cells and 48 valence bands and 24 valence bands for the 40-atom bixbyite unit cells. With these values, we identify an EBP of 4.06 eV above the VBM for Ga2O3 and 3.39 eV above the VBM for In2O3, indicating a valence band offset of 0.67 eV, in good agreement with the value ∼0.6 eV derived experimentally. (22,24,29) We note that the overall quantitative conclusions do depend on the choice of the numbers of bands included in the averaging, (72) but other comparable choices (e.g., 4 conduction and 8 valence bands for Ga2O3 and 16 conduction and 32 valence bands for In2O3) led to changes in the relative offsets by ∼0.05 eV. These values were also computed for Ga–V and In–V compounds, explicitly accounting for spin–orbit effects, with the results summarized in Table 1.
Table 1. Summary of Calculated Properties, Including Deformation Potentials for the Band Gaps (aVΓ) and Band Edges (aVΓ,c and aVΓ,v)a
 Ga2O3GaNGaPGaAsGaSbIn2O3InNInPInAsInSbInGaO3
α (%)3228283233282526293032
EBP (eV)4.062.380.680.42–0.113.391.650.750.470.043.79
EgΓ (eV)4.853.462.901.530.822.860.731.420.410.224.13
B0 (kbar)18442003885751552171514236965964552026
volume (Å3)104.3145.2040.6245.6257.78522.2462.1051.3656.8870.23113.99
V/atom (Å3)10.4311.3020.3122.8128.8913.0615.5225.6828.4435.1111.40
aVΓ (eV)–8.66–7.82–9.08–8.54–8.46–6.34–4.04–6.14–5.92–6.77–8.70
aVΓ,c (eV)–4.96–6.48–9.86–8.86–9.27–3.70–3.23–6.33–6.66–7.79–5.43
aVΓ,v (eV)3.701.34–0.78–0.32–0.812.640.81–0.19–0.73–1.023.27
a

The band edge deformation potentials were determined by using the branch-point energy (EBP, defined relative to each material’s VBM) as a reference level, as described in the text. The α parameter represents the fraction of Hartree–Fock exact exchange incorporated into the HSE06 range-separated hybrid functional, which was tuned to reproduce the experimental band gaps.

We additionally assessed the validity of EBP as a suitable reference scale through tests with InP by aligning the average electrostatic potential with the vacuum level for a series of hydrostatically strained unrelaxed, 8-layer slab models of the (111) surface. Aligned relative to the vacuum level, the deformation potentials of EBP and the VBM were found to be −0.19 and −0.41 eV, respectively. The value for EBP is on the same order as the deformation potential calculated for the VBM when aligned relative to EBP (∼−0.2 eV), indicating the uncertainty in this approach. This suggests that EBP, similar to core levels, (69) are not fully robust reference levels for absolute volume deformation potentials. Nonetheless, we use it to consider qualitative trends in the spirit of Cardona and Christensen. We include the calculated band gap and band edge components of the deformation potentials as a function of the equilibrium volume per atom in Figures S7 and S8, with the results summarized in Figure 10c and Table 1.
When considering the absolute deformation potentials of the band edges, as referenced to the EBP in Figure 10c, we see that aVΓ,v (the deformation potential of the VBM) is positive for oxides and decreases as the coupling from the anion p orbitals increases, becoming negative for phosphides, arsenides, and antimonides, which all share similar values. This seems to have only a small effect on the position of the VBM due to lattice compression, which is instead mostly dependent on the anion p-orbital position as mentioned. aVΓ,c tends to be a relatively large negative number, contributing much more heavily to |aV| and to the position of the CBM. (55,56,73) In this respect, we also find that the oxides do exhibit significant differences compared to the III–Vs. We find that the aVΓ,c values are again negative for the oxides, but they exhibit a weaker dependence than that in III–Vs. We find that the conduction and valence band deformation potentials are of more comparable magnitudes in the oxides, whereas III–Vs tend to exhibit much larger magnitudes of the aVΓ,c values relative to the aVΓ,v.
When compared to the III–Vs, we find that the oxide aVΓ (where aVΓ = aVΓ,c – aVΓ,v) values are similar, but with the nitride values deviating more than those of the oxides. Our results for the Ga–V and In–V band gap deformation potentials largely follow those in ref (69), exhibiting the same qualitative trends, but with slightly larger absolute values. This may be a consequence of our results being obtained with tuned hybrid functionals, as opposed to the density functional theory within the local density approximation. (69) Another apparent outlier in Figure 10a,c is InN, which was discussed previously by Wei et al. (55) to be in no small part due to the relatively small value of aVΓ,c and hence small aVΓ.
Hence, the shifts in the energy gap are heavily affected not only by the positions of the constituent orbital energy levels but also by the changes in the CBM position, the changes in aV, and on the size of the unit cell. We plot the unit cell volumes and the volume per atom (unit cell volume divided by its occupany) in Figure 10d to visualize how these sizes vary between materials. Note that In2O3 has an extremely large unit cell, but it contains many atoms, and so its volume per atom is actually rather small in comparison to those in III–Vs. We see in Figure 10c that the Ga-anion semiconductors have larger |aV| than the respective In-anion semiconductors. This coupled with the fact that the Ga-based semiconductors have lower unit cell volumes than the respective indium-based compounds means Ga-based materials tend to have larger band gaps.
To summarize, our results are consistent with previous results on the III–V materials that identify (1) band gap deformation potentials are negative and (2) this dependence has a far larger contribution from the deformation potential of the conduction band minima (aVΓ,c) rather than for the valence band maxima (aVΓ,v). (56,67−69) However, in contrast to the III–Vs, we find (3) larger contributions to the aVΓ,v for the more ionic oxide semiconductors which are of comparable magnitude to the aVΓ,c.

5. Conclusion

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The evolution of the electronic structure of a variable composition (InxGa1–x)2O3 alloy has been determined by using a combination of combinatorial XPS, measurements of the optical gap, and state-of-the-art hybrid functional DFT calculations. We show the simple trends regarding the evolution of the valence band edges in (InxGa1–x)2O3. The energy of the valence band edge is reduced considerably with increasing In content when the bixbyite phase is realized. This affects the surface space-charge properties of the material. As shown, β-Ga2O3 displays upward surface band bending and electron depletion, which diminishes and subsequently changes to electron accumulation with increasing In content. The bixbyite phase displays electron accumulation and downward band bending consistent with previous findings. The transition from accumulation to depletion for Ga-rich alloys occurs at approximately x = 0.35. Furthermore, we investigate the electronic structure of these materials through calculated electronic density of states for the phase-pure materials and compare this to XPS valence band spectra. All phases are found to have O 2p-dominated valence bands and have contributions from In 4d/Ga 3d states at the VBM, associated with p–d repulsion. The CBM are In 5s/Ga 4s-dominated with a strong O 2s contribution. s–d hybridization of the systems is explored looking at the semicore levels. Finally, In2O3 and β-Ga2O3 are placed in the context of other materials to shed light on the origin of the optoelectronic performance of these materials, such as their wide gap nature and the n-type dopability of In2O3 and β-Ga2O3. The electronic structure and band edge positions of In2O3, Ga2O3, and (InxGa1–x)2O3 alloys have been understood in terms of their place within chemical trends of common-cation and common-anion materials. Trends in atomic orbital energies have been examined, and deformation potentials have been calculated. Compared with III–V semiconductors, it was found that there is a larger contribution to the aVΓ values from the aVΓ,v terms for the more ionic oxide semiconductors which are of comparable magnitude to the aVΓ,c terms. The volume deformation potentials are −8.66 eV for Ga2O3, −8.70 eV for InGaO3, and −6.34 eV for In2O3.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.0c16021.

  • XPS core levels and associated analysis, optical transmission and absorption spectra, XPS semicore levels, DFT of semicore levels accounting for SOC, DFT calculated band gap deformation potentials (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Jack E. N. Swallow - Stephenson Institute for Renewable Energy and Department of Physics, University of Liverpool, Liverpool L69 7ZF, U.K.;  Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, U.K.
    • Robert G. Palgrave - Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.;  Orcidhttp://orcid.org/0000-0003-4522-2486
    • Philip A. E. Murgatroyd - Stephenson Institute for Renewable Energy and Department of Physics, University of Liverpool, Liverpool L69 7ZF, U.K.
    • Anna Regoutz - Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.;  Orcidhttp://orcid.org/0000-0002-3747-3763
    • Michael Lorenz - Felix Bloch Institute for Solid State Physics, Universität Leipzig, Leipzig, Germany;  Orcidhttp://orcid.org/0000-0003-2774-6040
    • Anna Hassa - Felix Bloch Institute for Solid State Physics, Universität Leipzig, Leipzig, Germany
    • Marius Grundmann - Felix Bloch Institute for Solid State Physics, Universität Leipzig, Leipzig, Germany;  Orcidhttp://orcid.org/0000-0001-7554-182X
    • Holger von Wenckstern - Felix Bloch Institute for Solid State Physics, Universität Leipzig, Leipzig, Germany
    • Joel B. Varley - Lawrence Livermore National Laboratory, Livermore, California 94550, United States;  Orcidhttp://orcid.org/0000-0002-5384-5248
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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J.E.N.S. acknowledges funding through the Engineering and Physical Sciences Research Council (EPSRC) Centre for Doctoral Training in New and Sustainable Photovoltaics (EP/L01551X/1). James Gibbon is acknowledged for preliminary analysis of the photoemission data. T.D.V. acknowledges funding from EPSRC Grant EP/N015800/1. P.A.E.M. acknowledges undergraduate vacation bursary funding from the EPSRC. This work was partially performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. DOE, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. This work was also supported by the Air Force Office of Scientific Research under Award FA9550-18-1-0024.

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    Journal of Physics D: Applied Physics (2016), 49 (43), 433001/1-433001/53CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
    A review. Oxide electronic materials provide a plethora of possible applications and offer ample opportunity for scientists to probe into some of the exciting and intriguing phenomena exhibited by oxide systems and oxide interfaces. In addn. to the already diverse spectrum of properties, the nanoscale form of oxides provides a new dimension of hitherto unknown phenomena due to the increased surface-to-vol. ratio. Oxide electronic materials are becoming increasingly important in a wide range of applications including transparent electronics, optoelectronics, magnetoelectronics, photonics, spintronics, thermoelecs., piezoelecs., power harvesting, hydrogen storage and environmental waste management. Synthesis and fabrication of these materials, as well as processing into particular device structures to suit a specific application is still a challenge. Further, characterization of these materials to understand the tunability of their properties and the novel properties that evolve due to their nanostructured nature is another facet of the challenge. The research related to the oxide electronic field is at an impressionable stage, and this has motivated us to contribute with a roadmap on 'oxide electronic materials and oxide interfaces'. This roadmap envisages the potential applications of oxide materials in cutting edge technologies and focuses on the necessary advances required to implement these materials, including both conventional and novel techniques for the synthesis, characterization, processing and fabrication of nanostructured oxides and oxide-based devices. The contents of this roadmap will highlight the functional and correlated properties of oxides in bulk, nano, thin film, multilayer and heterostructure forms, as well as the theor. considerations behind both present and future applications in many technol. important areas as pointed out by Venkatesan. The contributions in this roadmap span several thematic groups which are represented by the following authors: novel field effect transistors and bipolar devices by Fortunato, Grundmann, Boschker, Rao, and Rogers; energy conversion and saving by Zaban, Weidenkaff, and Murakami; new opportunities of photonics by Fompeyrine, and Zuniga-Perez; multiferroic materials including novel phenomena by Ramesh, Spaldin, Mertig, Lorenz, Srinivasan, and Prellier; and concepts for topol. oxide electronics by Kawasaki, Pentcheva, and Gegenwart. Finally, Miletto Granozio presents the European action 'towards oxide-based electronics' which develops an oxide electronics roadmap with emphasis on future nonvolatile memories and the required technologies. In summary, we do hope that this oxide roadmap appears as an interesting up-to-date snapshot on one of the most exciting and active areas of solid state physics, materials science, and chem., which even after many years of very successful development shows in short intervals novel insights and achievements.
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  5. 5
    Gonçalves, G.; Barquinha, P.; Pereira, L.; Franco, N.; Alves, E.; Martins, R.; Fortunato, E. High Mobility a-IGO Films Produced at Room Temperature and Their Application in TFTs. Electrochem. Solid-State Lett. 2010, 13, H20– H22,  DOI: 10.1149/1.3257613
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  6. 6
    Huang, W.-L.; Hsu, M.-H.; Chang, S.-P.; Chang, S.-J.; Chiou, Y.-Z. Indium Gallium Oxide Thin Film Transistor for Two-Stage UV Sensor Application. ECS J. Solid State Sci. Technol. 2019, 8, Q3140– Q3143,  DOI: 10.1149/2.0251907jss
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    Indium gallium oxide thin film transistor for two-stage UV sensor application
    Huang, Wei-Lun; Hsu, Ming-Hung; Chang, Sheng-Po; Chang, Shoou-Jinn; Chiou, Yu-Zung
    ECS Journal of Solid State Science and Technology (2019), 8 (7), Q3140-Q3143CODEN: EJSSBG; ISSN:2162-8769. (Electrochemical Society)
    In this work, indium gallium oxide (IGO) thin film transistor (TFT) was fabricated by radio-frequency (RF) sputtering. The transmittance of the TFT shows larger than 80% cross the visible light region. As a wide bandgap and high transparency semiconducting material, IGO is a potential candidate for UV-detection applications.Measured in the dark, the IGO TFT exhibits a threshold voltage of 0.9 V, mobility of 2.66 cm2/Vs, on-off ratio of 1.21×106, subthreshold swing of 0.41 V/dec. The TFT was then employed to detect UV light and the sensing properties are investigated. The IGO phototransistor has a high responsivity of 5.012 A/W and a rejection ratio of 1.65×105. The above results reveal that IGO phototransistor is a brilliant multi-functional device, which can serve as either a switch component or a UV sensor.
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  7. 7
    Kim, Y. G.; Kim, T.; Avis, C.; Lee, S.-H.; Jang, J. Stable and High-Performance Indium Oxide Thin-Film Transistor by Ga Doping. IEEE Trans. Electron Devices 2016, 63, 1078– 1084,  DOI: 10.1109/TED.2016.2518703
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    7
    Stable and high-performance indium oxide thin-film transistor by Ga doping
    Kim, Youn Goo; Kim, Taehun; Avis, Christophe; Lee, Seung-Hun; Jang, Jin
    IEEE Transactions on Electron Devices (2016), 63 (3), 1078-1084CODEN: IETDAI; ISSN:0018-9383. (Institute of Electrical and Electronics Engineers)
    Research on a replacement of amorphous silicon for a thin-film transistor (TFT) and large area electronics has been driven by costly vacuum processed indium-gallium-zinc oxide (IGZO). Even though widely studied, the performances still require improvement, and a wide no. of other materials have been tested. While indium-zinc oxide, IGZO, indium-zinc-tin oxide (ZTO), and ZTO have been widely investigated, gallium-doped indium oxide (IGO) has not been under highlight. Here, we report the use of simple and cost effective spin-coated IGO TFT using spin-coated AlOx gate dielec. We achieved high mobility over 50 cm2/Vs and high stability. The thin films are studied by transmission electron microscopy, X-ray diffraction, XPS, Raman spectra, at. force microscopy, and field-effect measurements. Analyses reveal the strong dependence between crystallinity, mobility, and stability. All TFTs show excellent operation, with champion characteristics for the 10% Ga-doped InOx, revealing a mobility of 52.6 cm2/Vs, on/off ratios of 108, and VTH variation of <0.1 V during 1 h of stress measurement.
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  8. 8
    von Wenckstern, H.; Splith, D.; Werner, A.; Müller, S.; Lorenz, M.; Grundmann, M. Properties of Schottky Barrier Diodes on (InxGa1-x)2O3 for 0.01 ≤ x ≤ 0.85 Determined by a Combinatorial Approach. ACS Comb. Sci. 2015, 17, 710– 715,  DOI: 10.1021/acscombsci.5b00084
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    8
    Properties of Schottky Barrier Diodes on (InxGa1-x)2O3 for 0.01 ≤ x ≤ 0.85 Determined by a Combinatorial Approach
    von Wenckstern, H.; Splith, D.; Werner, A.; Mueller, S.; Lorenz, M.; Grundmann, M.
    ACS Combinatorial Science (2015), 17 (12), 710-715CODEN: ACSCCC; ISSN:2156-8944. (American Chemical Society)
    We investigated properties of an (InxGa1-x)2O3 thin film with laterally varying cation compn. that was realized by a large-area offset pulsed laser deposition approach. Within a two inch diam. thin film, the compn. varies between 0.01 ≤ x ≤ 0.85, and three crystallog. phases (cubic, hexagonal, and monoclinic) were identified. We obsd. a correlation between characteristic parameters of Schottky barrier diodes fabricated on the thin film and its chem. and structural material properties. The highest Schottky barriers and rectification of the diodes were found for low indium contents. The thermal stability of the diodes is also best for Ga-rich parts of the sample. Conversely, the series resistance is lowest for large In content. Overall, the (InxGa1-x)2O3 alloy is well-suited for potential applications such as solar-blind photodetectors with a tunable absorption edge.
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  9. 9
    Kranert, C.; Lenzner, J.; Jenderka, M.; Lorenz, M.; von Wenckstern, H.; Schmidt-Grund, R.; Grundmann, M. Lattice Parameters and Raman-active Phonon Modes of (InxGa1-x)2O3 for x < 0.4. J. Appl. Phys. 2014, 116, 013505,  DOI: 10.1063/1.4886895
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    9
    Lattice parameters and Raman-active phonon modes of (InxGa1-x)2O3 for x < 0.4
    Kranert, Christian; Lenzner, Joerg; Jenderka, Marcus; Lorenz, Michael; von Wenckstern, Holger; Schmidt-Grund, Ruediger; Grundmann, Marius
    Journal of Applied Physics (Melville, NY, United States) (2014), 116 (1), 013505/1-013505/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    We present X-ray diffraction and Raman spectroscopy investigations of (InxGa1-x)2O3 thin films and bulk-like ceramics in dependence of their compn. The thin films grown by pulsed laser deposition have a continuous lateral compn. spread allowing the detn. of phonon mode properties and lattice parameters with high sensitivity to the compn. from a single 2-in. wafer. In the regime of low indium concn., the phonon energies depend linearly on the compn. and show a good agreement between both sample types. We detd. the slopes of these dependencies for eight different Raman modes. While the lattice parameters of the ceramics follow Vegard's rule, deviations are obsd. for the thin films. Further, we found indications of the high-pressure phase InGaO3 II in the thin films above a crit. indium concn., its value depending on the type of substrate. (c) 2014 American Institute of Physics.
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  10. 10
    Shannon, R. D.; Prewitt, C. T. Synthesis and Structure of Phases in the In2O3-Ga2O3 System. J. Inorg. Nucl. Chem. 1968, 30, 1389– 1398,  DOI: 10.1016/0022-1902(68)80277-5
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    10
    Synthesis and structure of phases in the indium sesquioxide-gallium sesquioxide system
    Shannon, R. D.; Prewitt, C. T.
    Journal of Inorganic and Nuclear Chemistry (1968), 30 (6), 1389-98CODEN: JINCAO; ISSN:0022-1902.
    Compns. Ga2-xInxO3 (x = 0.0-1.0) having the β-Ga2O3 structure were prepd. in the powder form and as single crystals by several techniques and characterized by x-ray and resistivity measurements. The compn. InGaO3 transforms at high pressures to a new phase, InGaO3 II. InGaO3 II has a simple hexagonal oxide structure which is the 1st structure reported contg. Ga3+ in 5-fold coordination. The cell parameters are: a 3.310 ± 0.002, c 12.039 ± 0.002 A.; Z = 2; ρ(calcd.) = 6.756; and the space group is P63/mmc. Interat. distances obtained from least-sqs. refinement results are In-O(1)[×2], 3.010 A.; In-O(2)[×6], 2.174 A.; Ga-O(1)[×3], 1.911 A.; and Ga-O(2)[×2], 1.973 A. InGaO3 II is isotypic with hexagonal YAlO3, and its structure is related to that of YMnO3, which contains 5 coordinated Mn3+. 20 references.
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  11. 11
    Kneiß, M.; Hassa, A.; Splith, D.; Sturm, C.; von Wenckstern, H.; Lorenz, M.; Grundmann, M. Epitaxial Stabilization of Single Phase κ-(InxGa1-x)2O3 Thin Films up to x = 0.28 on c-Sapphire and κ-Ga2O3(001) Templates by Tin-Assisted VCCS-PLD. APL Mater. 2019, 7, 101102,  DOI: 10.1063/1.5120578
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    11
    Epitaxial stabilization of single phase κ (InxGa1-x)2O3 thin films up to x = 0.28 on c-sapphire and κ -Ga2O3(001) templates by tin-assisted VCCS-PLD
    Kneiss, M.; Hassa, A.; Splith, D.; Sturm, C.; von Wenckstern, H.; Lorenz, M.; Grundmann, M.
    APL Materials (2019), 7 (10), 101102/1-101102/10CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    High-quality (InxGa1-x)2O3 thin films in the orthorhombic κ -phase were grown by pulsed-laser deposition (PLD) on c-sapphire substrates as well as PLD-grown κ -Ga2O3 thin film templates. We varied the In-content 0 = x = 0.38 of the layers using a single, elliptically segmented, and tin-doped (In0.4Ga0.6)2O3/Ga2O3 target, employing the vertical continuous compn. spread (VCCS) PLD-technique. A stoichiometric transfer of In and Ga from the target to the thin films has been confirmed, suggesting that the formation of volatile Ga2O and In2O suboxides is not a limiting factor in the tin-assisted growth mode. For all x, the thin films crystd. predominantly in the κ -modification as demonstrated by XRD 2θ -ω scans. However, for x > 0.28, phase sepn. of the cubic bixbyite and the κ -phase occurred. The κ -Ga2O3 template increased the cryst. quality of the κ -(InxGa1-x)2O3 thin film layers remarkably. Epitaxial, but relaxed growth with three in-plane rotational domains has been found for all thin films by XRD κ -scans or reciprocal space map measurements. Smooth surface morphologies (Rq < 3 nm) for all phase pure thin films were evidenced by at. force microscopy measurements, making them suitable for multilayer heterostructures. The compn.-dependent in- and out-of plane lattice consts. follow a linear behavior according to Vegard's law. A linear relationship can also be confirmed for the optical bandgaps that demonstrate the feasibility of bandgap engineering in the energy range of 4.1-4.9 eV. The results suggest κ -(InxGa1-x)2O3 as a promising material for heterostructure device applications or photodetectors. (c) 2019 American Institute of Physics.
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    Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207– 8215,  DOI: 10.1063/1.1564060
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    Hybrid functionals based on a screened Coulomb potential
    Heyd, Jochen; Scuseria, Gustavo E.; Ernzerhof, Matthias
    Journal of Chemical Physics (2003), 118 (18), 8207-8215CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    Hybrid d. functionals are very successful in describing a wide range of mol. properties accurately. In large mols. and solids, however, calcg. the exact (Hartree-Fock) exchange is computationally expensive, esp. for systems with metallic characteristics. In the present work, we develop a new hybrid d. functional based on a screened Coulomb potential for the exchange interaction which circumvents this bottleneck. The results obtained for structural and thermodn. properties of mols. are comparable in quality to the most widely used hybrid functionals. In addn., we present results of periodic boundary condition calcns. for both semiconducting and metallic single wall carbon nanotubes. Using a screened Coulomb potential for Hartree-Fock exchange enables fast and accurate hybrid calcns., even of usually difficult metallic systems. The high accuracy of the new screened Coulomb potential hybrid, combined with its computational advantages, makes it widely applicable to large mols. and periodic systems.
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  13. 13
    Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953– 17979,  DOI: 10.1103/PhysRevB.50.17953
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    Projector augmented-wave method
    Blochl
    Physical review. B, Condensed matter (1994), 50 (24), 17953-17979 ISSN:0163-1829.
    There is no expanded citation for this reference.
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    Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-energy Calculations using a Plane-wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169– 11186,  DOI: 10.1103/PhysRevB.54.11169
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    Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
    Kresse, G.; Furthmueller, J.
    Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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    Blöchl, P. E.; Jepsen, O.; Andersen, O. K. Improved Tetrahedron Method for Brillouin-zone Integrations. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 16223– 16233,  DOI: 10.1103/PhysRevB.49.16223
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    Improved tetrahedron method for Brillouin-zone integrations
    Blochl, Peter E.; Jepsen, O.; Andersen, O. K.
    Physical Review B: Condensed Matter and Materials Physics (1994), 49 (23), 16223-33CODEN: PRBMDO; ISSN:0163-1829.
    Several improvements of the tetrahedron method for Brillouin-zone integrations are presented. (1) A translational grid of k points and tetrahedra is suggested that renders the results for insulators identical to those obtained with special-point methods with the same no. of k points. (2) A simple correction formula goes beyond the linear approxn. of matrix elements within the tetrahedra and also improves the results for metals significantly. For a required accuracy this reduces the no. of k points by orders of magnitude. (3) Irreducible k points and tetrahedra are selected by a fully automated procedure, requiring as input only the space-group operations. (4) The integration is formulated as a weighted sum over irreducible k points with integration wts. calcd. using the tetrahedron method once for a given band structure. This allows an efficient use of the tetrahedron method also in plane-wave-based electronic-structure methods.
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  16. 16
    Onuma, T.; Saito, S.; Sasaki, K.; Goto, K.; Masui, T.; Yamaguchi, T.; Honda, T.; Kuramata, A.; Higashiwaki, M. Temperature-Dependent Exciton Resonance Energies and their Correlation with IR-active Optical Phonon Modes in β-Ga2O3 Single Crystals. Appl. Phys. Lett. 2016, 108, 101904,  DOI: 10.1063/1.4943175
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    16
    Temperature-dependent exciton resonance energies and their correlation with IR-active optical phonon modes in β-Ga2O3 single crystals
    Onuma, T.; Saito, S.; Sasaki, K.; Goto, K.; Masui, T.; Yamaguchi, T.; Honda, T.; Kuramata, A.; Higashiwaki, M.
    Applied Physics Letters (2016), 108 (10), 101904/1-101904/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Temp.-dependent exciton resonance energies Eexciton in β-Ga2O3 single crystals are studied by using polarized reflectance measurement. The Eexciton values exhibit large energy changes at 179-268 meV from 5 to 300 K. The IR-active Au and Bu optical phonon modes are selectively obsd. in the IR ellipsometry spectra by reflecting the polarization selection rules. The LO phonon energies can be divided into 3 ranges: ℏωLO = 35-48, 70-73, and 88-99 meV. The broadening parameters, which are obtained from the reflectance measurements, correspond to the lower 2 ranges of ℏωLO at low temp. and 75 meV >150 K. The large Eexciton changes with temp. in β-Ga2O3 are originated from the exciton-LO-phonon interaction. (c) 2016 American Institute of Physics.
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  17. 17
    Ingebrigtsen, M. E.; Varley, J. B.; Kuznetsov, A. Y.; Svensson, B. G.; Alfieri, G.; Mihaila, A.; Badstübner, U.; Vines, L. Iron and Intrinsic Deep Level States in Ga2O3. Appl. Phys. Lett. 2018, 112, 042104,  DOI: 10.1063/1.5020134
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    17
    Iron and intrinsic deep level states in Ga2O3
    Ingebrigtsen, M. E.; Varley, J. B.; Kuznetsov, A. Yu.; Svensson, B. G.; Alfieri, G.; Mihaila, A.; Badstubner, U.; Vines, L.
    Applied Physics Letters (2018), 112 (4), 042104/1-042104/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Using a combination of deep level transient spectroscopy, secondary ion mass spectrometry, proton irradn., and hybrid functional calcns., we identify two similar deep levels that are assocd. with Fe impurities and intrinsic defects in bulk crystals and mol. beam epitaxy and hydride vapor phase epitaxi-grown epilayers of β-Ga2O3. First, our results indicate that FeGa, and not an intrinsic defect, acts as the deep acceptor responsible for the often dominating E2 level at ∼0.78 eV below the conduction band min. Second, by provoking addnl. intrinsic defect generation via proton irradn., we identified the emergence of a new level, labeled as E2*, having the ionization energy very close to that of E2, but exhibiting an order of magnitude larger capture cross section. Importantly, the properties of E2* are found to be consistent with its intrinsic origin. As such, contradictory opinions of a long standing literature debate on either extrinsic or intrinsic origin of the deep acceptor in question converge accounting for possible contributions from E2 and E2* in different exptl. conditions. (c) 2018 American Institute of Physics.
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    Geller, S. Crystal Structure of β-Ga2O3. J. Chem. Phys. 1960, 33, 676– 684,  DOI: 10.1063/1.1731237
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    18
    Crystal structure of β-Ga2O3
    Geller, S.
    Journal of Chemical Physics (1960), 33 (), 676-84CODEN: JCPSA6; ISSN:0021-9606.
    The monoclinic crystal has cell dimensions a = 12.23, b = 3.04, c = 5.80 A., β = 103.7°. There are 4 mols. per unit cell. The most probable space group is C32h-C2/m; the atoms are in 5 sets of special positions 4i: (000, 1/2 1/2 0) ± (x0z). There are 2 kinds of co.ovrddot.ordination for Ga+++ in this structure, tetrahedral and octahedral. Av. interionic distances are: tetrahedral Ga-O; 1.83 A.; octahedral Ga-O, 2.00 A.; tetrahedron edge O-O, 3.02 A.; and octahedron edge O-O, 2.84 A. Because of the reduced co.ovrddot.ordination of half of the metal ions, the d. of β-Ga2O3 is lower than that of α-Ga2O3. The av. Ga-O distances in the structure account for the fact that although the Ga+++ is substantially larger than the Al+++, its quant. preference for tetrahedrally co.ovrddot.ordinated sites when substituted for Fe+++ in the Fe garnets is nearly the same as that of the Al+++ ion. The magnetic aspects of the β-Ga2O3 structure are discussed, and a possible Fe2O3 isomorph may be expected to be at least antiferromagnetic with a Neel temp. of about 700°K.
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  19. 19
    Prozheeva, V.; Hölldobler, R.; von Wenckstern, H.; Grundmann, M.; Tuomisto, F. Effects of Alloy Composition and Si-doping on Vacancy Defect Formation in (InxGa1-x)2O3 Thin Films. J. Appl. Phys. 2018, 123, 125705,  DOI: 10.1063/1.5022245
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    19
    Effects of alloy composition and Si-doping on vacancy defect formation in (InxGa1-x)2O3 thin films
    Prozheeva, V.; Holldobler, R.; von Wenckstern, H.; Grundmann, M.; Tuomisto, F.
    Journal of Applied Physics (Melville, NY, United States) (2018), 123 (12), 125705/1-125705/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Various nominally undoped and Si-doped (InxGa1-x)2O3 thin films were grown by pulsed laser deposition in a continuous compn. spread mode on c-plane α-sapphire and (100)-oriented MgO substrates. Positron annihilation spectroscopy in the Doppler broadening mode was used as the primary characterization technique in order to investigate the effect of alloy compn. and dopant atoms on the formation of vacancy-type defects. In the undoped samples, we observe a Ga2O3-like trend for low indium concns. changing to In2O3-like behavior along with the increase in the indium fraction. Increasing indium concn. is found to suppress defect formation in the undoped samples at [In] > 70 at. %. Si doping leads to positron satn. trapping in VIn-like defects, suggesting a vacancy concn. of at least mid-1018 cm-3 independent of the indium content. This is a solid soln. study. (c) 2018 American Institute of Physics.
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  20. 20
    Schmidt-Grund, R.; Kranert, C.; Bontgen, T.; von Wenckstern, H.; Krauß, H.; Grundmann, M. Dielectric Function in the NIR-VUV Spectral Range of (InxGa1-x)2O3 Thin Films. J. Appl. Phys. 2014, 116, 053510,  DOI: 10.1063/1.4891521
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    20
    Dielectric function in the NIR-VUV spectral range of (InxGa1-x)2O3 thin films
    Schmidt-Grund, R.; Kranert, C.; Boentgen, T.; von Wenckstern, H.; Krauss, H.; Grundmann, M.
    Journal of Applied Physics (Melville, NY, United States) (2014), 116 (5), 053510/1-053510/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    We detd. the dielec. function of the alloy system (InxGa1-x)2O3 by spectroscopic ellipsometry in the wide spectral range from 0.5 eV to 8.5 eV and for In contents ranging from x = 0.02 to x = 0.61. The predicted optical transitions for binary, monoclinic β-Ga2O3, and cubic bcc-In2O3 are well reflected by the change of the dielec. functions' lineshape as a function of the In content. In an intermediate compn. range with phase-sepd. material (x ≈ 0.3...0.4), the lineshape differs considerably, which we assign to the presence of the high-pressure rhombohedral InGaO3-II phase, which we also observe in Raman expts. in this range. By model anal. of the dielec. function, we derived spectra of the refractive index and the absorption coeff. and energy parameters of electronic band-band transitions. We discuss the sub-band gap absorption tail in relation to the influence of the In 4d orbitals on the valence bands. The data presented here provide a basis for a deeper understanding of the electronic properties of this technol. important material system and may be useful for device engineering. (c) 2014 American Institute of Physics.
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  21. 21
    von Wenckstern, H.; Kneiß, M.; Hassa, A.; Storm, P.; Splith, D.; Grundmann, M. A Review of the Segmented-Target Approach to Combinatorial Material Synthesis by Pulsed-Laser Deposition. Phys. Status Solidi B 2020, 257, 1900626,  DOI: 10.1002/pssb.201900626
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    21
    A Review of the Segmented-Target Approach to Combinatorial Material Synthesis by Pulsed-Laser Deposition
    von Wenckstern, Holger; Kneiss, Max; Hassa, Anna; Storm, Philipp; Splith, Daniel; Grundmann, Marius
    Physica Status Solidi B: Basic Solid State Physics (2020), 257 (7), 1900626CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Combinatorial material synthesis has led to a significant acceleration in the optimization of multinary compds. and a more efficient usage of source and substrate materials. Various growth methods, including phys. vapor deposition, can be adopted to realize material libraries. Herein, two approaches to combinatorial material synthesis based on ablation of segmented targets during pulsed-laser deposition are reviewed. For these two processes, either laterally or radially segmented targets are utilized and allow the creation of lateral and vertical compn. spreads, resp. Radially segmented targets can addnl. be used to synthesize a discrete binary material library. Both approaches are introduced by calcg. the expected material distribution with a simple geometric plasma expansion model. Then, exptl. detd. elemental distributions and growth rates are compared to predictions and it is demonstrated that differences between calcd. and exptl. data contain vital information on the influence of, for example, thermodn. processes on the growth mechanism.
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  22. 22
    Vines, L.; Bhoodoo, C.; von Wenckstern, H.; Grundmann, M. Electrical Conductivity of In2O3 and Ga2O3 after Low Temperature Ion Irradiation; Implications for Intrinsic Defect Formation and Charge Neutrality Level. J. Phys.: Condens. Matter 2018, 30, 025502,  DOI: 10.1088/1361-648X/aa9e2a
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    22
    Electrical conductivity of In2O3 and Ga2O3 after low temperature ion irradiation; implications for instrinsic defect formation and charge neutrality level
    Vines, L.; Bhoodoo, C.; von Wenckstern, H.; Grundmann, M.
    Journal of Physics: Condensed Matter (2018), 30 (2), 025502/1-025502/6CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
    The evolution of sheet resistance of n-type In2O3 and Ga2O3 exposed to bombardment with MeV 12C and 28Si ions at 35 K is studied in situ. While the sheet resistance of Ga2O3 increased by more than 8-fold as a result of ion irradn., In2O3 showed a more complex defect evolution and became more conductive when irradiated at the highest doses. Heating up to room temp. reduced the sheet resistivity somewhat, but Ga2O3 remained highly resistive, while In2O3 showed a lower resistance than as deposited samples. Thermal admittance spectroscopy and deep level transient spectroscopy did not reveal new defect levels for irradn. up to 2 × 1012 cm-2. A model where larger defect complexes preferentially produce donor like defects in In2O3 is proposed, and may reveal a microscopic view of a charge neutrality level within the conduction band, as previously proposed.
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  23. 23
    King, P. D. C.; Veal, T. D.; Fuchs, F.; Wang, C. Y.; Payne, D. J.; Bourlange, A.; Zhang, H.; Bell, G. R.; Cimalla, V.; Ambacher, O.; Egdell, R. G.; Bechstedt, F.; McConville, C. F. Band Gap, Electronic Structure, and Surface Electron Accumulation of Cubic and Rhombohedral In2O3. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 205211,  DOI: 10.1103/PhysRevB.79.205211
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    23
    Band gap, electronic structure, and surface electron accumulation of cubic and rhombohedral In2O3
    King, P. D. C.; Veal, T. D.; Fuchs, F.; Wang, Ch. Y.; Payne, D. J.; Bourlange, A.; Zhang, H.; Bell, G. R.; Cimalla, V.; Ambacher, O.; Egdell, R. G.; Bechstedt, F.; McConville, C. F.
    Physical Review B: Condensed Matter and Materials Physics (2009), 79 (20), 205211/1-205211/10CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The bulk and surface electronic structure of In2O3 has proved controversial, prompting the current combined exptl. and theor. investigation. The band gap of single-cryst. In2O3 is detd. as 2.93±0.15 and 3.02±0.15 eV for the cubic bixbyite and rhombohedral polymorphs, resp. The valence-band d. of states is investigated from x-ray photoemission spectroscopy measurements and d.-functional theory calcns. These show excellent agreement, supporting the absence of any significant indirect nature of the In2O3 band gap. Clear exptl. evidence for an s-d coupling between In 4d and O 2s derived states is also obsd. Electron accumulation, recently reported at the (001) surface of bixbyite material, is also shown to be present at the bixbyite (111) surface and the (0001) surface of rhombohedral In2O3.
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  24. 24
    Swallow, J. E. N.; Varley, J. B.; Jones, L. A. H.; Gibbon, J. T.; Piper, L. F. J.; Dhanak, V. R.; Veal, T. D. Transition from Electron Accumulation to Depletion at β-Ga2O3 Surfaces: The Role of Hydrogen and the Charge Neutrality Level. APL Mater. 2019, 7, 022528,  DOI: 10.1063/1.5054091
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    Transition from electron accumulation to depletion at β-Ga2O3 surfaces: The role of hydrogen and the charge neutrality level
    Swallow, J. E. N.; Varley, J. B.; Jones, L. A. H.; Gibbon, J. T.; Piper, L. F. J.; Dhanak, V. R.; Veal, T. D.
    APL Materials (2019), 7 (2), 022528/1-022528/8CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    The surface electronic properties of bulk-grown β-Ga2O3 (201) single crystals are investigated. The band gap is found using optical transmission to be 4.68 eV. High-resoln. x-ray photoemission coupled with hybrid d. functional theory calcn. of the valence band d. of states provides insights into the surface band bending. Importantly, the std. linear extrapolation method for detg. the surface valence band max. (VBM) binding energy is found to underestimate the sepn. from the Fermi level by ∼0.5 eV. According to our interpretation, most reports of surface electron depletion and upward band bending based on photoemission spectroscopy actually provide evidence of surface electron accumulation. For uncleaned surfaces, the surface VBM to Fermi level sepn. is found to be 4.95 ± 0.10 eV, corresponding to downward band bending of ∼0.24 eV and an electron accumulation layer with a sheet d. of ∼5 × 1012 cm-2. Uncleaned surfaces possess hydrogen termination which acts as surface donors, creating electron accumulation and downward band bending at the surface. In situ cleaning by thermal annealing removes H from the surface, resulting in a ∼0.5 eV shift of the surface VBM and formation of a surface electron depletion layer with upward band bending of ∼0.26 eV due to native acceptor surface states. These results are discussed in the context of the charge neutrality level, calcd. bulk interstitial hydrogen transition levels, and related previous exptl. findings. (c) 2019 American Institute of Physics.
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    Nagata, T.; Hoga, T.; Yamashita, A.; Asahi, T.; Yagyu, S.; Chikyow, T. Valence Band Modification of a (GaxIn1-x)2O3 Solid Solution System Fabricated by Combinatorial Synthesis. ACS Comb. Sci. 2020, 22, 433439,  DOI: 10.1021/acscombsci.0c00033
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    King, P. D. C.; Veal, T. D.; Payne, D. J.; Bourlange, A.; Egdell, R. G.; McConville, C. F. Surface Electron Accumulation and the Charge Neutrality Level in In2O3. Phys. Rev. Lett. 2008, 101, 116808,  DOI: 10.1103/PhysRevLett.101.116808
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    Surface electron accumulation and the charge neutrality level in In2O3
    King, P. D. C.; Veal, T. D.; Payne, D. J.; Bourlange, A.; Egdell, R. G.; McConville, C. F.
    Physical Review Letters (2008), 101 (11), 116808/1-116808/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    High-resoln. x-ray photoemission spectroscopy, IR reflectivity and Hall effect measurements, combined with surface space-charge calcns., are used to show that electron accumulation occurs at the surface of undoped single-cryst. In2O3. From a combination of measurements performed on undoped and heavily Sn-doped samples, the charge neutrality level is shown to lie ∼0.4 eV above the conduction band min. in In2O3, explaining the electron accumulation at the surface of undoped material, the propensity for n-type cond., and the ease of n-type doping in In2O3, and hence its use as a transparent conducting oxide material.
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    Lovejoy, T. C.; Chen, R.; Zheng, X.; Villora, E. G.; Shimamura, K.; Yoshikawa, H.; Yamashita, Y.; Ueda, S.; Kobayashi, K.; Dunham, S. T.; Ohuchi, F. S.; Olmstead, M. A. Band Bending and Surface Defects in β-Ga2O3. Appl. Phys. Lett. 2012, 100, 181602,  DOI: 10.1063/1.4711014
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    Band bending and surface defects in β-Ga2O3
    Lovejoy, T. C.; Chen, Renyu; Zheng, X.; Villora, E. G.; Shimamura, K.; Yoshikawa, H.; Yamashita, Y.; Ueda, S.; Kobayashi, K.; Dunham, S. T.; Ohuchi, F. S.; Olmstead, M. A.
    Applied Physics Letters (2012), 100 (18), 181602/1-181602/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Surface band bending and surface defects on the UV-transparent conducting oxide β-Ga2O3 (100) are studied with hard x-ray photoemission spectroscopy and scanning tunneling microscopy. Highly doped β-Ga2O3 shows flat bands near the surface, while the bands on nominally undoped (but still n-type), air-cleaved β-Ga2O3 are bent upwards by > 0.5 eV. Neg. charged surface defects are obsd. on vacuum annealed β-Ga2O3, which also shows upward band bending. D. functional calcns. show O vacancies are not likely to be ionized in the bulk, but could be activated by surface band bending. The large band bending may also hinder formation of ohmic contacts. (c) 2012 American Institute of Physics.
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    Navarro-Quezada, A.; Alamé, S.; Esser, N.; Furthmüller, J.; Bechstedt, F.; Galazka, Z.; Skuridina, D.; Vogt, P. Near Valence-band Electronic Properties of Semiconducting β-Ga2O3 (100) Single Crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 195306,  DOI: 10.1103/PhysRevB.92.195306
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    Near valence-band electronic properties of semiconducting β-Ga2O3 (100) single crystals
    Navarro-Quezada, A.; Alame, S.; Esser, N.; Furthmueller, J.; Bechstedt, F.; Galazka, Z.; Skuridina, D.; Vogt, P.
    Physical Review B: Condensed Matter and Materials Physics (2015), 92 (19), 195306/1-195306/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    β-Ga2O3 is a transparent wide-band-gap semiconductor that has attracted considerable interest in recent years due to its suitable elec. cond. and transparency in the UV spectral region. In this work we investigate the electronic properties of the near valence-band-edge region for semiconducting β-Ga2O3 (100) bulk single crystals using core-level photoelectron spectroscopy and ab initio theory within the framework of d. functional theory and the GW approach. We find good agreement between the exptl. results and the theor. calcns. This is explained by the hybridization of the Ga 3d and O 2s states, similar as for In2O3.
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  29. 29
    King, P. D. C.; Veal, T. D.; Schleife, A.; Zúñiga-Pérez, J.; Martel, B.; Jefferson, P. H.; Fuchs, F.; Muñoz-Sanjosé, V.; Bechstedt, F.; McConville, C. F. Valence-band Electronic Structure of CdO, ZnO, and MgO from X-ray Photoemission Spectroscopy and Quasi-particle-corrected Density-functional Theory Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 205205,  DOI: 10.1103/PhysRevB.79.205205
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    Valence-band electronic structure of CdO, ZnO, and MgO from x-ray photoemission spectroscopy and quasi-particle-corrected density-functional theory calculations
    King, P. D. C.; Veal, T. D.; Schleife, A.; Zuniga-Perez, J.; Martel, B.; Jefferson, P. H.; Fuchs, F.; Munoz-Sanjose, V.; Bechstedt, F.; McConville, C. F.
    Physical Review B: Condensed Matter and Materials Physics (2009), 79 (20), 205205/1-205205/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The valence-band d. of states of single-cryst. rock-salt CdO(001), wurtzite c-plane ZnO, and rock- salt MgO(001) are investigated by high-resoln. x-ray photoemission spectroscopy. A classic two-peak structure is obsd. in the VB-DOS due to the anion 2p-dominated valence bands. Good agreement is found between the exptl. results and quasi-particle-cor. d.-functional theory calcns. Occupied shallow semicore d levels are obsd. in CdO and ZnO. While these exhibit similar spectral features to the calcns., they occur at slightly higher binding energies, detd. as 8.8 eV and 7.3 eV below the valence band max. in CdO and ZnO, resp. The implications of these on the electronic structure are discussed.
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    Moses, P. G.; Van de Walle, C. G. Band Bowing and Band Alignment in InGaN Alloys. Appl. Phys. Lett. 2010, 96, 021908,  DOI: 10.1063/1.3291055
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    Band bowing and band alignment in InGaN alloys
    Moses, Poul Georg; Van de Walle, Chris G.
    Applied Physics Letters (2010), 96 (2), 021908/1-021908/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    We use d. functional theory calcns. with the HSE06 hybrid exchange-correlation functional to investigate InGaN alloys and accurately det. band gaps and band alignments. We find a strong band-gap bowing at low In content. Band positions on an abs. energy scale are detd. from surface calcns. The resulting GaN/InN valence-band offset is 0.62 eV. The dependence of InGaN valence-band alignment on In content is found to be almost linear. Based on the values of band gaps and band alignments, we conclude that InGaN fulfills the requirements for a photoelectrochem. electrode for In contents up to 50%. (c) 2010 American Institute of Physics.
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    Peelaers, H.; Steiauf, D.; Varley, J. B.; Janotti, A.; Van de Walle, C. G. (InxGa1-x)2O3 Alloys for Transparent Electronics. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 085206,  DOI: 10.1103/PhysRevB.92.085206
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    (InxGa1-x)2O3 alloys for transparent electronics
    Peelaers, Hartwin; Steiauf, Daniel; Varley, Joel B.; Janotti, Anderson; Van de Walle, Chris G.
    Physical Review B: Condensed Matter and Materials Physics (2015), 92 (8), 085206/1-085206/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    (InxGa1-x)2O3 alloys show promise as transparent conducting oxides. Using hybrid d. functional calcns., band gaps, formation enthalpies, and structural parameters are detd. for monoclinic and bixbyite crystal structures. In the monoclinic phase the band gap exhibits a linear dependence on alloy concn., whereas in the bixbyite phase a large band-gap bowing occurs. The calcd. formation enthalpies show that the monoclinic structure is favorable for In compns. up to 50% and bixbyite for larger compns. This is caused by In strongly preferring sixfold oxygen coordination. The formation enthalpy of the 50:50 monoclinic alloy is much lower than the formation enthalpy of the 50:50 bixbyite alloy and also lower than most monoclinic alloys with lower In concn.; these trends are explained in terms of local strain. Consequences for expt. and applications are discussed.
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    Oshima, T.; Fujita, S. Properties of Ga2O3-based (InxGa1-x)2O3 Alloy Thin Films Grown by Molecular Beam Epitaxy. Phys. Status Solidi C 2008, 5, 3113– 3115,  DOI: 10.1002/pssc.200779297
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    Properties of Ga2O3-based (InxGa1-x)2O3 alloy thin films grown by molecular beam epitaxy
    Oshima, Takayoshi; Fujita, Shizuo
    Physica Status Solidi C: Current Topics in Solid State Physics (2008), 5 (9), 3113-3115CODEN: PSSCGL; ISSN:1862-6351. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A series of Ga2O3-based (InxGa1-x)2O3 alloy thin films were grown on c-plane Al2O3 substrates with a thin Ga2O3 buffer layer by plasma-assisted MBE. At growth temps. of 700° and higher, even with a slight inclusion of In2O3 to Ga2O3, for example, the film of (In0.08Ga0.92)2O3, exhibited a rough surface and degraded transmission spectrum resulting from phase sepn. of In2O3. Due to low temp. growth at 600°, however, the phase sepn. was suppressed for the In compn. ≤35%, which was confirmed by x-ray diffraction measurement, and the films exhibited high transmittance over 85% with sharp absorption edges. The bandgap could be tuned from 5.0-4.0 eV. The results encourage the application of (InxGa1-x)2O3 thin films in short-wavelength optical devices.
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    Regoutz, A.; Egdell, R.; Morgan, D.; Palgrave, R.; Téllez, H.; Skinner, S.; Payne, D.; Watson, G.; Scanlon, D. Electronic and Surface Properties of Ga-doped In2O3 Ceramics. Appl. Surf. Sci. 2015, 349, 970– 982,  DOI: 10.1016/j.apsusc.2015.04.106
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    Electronic and surface properties of Ga-doped In2O3 ceramics
    Regoutz, A.; Egdell, R. G.; Morgan, D. J.; Palgrave, R. G.; Tellez, H.; Skinner, S. J.; Payne, D. J.; Watson, G. W.; Scanlon, D. O.
    Applied Surface Science (2015), 349 (), 970-982CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    The limit of soly. of Ga2O3 in the cubic bixbyite In2O3 phase was established by x-ray diffraction and Raman spectroscopy to correspond to replacement of around 6% of In cations by Ga for samples prepd. at 1250°. D. functional theory calcns. suggest that Ga substitution should lead to widening of the bulk bandgap, as expected from the much larger gap of Ga2O3 as compared to In2O3. However both diffuse reflectance spectroscopy and valence band x-ray photoemission reveal an apparent narrowing of the gap with Ga doping. It is tentatively concluded that this anomaly arises from introduction of Ga+ surface lone pair states at the top of the valence band and structure at the top of the valence band in Ga-segregated samples is assigned to these lone pair states. In addn. photoemission reveals a broadening of the valence band edge. Core x-ray photoemission spectra and low energy ion scattering spectroscopy both reveal pronounced segregation of Ga to the ceramic surface, which may be linked to both relief of strain in the bulk and the preferential occupation of surface sites by lone pair cations. Surprisingly Ga segregation is not accompanied by the development of chem. shifted structure in Ga 2p core XPS assocd. with Ga+. However expts. on ion bombarded Ga2O3, where a shoulder at the top edge of the valence band spectra provide a clear signature of Ga+ at the surface, show that the chem. shift between Ga+ and Ga3+ is too small to be resolved in Ga 2p core level spectra. Thus the failure to observe chem. shifted structure assocd. with Ga+ is not inconsistent with the proposal that band gap narrowing is assocd. with lone pair states at surfaces and interfaces.
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    von Wenckstern, H.; Splith, D.; Purfürst, M.; Zhang, Z.; Kranert, C.; Müller, S.; Lorenz, M.; Grundmann, M. Structural and Optical Properties of (In,Ga)2O3 Thin Films and Characteristics of Schottky Contacts Thereon. Semicond. Sci. Technol. 2015, 30, 024005,  DOI: 10.1088/0268-1242/30/2/024005
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    Structural and optical properties of (In, Ga)2O3 thin films and characteristics of Schottky contacts thereon
    von Wenckstern, H.; Splith, D.; Purfuerst, M.; Zhang, Z.; Kranert, Ch.; Mueller, S.; Lorenz, M.; Grundmann, M.
    Semiconductor Science and Technology (2015), 30 (2), 24005/1-24005/7, 7 pp.CODEN: SSTEET; ISSN:0268-1242. (IOP Publishing Ltd.)
    We report on structural and optical properties of a (InxGa1-x)2O3 thin film having a monotonic lateral variation of the indium content x (0 ≤ x ≤ 0.9). The growth condition for each In content is similar allowing precise detn. of the dependence of material properties on x. For low In content (x < 0.15) the thin film has monoclinic crystal structure; for highest In contents (x > 0.8) the cubic bixbyite phase is predominant. For intermediate alloying we observe addnl. the rhombohedral InGaO3(II) crystallog. phase. The optical band-gap decreases systematically with increasing indium content and has a linear dependency on x for parts of the sample having the monoclinic phase, only. Further, properties of Pt Schottky diodes are reported for monoclinic (InxGa1-x)2O3 and photo response measurements for x < 0.1.
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  35. 35
    Yang, F.; Ma, J.; Luan, C.; Kong, L. Structural and Optical Properties of Ga2(1-x)In2xO3 films Prepared on α-Al2O3 (0001) by MOCVD. Appl. Surf. Sci. 2009, 255, 4401– 4404,  DOI: 10.1016/j.apsusc.2008.10.129
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    35
    Structural and optical properties of Ga2(1-x)In2xO3 films prepared on α-Al2O3 (0001) by MOCVD
    Yang, Fan; Ma, Jin; Luan, Caina; Kong, Lingyi
    Applied Surface Science (2009), 255 (8), 4401-4404CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Ga2(1-x)In2xO3 thin films with different indium content x [In/(Ga + In) at. ratio] were prepd. on α-Al2O3 (0 0 0 1) substrates by the metal org. chem. vapor deposition (MOCVD). The structural and optical properties of the Ga2(1-x)In2xO3 films were investigated in detail. Microstructure anal. revealed that the film deposited with compn. x = 0.2 was polycryst. structure and the sample prepd. with x up to 0.8 exhibited single cryst. structure of In2O3. The optical band gap of the films varied with increasing Ga content from 3.72 to 4.58 eV. The av. transmittance for the films in the visible range was over 90%.
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  36. 36
    Zhang, Z.; von Wenckstern, H.; Lenzner, J.; Lorenz, M.; Grundmann, M. Visible-blind and Solar-blind Ultraviolet Photodiodes Based on (InxGa1-x)2O3. Appl. Phys. Lett. 2016, 108, 123503,  DOI: 10.1063/1.4944860
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    Visible-blind and solar-blind ultraviolet photodiodes based on (InxGa1-x)2O3
    Zhang, Zhipeng; von Wenckstern, Holger; Lenzner, Joerg; Lorenz, Michael; Grundmann, Marius
    Applied Physics Letters (2016), 108 (12), 123503/1-123503/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    UV and deep-UV selective photodiodes from visible-blind to solar-blind were realized based on a Si-doped (InxGa1-x)2O3 thin film with a monotonic lateral variation of 0.0035 < x < 0.83. Such layer was deposited by employing a continuous compn. spread approach relying on the ablation of a single segmented target in pulsed-laser deposition. The photo response signal is provided from a metal-semiconductor-metal structure upon backside illumination. The absorption onset was tuned from 4.83 to 3.22 eV for increasing x. Higher responsivities were obsd. for photodiodes fabricated from indium-rich part of the sample, for which an internal gain mechanism could be identified. (c) 2016 American Institute of Physics.
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  37. 37
    Michel, J.; Splith, D.; Rombach, J.; Papadogianni, A.; Berthold, T.; Krischok, S.; Grundmann, M.; Bierwagen, O.; von Wenckstern, H.; Himmerlich, M. Processing Strategies for High-Performance Schottky Contacts on n-Type Oxide Semiconductors: Insights from In2O3. ACS Appl. Mater. Interfaces 2019, 11, 27073– 27087,  DOI: 10.1021/acsami.9b06455
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    37
    Processing strategies for high-performance Schottky contacts on n-type oxide semiconductors: insights from In2O3
    Michel, Jonas; Splith, Daniel; Rombach, Julius; Papadogianni, Alexandra; Berthold, Theresa; Krischok, Stefan; Grundmann, Marius; Bierwagen, Oliver; von Wenckstern, Holger; Himmerlich, Marcel
    ACS Applied Materials & Interfaces (2019), 11 (30), 27073-27087CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Prepn. of rectifying Schottky contacts on n-type oxide semiconductors, such as indium oxide (In2O3), is often challenged by the presence of a distinct surface electron accumulation layer. Here, the authors investigated the material properties and elec. transport characteristics of platinum contact/indium oxide heterojunctions to define routines for the prepn. of high-performance Schottky diodes on n-type oxide semiconductors. Combining the evaluation of different Pt deposition methods, such as electron-beam evapn. and (reactive) sputtering in an (O and) Ar atm., with oxygen plasma interface treatments, the authors identify key parameters to obtain Schottky-type contacts with high electronic barrier height and high rectification ratio. Different photoelectron spectroscopy approaches are compared to characterize the chem. properties of the contact layers and the interface region toward In2O3, to analyze charge transfer and plasma oxidn. processes as well as to evaluate the precision and limits of different methodologies to det. heterointerface energy barriers. An oxygen-plasma-induced passivation of the semiconductor surface, which induces electron depletion and generates an intrinsic interface energy barrier, is found to be not sufficient to generate rectifying platinum contacts. The dissoln. of the functional interface oxide layer within the Pt film results in an energy barrier of ∼0.5 eV, which is too low for an In2O3 electron concn. of ∼1018 cm-3. A reactive sputter process in an Ar and O atm. is required to fabricate rectifying contacts that are composed of platinum oxide (PtOx). Combining oxygen plasma interface oxidn. of the semiconductor surface with reactive sputtering of PtOx layers results in the generation of a high Schottky barrier of ∼0.9 eV and a rectification ratio of up to 106. An addnl. oxygen plasma treatment after contact deposition further reduced the reverse leakage current, likely by eliminating a surface conduction path between the coplanar Ohmic and Schottky contacts. We conclude that processes that allow us to increase the oxygen content in the interface and contact region are essential for fabrication of device-quality-rectifying contacts on various oxide semiconductors.
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  38. 38
    Rombach, J.; Papadogianni, A.; Mischo, M.; Cimalla, V.; Kirste, L.; Ambacher, O.; Berthold, T.; Krischok, S.; Himmerlich, M.; Selve, S.; Bierwagen, O. The Role of Surface Electron Accumulation and Bulk Doping for Gas-Sensing Explored with Single-Crystalline In2O3 Thin Films. Sens. Actuators, B 2016, 236, 909– 916,  DOI: 10.1016/j.snb.2016.03.079
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    38
    The role of surface electron accumulation and bulk doping for gas-sensing explored with single-crystalline In2O3 thin films
    Rombach, Julius; Papadogianni, Alexandra; Mischo, Markus; Cimalla, Volker; Kirste, Lutz; Ambacher, Oliver; Berthold, Theresa; Krischok, Stefan; Himmerlich, Marcel; Selve, Soeren; Bierwagen, Oliver
    Sensors and Actuators, B: Chemical (2016), 236 (), 909-916CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
    Single cryst. and textured In2O3 thin films with (1 1 1) surface orientation, grown by plasma-assisted mol. beam epitaxy, were used as a model system to study the role of bulk and surface electron accumulation layer conductance for ozone sensing. Both conductance contributions, which add to the total film conductance, were systematically varied. The resulting ozone sensitivity was detd. by total conductance measurements in synthetic air with defined ozone concn. using UV irradn. instead of heating to regenerate the In2O3 surface. Depletion of the surface electron accumulation by an oxygen plasma treatment, confirmed by XPS, rendered the films ozone insensitive. The ozone response of films with an accumulation layer was increased by thickness redn. or by designing the bulk of the film semi-insulating using deep acceptor doping by Mg. Our results of using electron accumulation layers for gas sensing and bulk doping by deep acceptors to increase sensitivity can be generalized to other gas sensing materials. The use of single cryst. films allows selecting the most sensitive crystallog. surface orientation and may have further advantages over polycryst. films, such as increased stability and sensing speed.
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  39. 39
    Veal, T. D.; Jefferson, P. H.; Piper, L. F. J.; McConville, C. F.; Joyce, T. B.; Chalker, P. R.; Considine, L.; Lu, H.; Schaff, W. J. Transition from Electron Accumulation to Depletion at InGaN Surfaces. Appl. Phys. Lett. 2006, 89, 202110,  DOI: 10.1063/1.2387976
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    Transition from electron accumulation to depletion at InGaN surfaces
    Veal, T. D.; Jefferson, P. H.; Piper, L. F. J.; McConville, C. F.; Joyce, T. B.; Chalker, P. R.; Considine, L.; Lu, Hai; Schaff, W. J.
    Applied Physics Letters (2006), 89 (20), 202110/1-202110/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The compn. dependence of the Fermi-level pinning at the oxidized (0001) surfaces of n-type InxGa1-xN films (0 ≤ x ≤ 1) was studied using x-ray photoemission spectroscopy. The surface Fermi-level position varies from high above the conduction band min. (CBM) at InN surfaces to significantly below the CBM at GaN surfaces, with the transition from electron accumulation to depletion occurring at approx. x = 0.3. The results are consistent with the compn. dependence of the band edges with respect to the charge neutrality level.
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    Kajiyama, K.; Mizushima, Y.; Sakata, S. Schottky Barrier Height of n-InxGa1-xAs Diodes. Appl. Phys. Lett. 1973, 23, 458,  DOI: 10.1063/1.1654957
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    Schottky barrier height of n-indium gallium arsenide (n-InxGa1-xAs) diodes
    Kajiyama, K.; Mizushima, Y.; Sakata, S.
    Applied Physics Letters (1973), 23 (8), 458-9CODEN: APPLAB; ISSN:0003-6951.
    The barrier heights, ΦB, of Au/n-InxGa1-xAs diodes were measured by the capacitance-voltage and satn. current methods. The compn. dependence of the barrier height is ΦB (eV) = 0.95 - 1.90x + 0.90x2. A low barrier height with a relatively wide band gap is obtained in this system.
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    Lüth, H. Research on III-V Semiconductor Interfaces: Its Impact on Technology and Devices. Physica Status Solidi (a) 2001, 187, 33– 44,  DOI: 10.1002/1521-396X(200109)187:1<33::AID-PSSA33>3.0.CO;2-9
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    Wei, S.-H.; Zunger, A. Role of Metal d States in II-VI Semiconductors. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 8958,  DOI: 10.1103/PhysRevB.37.8958
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    Role of metal d states in II-VI semiconductors
    Wei, S. H.; Zunger, Alex
    Physical Review B: Condensed Matter and Materials Physics (1988), 37 (15), 8958-81CODEN: PRBMDO; ISSN:0163-1829.
    All-electron band-structure calcns. and photoemission expts. on II-VI semiconductors both exhibit a metal d subband inside the main valence band. It has nevertheless been customary in pseudopotential and tight-binding approaches to neglect the metal d band by choosing Hamiltonian parameters which place this band inside the chem. inert at. cores. By using all-electron self-consistent electronic-structure techniques (which treat the outermost d electrons in the same way as other valence electrons) and by comparing the results to those obtained by methods which remove the d band from the valence spectrum, the effects on valence properties were studied. For II-VI semiconductors p-d repulsion and hybridization (1) lower the band gaps, (2) reduce the cohesive energy, (3) increase the equil. lattice parameters, (4) reduce the spin-orbit splitting, (5) alter the sign of the crystal-field splitting, (6) increase the valence-band offset between common-anion II-VI semiconductors, and (7) modify the charge distributions of various II-VI systems and their alloys. P-d repulsion is also responsible for the occurrence of deep Cu acceptor levels in II-VI semiconductors (compared with shallow acceptors of Zn in III-V), for the anomalously small band gaps in chalcopyrites, and for the neg. exchange splitting in ferromagnetic MnTe.
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    Varley, J. B.; Weber, J. R.; Janotti, A.; Van de Walle, C. G. Oxygen Vacancies and Donor Impurities in β-Ga2O3. Appl. Phys. Lett. 2010, 97, 142106,  DOI: 10.1063/1.3499306
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    Oxygen vacancies and donor impurities in β-Ga2O3
    Varley, J. B.; Weber, J. R.; Janotti, A.; Van de Walle, C. G.
    Applied Physics Letters (2010), 97 (14), 142106/1-142106/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Using hybrid functionals we have investigated the role of O vacancies and various impurities in the elec. and optical properties of the transparent conducting oxide β-Ga2O3. We find that O vacancies are deep donors, and thus cannot explain the unintentional n-type cond. Instead, we attribute the cond. to common background impurities such as Si and H. Monoat. H has low formation energies and acts as a shallow donor in both interstitial and substitutional configurations. We also explore other dopants, where substitutional forms of Si, Ge, Sn, F, and Cl are shown to behave as shallow donors. (c) 2010 American Institute of Physics.
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    Karazhanov, S. Z.; Ravindran, P.; Vajeeston, P.; Ulyashin, A.; Finstad, T. G.; Fjellvag, H. Phase Stability, Electronic Structure, and Optical Properties of Indium Oxide Polytypes. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 075129,  DOI: 10.1103/PhysRevB.76.075129
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    Phase stability, electronic structure, and optical properties of indium oxide polytypes
    Karazhanov, S. Zh.; Ravindran, P.; Vajeeston, P.; Ulyashin, A.; Finstad, T. G.; Fjellvag, H.
    Physical Review B: Condensed Matter and Materials Physics (2007), 76 (7), 075129/1-075129/13CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Structural phase stability, electronic structure, optical properties, and high-pressure behavior of polytypes of In2O3 in three space group symmetries I213, Ia3, and R3 were studied by 1st-principles d.-functional calcns. From structural optimization based on total energy calcns., lattice and positional parameters were established, which are in good agreement with the corresponding exptl. data except for I213, where the symmetry anal. for optimized structure indicates that it arrived at the Ia3 phase. In2O3 of space group symmetry Ia3 is found to undergo a pressure-induced phase transition to the R3 phase at pressures around 3.8 GPa. From the anal. of band structure coming out from the calcns. within the local d. and generalized gradient approxns., In2O3 of space group symmetry I213 and R3 are indirect band gap semiconductors, while the other phase of space group Ia3 is having direct band gap. The calcd. carrier effective masses for all these three phases are compared with available exptl. and theor. values. From charge-d. and electron localization function anal., these phases have dominant ionic bonding with noticeable covalent interaction between In and O. The magnitudes of the absorption and reflection coeffs. for In2O3 with space groups Ia3 and R3 are small in the energy range 0-5 eV, indicating that these phases can be regarded and classified as transparent.
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    Mryasov, O. N.; Freeman, A. J. Electronic Band Structure of Indium Tin Oxide and Criteria for Transparent Conducting Behavior. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 64, 233111,  DOI: 10.1103/PhysRevB.64.233111
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    Electronic band structure of indium tin oxide and criteria for transparent conducting behavior
    Mryasov, O. N.; Freeman, A. J.
    Physical Review B: Condensed Matter and Materials Physics (2001), 64 (23), 233111/1-233111/3CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    Indium-based transparent conductors, notably indium tin oxide (ITO), have a wide range of applications due to a unique combination of visible light transparency and modest cond. A fundamental understanding of such an unusual combination of properties is strongly motivated by the great demand for materials with improved transparent conducting properties. Here we formulate conditions for transparent conducting behavior on the basis of the local d. full-potential linear muffin-tin orbital electronic band structure calcns. for Sn-doped In2O3 and available exptl. data. We conclude that the position, dispersion, and character of the lowest conduction band are the key characteristics of the band structure responsible for its electro-optical properties. Further, we find that this lowest band is split with Sn doping due to the strong hybridization with dopant s-type states and this splitting contributes to both the decrease of the plasma frequency and the mobility of the carriers.
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    Furthmüller, J.; Bechstedt, F. Quasiparticle Bands and Spectra of Ga2O3 Polymorphs. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 115204,  DOI: 10.1103/PhysRevB.93.115204
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    Hajnal, Z.; Miró, J.; Kiss, G.; Réti, F.; Deák, P.; Herndon, R. C.; Kuperberg, J. M. Role of Oxygen Vacancy Defect States in the n-type Conduction of β-Ga2O3. J. Appl. Phys. 1999, 86, 3792,  DOI: 10.1063/1.371289
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    Role of oxygen vacancy defect states in the n-type conduction of β-Ga2O3
    Hajnal, Zoltan; Miro, Jozsef; Kiss, Gabor; Reti, Ferenc; Deak, Peter; Herndon, Roy C.; Kuperberg, J. Michael
    Journal of Applied Physics (1999), 86 (7), 3792-3796CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Based on semiempirical quantum-chem. calcns., the electronic band structure of β-Ga2O3 is presented and the formation and properties of oxygen vacancies are analyzed. The equil. geometries and formation energies of neutral and doubly ionized vacancies were calcd. Using the calcd. donor level positions of the vacancies, the high temp. n-type conduction is explained. The vacancy concn. is obtained by fitting to the exptl. resistivity and electron mobility.
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    Swallow, J. E. N. Influence of Polymorphism on the Electronic Structure of Ga2O3. Chem. Mater. 2020, 32, 8460– 8470,  DOI: 10.1021/acs.chemmater.0c02465
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    Influence of Polymorphism on the Electronic Structure of Ga2O3
    Swallow, Jack E. N.; Vorwerk, Christian; Mazzolini, Piero; Vogt, Patrick; Bierwagen, Oliver; Karg, Alexander; Eickhoff, Martin; Schormann, Jorg; Wagner, Markus R.; Roberts, Joseph W.; Chalker, Paul R.; Smiles, Matthew J.; Murgatroyd, Philip; Razek, Sara A.; Lebens-Higgins, Zachary W.; Piper, Louis F. J.; Jones, Leanne A. H.; Thakur, Pardeep K.; Lee, Tien-Lin; Varley, Joel B.; Furthmuller, Jurgen; Draxl, Claudia; Veal, Tim D.; Regoutz, Anna
    Chemistry of Materials (2020), 32 (19), 8460-8470CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The search for new wide-band-gap materials is intensifying to satisfy the need for more advanced and energy-efficient power electronic devices. Ga2O3 has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main β-phase. Here, three polymorphs of Ga2O3, α, β, and ε, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theor. approaches to gain insights into their structure-electronic structure relationships. Valence and conduction electronic structure as well as semicore and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron d. functional theory and many-body perturbation theory, provides detailed understanding of the spectroscopic results. The calcd. spectra provide very accurate descriptions of all exptl. spectra and addnl. illuminate the origin of obsd. spectral features. This work provides a strong basis for the exploration of the Ga2O3 polymorphs as materials at the heart of future electronic device generations.
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    Persson, C.; Zunger, A. s-d Coupling in Zinc-blende Semiconductors. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 073205,  DOI: 10.1103/PhysRevB.68.073205
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    s-d coupling in zinc-blende semiconductors
    Persson, Clas; Zunger, Alex
    Physical Review B: Condensed Matter and Materials Physics (2003), 68 (7), 073205/1-073205/4CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    Most zinc blende semiconductors have a single anion-like s state near the bottom of the valence band, found in d.-of-states (DOS) calcns., and seen in photoemission. Here, we discuss the case where two s-like peaks appear, due to strong s-d coupling. Indeed, away from the k = 0 Brillouin zone center, cation d states and anion s states can couple in zinc blende symmetry. Depending on the energy difference ΔEsd = Esanion - Edcation, this interaction can lead to either a single or two s-like peaks in the DOS and photoemission. We find four types of behaviors: (i) in GaP, GaAs, InP, and InAs, ΔEsd is large, giving rise to a single cation d peak well below the single anion s peak; (ii) similarly, in CdS, CdSe, ZnS, ZnSe, and ZnTe, we see also a single s peak, but now the cation d is above the anion s. In both (i) and (ii) the s-d coupling is very weak. For (iii) in GaN and InN, the local d. approxn. (LDA) predicts two s-like peaks bracketing below and above the cation d-like state. Correcting the too low binding energies of LDA by LDA + SIC (self-interaction correction) still leaves the two s-like peaks. The occurrence of two s-like peaks represents the fingerprint of strong s-d coupling. And (iv) in CdTe, LDA predicts a single s-like peak just as in case (ii) above. However, LDA + SIC correction shifts down the cation d state closer to the anion s band, enhancing the s-d coupling, and leading to the appearance of two s-like peaks. Case (iv) is a remarkable situation where LDA errors cause not only quant. energetic errors, but actually leads to a qual. effect of a DOS peak that exists in LDA + SIC but is missing in LDA. We predict that the double-s peak should be obsd. in photoemission for GaN, InN, and CdTe.
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    Fuchs, F.; Bechstedt, F. Indium-oxide Polymorphs from First Principles: Quasiparticle Electronic States. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 155107,  DOI: 10.1103/PhysRevB.77.155107
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    Indium-oxide polymorphs from first principles: Quasiparticle electronic states
    Fuchs, F.; Bechstedt, F.
    Physical Review B: Condensed Matter and Materials Physics (2008), 77 (15), 155107/1-155107/10CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The electronic structure of In2O3 polymorphs is calcd. from first principles using d. functional theory (DFT) and many-body perturbation theory (MBPT). DFT calcns. with a local exchange-correlation (XC) functional give the relaxed at. coordinates of the two stable polymorphs. Their electronic structure, i.e., the band structure and d. of states, is studied within MBPT. The quasiparticle equation is solved in two steps. As the zeroth approxn. for the XC self-energy the nonlocal potential resulting from a HSE03 hybrid functional is used. In the sense of a self-consistent procedure G0W0 quasiparticle corrections are computed on top. The calcd. direct quasiparticle gaps at Γ amt. to 3.3 eV (rhombohedral) and 3.1 eV (cubic). The rhombohedral polymorph is found to exhibit a near degeneracy of the valence-band maxima at the Γ point and on the Γ-L line, while the valence-band max. of the cubic polymorph occurs near Γ. Interconduction band transitions are identified as possible origin of conflicting exptl. reports, claiming a much larger difference between the direct and indirect gap. The results for gaps, d-band positions, and d. of states are compared with available exptl. data.
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    Fuchs, F.; Furthmüller, J.; Bechstedt, F.; Shishkin, M.; Kresse, G. Quasiparticle Band Structure based on a Generalized Kohn-Sham Scheme. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 115109,  DOI: 10.1103/PhysRevB.76.115109
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    Quasiparticle band structure based on a generalized Kohn-Sham scheme
    Fuchs, F.; Furthmuller, J.; Bechstedt, F.; Shishkin, M.; Kresse, G.
    Physical Review B: Condensed Matter and Materials Physics (2007), 76 (11), 115109/1-115109/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We present a comparative full-potential study of generalized Kohn-Sham (gKS) schemes with explicit focus on their suitability as starting point for the soln. of the quasiparticle equation. We compare G0W0 quasiparticle band structures calcd. upon local-d. approxn. (LDA), screened-exchange, HSE03, PBE0, and Hartree-Fock functionals for exchange and correlation (XC) for Si, InN, and ZnO. Furthermore, the HSE03 functional is studied and compared to the generalized gradient approxn. (GGA) for 15 nonmetallic materials for its use as a starting point in the calcn. of quasiparticle excitation energies. For this case, the effects of self-consistency in the GW self-energy are also analyzed. It is shown that the use of a gKS scheme as a starting point for a perturbative quasiparticle correction can improve upon the deficiencies found for LDA or GGA starting points for compds. with shallow d bands. For these solids, the order of the valence and conduction bands is often inverted using local or semilocal approxns. for XC, which makes perturbative G0W0 calcns. unreliable. The use of a gKS starting point allows for the calcn. of fairly accurate band gaps even in these difficult cases, and generally single-shot G0W0 calcns. following calcns. using the HSE03 functional are very close to expt.
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    Ley, L.; Pollak, R. A.; McFeely, F. R.; Kowalczyk, S. P.; Shirley, D. A. Total Valence-band Densities of States of III-V and II-VI Compounds from X-ray Photoemission Spectroscopy. Phys. Rev. B 1974, 9, 600– 621,  DOI: 10.1103/PhysRevB.9.600
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    Total valence-band densities of states of [Groups] III-V and II-VI compounds from x-ray photoemission spectroscopy
    Ley, L.; Pollak, R. A.; McFeely, F. R.; Kowalczyk, S. P.; Shirley, D. A.
    Physical Review B: Solid State (1974), 9 (2), 600-21CODEN: PLRBAQ; ISSN:0556-2805.
    A comprehensive survey of the total valence-band x-ray-photoemission spectra of 14 semiconductors is reported. The x-ray photoelectron spectra of cubic GaP, GaAs, GaSb, InP, InAs, InSb, ZnS, ZnSe, ZnTe, CdTe, and HgTe, and of hexagonal ZnO, CdS, and CdSe were obtained from freshly cleaved single crystals, in the 0-50-eV binding-energy range, by using monochromatized Al Kα (1486.6 eV) radiation. The binding energies of the outermost d shells are reported. They were detd. relative both to the top of the valence bands (EBV) and to the Fermi level of a thin layer of Au that was vapor deposited after each run (EBF). These data also yielded accurate measures of sample charging. The Fermi level fell near the center of the gap for 6 samples, near the top for 2, and near the bottom for 3. Evidence for an apparent increase in core d-level spin-orbit splitting over free-atom values was interpreted as a possible spreading of a Γ7 and a Γ8 level from the upper (d3/2) Γ8 level by a tetrahedral crystal field. The s, p valence-band spectra showed 3 main peaks, with considerable structure on the least-bound peak. A discussion is given of the validity of comparing the valence-band (VB) spectrum I'(E) with VB d. of states, including cross-section modulation, final-state modulation, and relaxation effects. Characteristic binding energies of spectra features in I'(E) are tabulated. In addn., the energies of the characteristic symmetry points L3, X5, W2, Σ1min., W1, X3(L1), X1, L1, and Γ1 are given for the 11 cubic compds. These are compared with uv photoemission spectroscopy results where available and with theor. band-structure results where available. The energies calcd. by using the relativistic-orthogonalized-plane-wave approach with Xαβ exchange agree very well with expt., on the whole. In particular, they predict the important ionicity gap X3-X1 quite accurately. The ds. of states calcd. by using the empirical-pseudopotential method provided a useful basis for relating features in I'(E) to energies of the characteristic symmetry points. Band-structure calcns. in combination with x-ray-photoemission spectra appears to provide a very powerful approach to establishing the total valence-band structure of semiconductors.
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    Erhart, P.; Klein, A.; Egdell, R. G.; Albe, K. Band Structure of Indium Oxide: Indirect Versus Direct Band Gap. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 153205,  DOI: 10.1103/PhysRevB.75.153205
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    Band structure of indium oxide: Indirect versus direct band gap
    Erhart, Paul; Klein, Andreas; Egdell, Russell G.; Albe, Karsten
    Physical Review B: Condensed Matter and Materials Physics (2007), 75 (15), 153205/1-153205/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The nature of the band gap of In oxide is still a matter of debate. Based on optical measurements the presence of an indirect band gap was suggested, which is 0.9 to 1.1 eV smaller than the direct band gap at the Γ point. This could be caused by strong mixing of O 2p and In 4d orbitals off Γ. The authors have performed extensive d. functional theory calcns. using the LDA+U and the GGA+U methods to elucidate the contribution of the In 4d states and the effect of spin-orbit coupling on the valence band structure. Although an indirect band gap is obtained, the energy difference between the overall valence band max. and the highest occupied level at the Γ point is <50 meV. The exptl. observation cannot be related to the electronic structure of the defect free bulk material.
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    Wei, S.-H.; Nie, X.; Batyrev, I. G.; Zhang, S. B. Breakdown of the Band-gap-common-cation Rule: The Origin of the Small Band Gap of InN. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 165209,  DOI: 10.1103/PhysRevB.67.165209
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    Breakdown of the band-gap-common-cation rule: the origin of the small band gap of InN
    Wei, Su-Huai; Nie, Xiliang; Batyrev, Iskander G.; Zhang, S. B.
    Physical Review B: Condensed Matter and Materials Physics (2003), 67 (16), 165209/1-165209/4CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    It is well accepted that the band gap of a semiconductor compd. increases as the at. no. decreases. However, recent measurements of the small band gap of InN (Eg ∼ 0.9 eV) suggest that this rule may not hold for the common-cation In compds. Using a band-structure method that includes band-gap correction, we systematically study the chem. trends of the band-gap variation in III-V semiconductors. The calcd. InN band gap is 0.85 ± 0.1 eV, much smaller than previous exptl. value of ∼1.9 eV. The InN band-gap anomaly is explained in terms of at.-orbital energies and the band-gap deformation potentials.
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    Wei, S.-H.; Zunger, A. Predicted Band-gap Pressure Coefficients of all Diamond and Zinc-blende Semiconductors: Chemical Trends. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60, 5404,  DOI: 10.1103/PhysRevB.60.5404
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    Predicted band-gap pressure coefficients of all diamond and zinc-blende semiconductors: Chemical trends
    Wei, Su-Huai; Zunger, Alex
    Physical Review B: Condensed Matter and Materials Physics (1999), 60 (8), 5404-5411CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    We have studied systematically the chem. trends of the band-gap pressure coeffs. of all Group IVA, IIIA-VA, and IIB-VIA semiconductors using first-principles band-structure method. We have also calcd. the individual "abs." deformation potentials of the valence-band max. (VBM) and conduction-band min. (CBM). We find that (1) the vol. deformation potentials of the Γ6c CBM are usually large and always neg., while (2) the vol. deformation potentials of the Γ8v VBM state are usually small and neg. for compds. contg. occupied valence d state but pos. for compds. without occupied valence d orbitals. Regarding the chem. trends of the band-gap pressure coeffs., we find that (3) apΓ-Γ decreases as the ionicity increases (e.g., from Ge → GaAs → ZnSe), (4) apΓ-Γ increases significantly as anion at. no. increases (e.g., from GaN → GaP → GaAs → GaSb), (5) apΓ-Γ decreases slightly as cation at. no. increases (e.g., from AlAs → GaAs → InAs), (6) the variation of apΓ-L are relatively small and follow similar trends as apΓ-Γ, and (7) the magnitude of apΓ-X are small and usually neg., but are sometimes slightly pos. for compds. contg. first-row elements. Our calcd. chem. trends are explained in terms of the energy levels of the at. valence orbitals and coupling between these orbital. In light of the above, we suggest that "empirical rule" of the pressure coeffs. should be modified.
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    Li, Y.-H.; Gong, X. G.; Wei, S.-H. Ab Initio All-Electron Calculation of Absolute Volume Deformation Potentials of IV-IV, III-V, and II-VI Semiconductors: The Chemical Trends. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 245206,  DOI: 10.1103/PhysRevB.73.245206
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    Ab initio all-electron calculation of absolute volume deformation potentials of IV-IV, III-V, and II-VI semiconductors: The chemical trends
    Li, Yong-Hua; Gong, X. G.; Wei, Su-Huai
    Physical Review B: Condensed Matter and Materials Physics (2006), 73 (24), 245206/1-245206/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We calc. systematically the abs. vol. deformation potential (AVDP) of the Γ8v valence band max. (VBM) and the Γ6c conduction band min. (CBM) states for all group IV, III-V, and II-VI semiconductors. Unlike previous calcns. that involve various assumptions, the AVDPs are calcd. using a recently developed approach that is independent of the selection of the ref. energy levels. We find that although the vol. deformation potentials of the CBM state are usually large and always neg., those of the VBM state are usually small and always pos. The AVDP of the VBM state decreases as the p-d coupling increases, e.g., in the II-VI compds. The AVDP of CBM decreases as the ionicity increases. Our calcd. chem. trends of the AVDPs are explained in terms of the AO energy levels and coupling between these orbitals.
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    Mann, J. B.; Meek, T. L.; Allen, L. C. Configuration Energies of the Main Group Elements. J. Am. Chem. Soc. 2000, 122, 2780– 2783,  DOI: 10.1021/ja992866e
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    Configuration Energies of the Main Group Elements
    Mann, Joseph B.; Meek, Terry L.; Allen, Leland C.
    Journal of the American Chemical Society (2000), 122 (12), 2780-2783CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Configuration energies (CEs), formerly called spectroscopic electronegativities, are an attempt to quantum mech. define, and extend, the important chem. concept of electronegativity. In a previous paper (J. Am. Chem. Soc. 1989, 111, 9003) we reported high-resoln. exptl. values obtained from the National Institutes of Science and Technol. spectroscopic energy level tables using the formula CE = (nεs + mεp)/(n + m); n and m are the no. of s and p electrons and εs and εp are their multiplet-averaged one-electron energies, for the 34 s and p-block atoms H→Xe. This CE definition is a direct extension of N. Bohr's introduction of electron configurations to quantum mech. rationalize the periodic table (hence its designation as configuration energy). Here we give exptl. nos. for the remaining 8 sixth row representative atoms plus Zn, Cd, and Hg. In addn., we have carried out high accuracy numerical Dirac-Hartree-Fock solns. for all 45 atoms. Results from these calcns. closely parallel the exptl. values and enable us to est. some of the at. multiplet levels for which no exptl. data exist. CE leads to nos. which are interpretable as an "electron attracting power" in the same manner as the traditional scales of Pauling and Allred & Rochow. They are also strongly correlated with at. energy level spacings, therefore providing an addnl. interpretability compatible with energy level data and the MO diagrams that dominate much of contemporary chem. Likewise, CEs are able to rationalize the origin of the metalloid band (diagonal line sepg. metals from nonmetals) in the periodic table and the new detn. of sixth row CEs permit designation of bismuth and polonium as metalloids, clarifying their previous uncertain classification between metal and metalloid.
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    Fischer, C. F. Average-energy-of-configuration Hartree-Fock Results for the Atoms Helium to Radon. At. Data Nucl. Data Tables 1973, 12, 87– 99,  DOI: 10.1016/0092-640X(73)90014-4
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    Average-energy-of-configuration Hartree-Fock results for the atoms helium to radon
    Fischer, Charlotte Froese
    Atomic Data and Nuclear Data Tables (1973), 12 (1), 87-99CODEN: ADNDAT; ISSN:0092-640X.
    Table I of the paper [Atomic Data, 1972, 4, 301] is actually for the lowest term of the configuration rather than for the av.-energy-of-the-configuration, as the paper implies. For configurations contg. 4f electrons the results of that paper correspond to an incorrect energy expression. When the energy of the lowest term is also the av. energy as for complete groups or complete groups plus (or minus) one electron, all atomic parameters are correct except for the total energy which is too high by the amt. q(q - 1) [2/95 - 2/195]F2(4f,4f), where q is the no. of 4f electrons. In all other cases, the radial functions were detd. from an energy expression too low by the same amt. The at. properties are reasonably accurate but the energy is too low. Table I for the av.-energy-of-the-configuration is given.
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    Herman, F.; Skillman, S. Atomic Structure Calculations; Prentice-Hall: Englewood Cliffs, NJ, 1963.
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    Vurgaftman, I.; Meyer, J. R.; Ram-Mohan, L. R. Band Parameters for III-V Compound Semiconductors and their Alloys. J. Appl. Phys. 2001, 89, 5815– 5875,  DOI: 10.1063/1.1368156
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    Band parameters for III-V compound semiconductors and their alloys
    Vurgaftman, I.; Meyer, J. R.; Ram-Mohan, L. R.
    Journal of Applied Physics (2001), 89 (11, Pt. 1), 5815-5875CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    A review with 1001 refs. We present a comprehensive, up-to-date compilation of band parameters for the technol. important III-V zinc blende and wurtzite compd. semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calcns., we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temp. and alloy-compn. dependences where available. Heterostructure band offsets are also given, on an abs. scale that allows any material to be aligned relative to any other.
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    Wu, J.; Walukiewicz, W.; Shan, W.; Yu, K. M.; Ager, J. W.; Li, S. X.; Haller, E. E.; Lu, H.; Schaff, W. J. Temperature Dependence of the Fundamental Band Gap of InN. J. Appl. Phys. 2003, 94, 4457– 4460,  DOI: 10.1063/1.1605815
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    Temperature dependence of the fundamental band gap of InN
    Wu, J.; Walukiewicz, W.; Shan, W.; Yu, K. M.; Ager, J. W.; Li, S. X.; Haller, E. E.; Lu, Hai; Schaff, William J.
    Journal of Applied Physics (2003), 94 (7), 4457-4460CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    The fundamental band gap of InN films grown by MBE were measured by transmission and photoluminescence spectroscopy as a function of temp. The band edge absorption energy and its temp. dependence depend on the doping level. The band gap variation and Varshni parameters of InN are compared with other Group III nitrides. The energy of the photoluminescence peak is affected by the emission from localized states and cannot be used to det. the band gap energy. Based on the results obtained on two samples with distinctly different electron concns., the effect of degenerate doping on the optical properties of InN is discussed.
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    Tippins, H. H. Optical Absorption and Photoconductivity in the Band Edge of β-Ga2O3. Phys. Rev. 1965, 140, A316– A319,  DOI: 10.1103/PhysRev.140.A316
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    Optical absorption and photoconductivity in the band edge of β-Ga2O3
    Tippins, H. H.
    Physical Review (1965), 140 (1A), 316-19CODEN: PHRVAO; ISSN:0031-899X.
    Optical absorption and photocond. were observed in the uv in single crystals of nominally pure β-Ga2O3. At room temp. a steep absorption edge, characteristic of a band-to-band transition, is observed at 2700 A. The edge is shifted approx. 100 A. toward shorter wavelengths when the temp. is reduced to 77°K. Photocond. begins coincident with the absorption edge at 77°K., but could not be detected at room temp. A model is proposed in which the absorption arises as a result of excitation of an electron from the O 2p band to the Ga 4s band. Calcns. using this model and the Born-Haber cycle are in good agreement with the observed band gap of 4.7 ev. The much smaller band gap of β-Ga2O3 as compared with sapphire is due to the reduced coordination no. of the ions involved in the transition.
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    Varley, J. B.; Schleife, A. Bethe-Salpeter Calculation of Optical-absorption Spectra of In2O3 and Ga2O3. Semicond. Sci. Technol. 2015, 30, 024010,  DOI: 10.1088/0268-1242/30/2/024010
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    Bethe-Salpeter calculation of optical-absorption spectra of In2O3 and Ga2O3
    Varley, Joel B.; Schleife, Andre
    Semiconductor Science and Technology (2015), 30 (2), 24010/1-24010/5, 5 pp.CODEN: SSTEET; ISSN:0268-1242. (IOP Publishing Ltd.)
    Transparent conducting oxides keep attracting strong scientific interest not only due to their promising potential for 'transparent electronics' applications but also due to their intriguing optical absorption characteristics. Materials such as In2O3 and Ga2O3 have complicated unit cells and, consequently, are interesting systems for studying the physics of excitons and anisotropy of optical absorption. Since currently no exptl. data is available, for instance, for their dielec. functions across a large photon-energy range, we employ modern first-principles computational approaches based on many-body perturbation theory to provide theor.-spectroscopy results. Using the Bethe-Salpeter framework, we compute dielec. functions and we compare to spectra computed without excitonic effects. We find that the electron-hole interaction strongly modifies the spectra and we discuss the anisotropy of optical absorption that we find for Ga2O3 in relation to existing theor. and exptl. data.
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    Wei, S.-H.; Zunger, A. Calculated Natural Band Offsets of all II-VI and III-V Semiconductors: Chemical Trends and the Role of Cation d Orbitals. Appl. Phys. Lett. 1998, 72, 2011,  DOI: 10.1063/1.121249
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    Calculated natural band offsets of all II-VI and III-V semiconductors: Chemical trends and the role of cation d orbitals
    Wei, Su-Huai; Zunger, Alex
    Applied Physics Letters (1998), 72 (16), 2011-2013CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Using first-principles all-electron band structure method, we have systematically calcd. the natural band offsets ΔEv between all II-VI and sep. between III-V semiconductor compds. Fundamental regularities are uncovered: for common-cation systems ΔEv decreases when the cation at. no. increases, while for common-anion systems ΔEv decreases when the anion at. no. increases. We find that coupling between anion p and cation d states plays a decisive role in detg. the abs. position of the valence band max. and thus the obsd. chem. trends.
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    Wei, S.-H.; Zunger, A. Role of d Orbitals in Valence-Band Offsets of Common-Anion Semiconductors. Phys. Rev. Lett. 1987, 59, 144,  DOI: 10.1103/PhysRevLett.59.144
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    Role of d orbitals in valence-band offsets of common-anion semiconductors
    Wei, Su Huai; Zunger, Alex
    Physical Review Letters (1987), 59 (1), 144-7CODEN: PRLTAO; ISSN:0031-9007.
    All-electron first-principles electronic structure calcns. of core levels were made on common-anion semiconductors AlAs-GaAs and CdTe-HgTe and contrary to previous expectations, the valence-band offsets are decided primarily by intrinsic bulk effects and the interface charge transfer has but a small effect on these quantities. The failure of previous models results primarily from the omission of cation d orbitals.
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    Van de Walle, C. G.; Martin, R. M. Absolute Deformation Potentials: Formulation and Ab initio Calculations for Semiconductors. Phys. Rev. Lett. 1989, 62, 2028– 2031,  DOI: 10.1103/PhysRevLett.62.2028
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    "Absolute" deformation potentials: formulation and ab initio calculations for semiconductors
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    Physical Review Letters (1989), 62 (17), 2028-31CODEN: PRLTAO; ISSN:0031-9007.
    The subjects of this paper are the proper inclusion of long-range electrostatic terms in the theory of electronic deformation potentials, a way to include these terms by using supercells in ab initio d.-functional methods, and calcns. for selected semiconductors. The connection with the heterojunction problem is described. The results are compared with previous model theories and with expt.
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    Cardona, M.; Christensen, N. E. Acoustic Deformation Potentials and Heterostructure Band Offsets in Semiconductors. Phys. Rev. B: Condens. Matter Mater. Phys. 1987, 35, 6182– 6194,  DOI: 10.1103/PhysRevB.35.6182
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    Acoustic deformation potentials and heterostructure band offsets in semiconductors
    Cardona, Manuel; Christensen, Niels E.
    Physical Review B: Condensed Matter and Materials Physics (1987), 35 (12), 6182-94CODEN: PRBMDO; ISSN:0163-1829.
    The abs. hydrostatic deformation potentials calcd. by J. A. Verges, et al., (1982) for tetrahedral semiconductors with the linear muffin-tin-orbital method must be screened by the dielec. response of the material before using them to calc. electron-phonon interaction. This screening can be estd. by using the midpoint of an av. dielec. gap evaluated at special (Baldereschi) points of the band structure. This dielec. midgap energy (DME) was related to the charge-neutrality point (Tersoff, J., 1984) to evaluate band offsets in heterojunctions and Schottky-barrier heights. Band offsets obtained with this method for several heterojunctions are tabulated, and compared with existing exptl. results and other theor. calcns. The DME's are tabulated, and compared with those based on charge-neutrality points.
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    Li, Y.-H.; Gong, X. G.; Wei, S.-H. Ab initio All-electron Calculation of Absolute Volume Deformation Potentials of IV-IV, III-V, and II-VI Semiconductors: The Chemical Trends. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 245206,  DOI: 10.1103/PhysRevB.73.245206
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    Ab initio all-electron calculation of absolute volume deformation potentials of IV-IV, III-V, and II-VI semiconductors: The chemical trends
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    Figure 1

    Figure 1. Unit cells of (a) monoclinic β-Ga2O3, (b) hexagonal phase InGaO3, and (c) bixbyite In2O3. Ga atoms are shown in green, In atoms in purple, and O atoms in red. For (a) and (c), the inequivalent cation sites are shown with different color octahedra.

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    Figure 2

    Figure 2. (a) False color representation of wide-angle 2θ–ω XRD patterns, taken at ∼1 mm intervals (55 in total) across the compositionally graded strip in the direction highlighted by a black arrow in Figure 3. Black horizontal lines indicate the point in the film corresponding to the monoclinic phase (top line), the onset of the hexagonal phase (middle line), and the onset of the bixbyite phase (bottom line). (b), (c), and (d) show individual diffraction patterns taken at the horizontal lines at 40, 9, and 6 mm, showing primarily (b) the monoclinic β-Ga2O3 phase, (c) the hexagonal InGaO3 (II), and (d) the bixbyite In2O3 phase, respectively. A small peak corresponding to the 0004 peak of the hexagonal (InxGa1–x)2O3 phase can be seen at ∼29° in (d). Note the intensity scale in (b)–(d) is logarithmic.

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    Figure 3

    Figure 3. (a) EDX line scan of Ga and In content along the film. Black dashed lines represent positions of different crystallographic phases (bixbyite to mixed bixbyite–hexagonal–monoclinic to monoclinic). (b) EDX false color map of In content in the as deposited (InxGa1–x)2O3 film (51 mm diameter), the black arrow displaying the position where the line scan was taken, and a 4 mm wide strip was cut for further experiments.

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    Figure 4

    Figure 4. High-resolution XPS plots of the valence band of (InxGa1–x)2O3 film. A total of 46 spectra were recorded across the film at 1 mm intervals, matching the EDX measurement in Figure 3. The spectra recorded from the Ga-rich end of the film are at the top, with increasing In content down the plot.

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    Figure 5

    Figure 5. Energy positions of the VBM from XPS referenced to EF as a function of EDX indium content. Black dashed lines separate the region of different crystallographic phases.

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    Figure 6

    Figure 6. Optical gap vs indium content as determined via transmission spectroscopy. A fit to the data points was performed by using the bowing equation, (30,31) with the fitted parameters displayed in the equation. Importantly, for x > 0.6, because of transitions from the topmost valence bands being dipole forbidden, these values do not correspond to the fundamental band gap.

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    Figure 7

    Figure 7. (a) Variation of band gap (Eg) and barrier height (ΦB) at the (InxGa1–x)2O3 surface with respect to varying indium content (x). Energy gaps were determined in this study by transmission measurements (○) as well as values being taken from the literature (□, Oshima; (32) ⬡, Regoutz; (33) △, Wenckstern; (34) ▽, Yang; (35) ◇, Zhang; (36) ▷, Swallow (24)). Barrier heights (ΦB) are derived by subtracting the surface Fermi level to VBM (ξ) from the band gap (ΦB = Eg – ξ). The surface space-charge variation is schematically represented in (b) and (c) with positive ΦB indicating electron depletion and upward band bending (b), while negative ΦB indicated electron accumulation and downward band bending (c).

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    Figure 8

    Figure 8. Uncorrected partial (colored lines) and total (black lines) DOS for (a) β-Ga2O3, (b) hexagonal InGaO3, and (c) bixbyite In2O3, with the insets showing an expanded region of the CBM. The valence band regions are expanded in (d)–(f). For InGaO3 in (e), orbitals with strong overlap are shaded to aid in visualization.

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    Figure 9

    Figure 9. Valence and semicore levels. (a) Cross-section-corrected and broadened DFT partial and total VB-DOS of β-Ga2O3 (top), InGaO3 (middle), and In2O3 (bottom). (b) Measured XPS VB spectra aligned to the VBM. The top and bottom spectra were taken from refs (24) and (23) for single crystalline β-Ga2O3 (x = 0) and In2O3 (x = 1), respectively. The other spectra represent intermediate composition steps, working from top to bottom In(x) is x = 0.25, x = 0.5, and x = 0.75, respectively. (c) DFT calculated semicore levels for β-Ga2O3 (top), InGaO3 (middle), and In2O3 (bottom). The semicore levels are dominated by the d-orbital contributions from Ga and In. Note that SOC has been included for the In2O3 which splits the d band, better accounting for the broadness of the semicore level. (d) Measured XPS semicore level spectra of β-Ga2O3 (ref (24)) (top), InGaO3 (middle), and In2O3 (ref (23)) (bottom). Two Gaussians are overlaid in the InGaO3 spectra to aid comparison with the In 4d and Ga 3d hybrid semicore level in the theory. The high binding energy region in each case is shown magnified ×15.

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    Figure 10

    Figure 10. (a) Theoretically calculated band-edge positions for Ga and In cation compounds relative to the charge neutrality level ECNL. Γ–Γ transitions are shown, but in cases where the indirect transition is significant this is indicated. (b) Atomic orbital energy levels for the constituent elements taken experimentally (58) and theoretically (59) where experiment was not available. These results corroborate the trends seen in other works. (55,60) (c) Absolute volume deformation potential at the Γ-point aVΓ, CBM deformation potential aVΓ,CBM, and VBM deformation potential aVΓ,VBM for compounds. (d) Calculated equilibrium unit cell volume and volume per atom (unit cell volume normalized by the number of atoms it contains) for compounds.

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    This article references 73 other publications.

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      Walsh, A.; Da Silva, J. L. F.; Wei, S.-H.; Korber, C.; Klein, A.; Piper, L. F. J.; DeMasi, A.; Smith, K. E.; Panaccione, G.; Torelli, P.; Payne, D. J.; Bourlange, A.; Egdell, R. G. Nature of the Band Gap of In2O3 Revealed by First-Principles Calculations and X-Ray Spectroscopy. Phys. Rev. Lett. 2008, 100, 167402,  DOI: 10.1103/PhysRevLett.100.167402
      1
      Nature of the Band Gap of In2O3 Revealed by First-Principles Calculations and X-Ray Spectroscopy
      Walsh, Aron; Da Silva, Juarez L. F.; Wei, Su-Huai; Korber, C.; Klein, A.; Piper, L. F. J.; DeMasi, Alex; Smith, Kevin E.; Panaccione, G.; Torelli, P.; Payne, D. J.; Bourlange, A.; Egdell, R. G.
      Physical Review Letters (2008), 100 (16), 167402/1-167402/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Bulk and surface sensitive x-ray spectroscopic techniques are applied in tandem to show that the valence band edge for In2O3 is found significantly closer to the bottom of the conduction band than expected from the widely quoted bulk band gap of 3.75 eV. First-principles theory shows that the upper valence bands of In2O3 exhibit a small dispersion and the conduction band min. is positioned at Γ. However, direct optical transitions give a minimal dipole intensity until 0.8 eV below the valence band max. The results set an upper limit on the fundamental band gap of 2.9 eV.
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      Ellmer, K. Past Achievements and Future Challenges in the Development of Optically Transparent Electrodes. Nat. Photonics 2012, 6, 809– 817,  DOI: 10.1038/nphoton.2012.282
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      Past achievements and future challenges in the development of optically transparent electrodes
      Ellmer, Klaus
      Nature Photonics (2012), 6 (12), 809-817CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      A review. Transparent conductive electrodes play important roles in information and energy technologies. These materials, particularly transparent conductive oxides, are widely used as transparent electrodes across tech. fields such as low-emissivity coatings, flat-panel displays, thin-film solar cells and org. light-emitting diodes. This Review begins by summarizing the properties and applications of transparent conductive oxides such as In2O3, SnO2, ZnO and TiO2. Owing to the increasing demand for raw materials - esp. indium - scientists are currently searching for alternatives to indium tin oxide. Carbon nanotube and metal nanowire networks, as well as regular metal grids, have been investigated for use as transparent conductive electrodes. This Review compares these materials and the recently 'rediscovered' graphene with today's established transparent conductive oxides.
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      Pearton, S. J.; Yang, J.; Cary, P. H.; Ren, F.; Kim, J.; Tadjer, M. J.; Mastro, M. A. A Review of Ga2O3 Materials, Processing, and Devices. Appl. Phys. Rev. 2018, 5, 011301,  DOI: 10.1063/1.5006941
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      A review of Ga2O3 materials, processing, and devices
      Pearton, S. J.; Yang, Jiancheng; Cary, Patrick H.; Ren, F.; Kim, Jihyun; Tadjer, Marko J.; Mastro, Michael A.
      Applied Physics Reviews (2018), 5 (1), 011301/1-011301/56CODEN: APRPG5; ISSN:1931-9401. (American Institute of Physics)
      A review. Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (ε) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. The performance of technol. important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger crit. elec. field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielecs. for gate formation, and passivation are discussed. (c) 2018 American Institute of Physics.
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    4. 4
      Lorenz, M. The 2016 Oxide Electronic Materials and Oxide Interfaces Roadmap. J. Phys. D: Appl. Phys. 2016, 49, 433001,  DOI: 10.1088/0022-3727/49/43/433001
      4
      The 2016 oxide electronic materials and oxide interfaces roadmap
      Lorenz, M.; Rao, M. S. Ramachandra; Venkatesan, T.; Fortunato, E.; Barquinha, P.; Branquinho, R.; Salgueiro, D.; Martins, R.; Carlos, E.; Liu, A.; Shan, F. K.; Grundmann, M.; Boschker, H.; Mukherjee, J.; Priyadarshini, M.; DasGupta, N.; Rogers, D. J.; Teherani, F. H.; Sandana, E. V.; Bove, P.; Rietwyk, K.; Zaban, A.; Veziridis, A.; Weidenkaff, A.; Muralidhar, M.; Murakami, M.; Abel, S.; Fompeyrine, J.; Zuniga-Perez, J.; Ramesh, R.; A. Spaldin, N.; Ostanin, S.; Borisov, V.; Mertig, I.; Lazenka, V.; Srinivasan, G.; Prellier, W.; Uchida, M.; Kawasaki, M.; Pentcheva, R.; Gegenwart, P.; Granozio, F. Miletto; Fontcuberta, J. Pryds
      Journal of Physics D: Applied Physics (2016), 49 (43), 433001/1-433001/53CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
      A review. Oxide electronic materials provide a plethora of possible applications and offer ample opportunity for scientists to probe into some of the exciting and intriguing phenomena exhibited by oxide systems and oxide interfaces. In addn. to the already diverse spectrum of properties, the nanoscale form of oxides provides a new dimension of hitherto unknown phenomena due to the increased surface-to-vol. ratio. Oxide electronic materials are becoming increasingly important in a wide range of applications including transparent electronics, optoelectronics, magnetoelectronics, photonics, spintronics, thermoelecs., piezoelecs., power harvesting, hydrogen storage and environmental waste management. Synthesis and fabrication of these materials, as well as processing into particular device structures to suit a specific application is still a challenge. Further, characterization of these materials to understand the tunability of their properties and the novel properties that evolve due to their nanostructured nature is another facet of the challenge. The research related to the oxide electronic field is at an impressionable stage, and this has motivated us to contribute with a roadmap on 'oxide electronic materials and oxide interfaces'. This roadmap envisages the potential applications of oxide materials in cutting edge technologies and focuses on the necessary advances required to implement these materials, including both conventional and novel techniques for the synthesis, characterization, processing and fabrication of nanostructured oxides and oxide-based devices. The contents of this roadmap will highlight the functional and correlated properties of oxides in bulk, nano, thin film, multilayer and heterostructure forms, as well as the theor. considerations behind both present and future applications in many technol. important areas as pointed out by Venkatesan. The contributions in this roadmap span several thematic groups which are represented by the following authors: novel field effect transistors and bipolar devices by Fortunato, Grundmann, Boschker, Rao, and Rogers; energy conversion and saving by Zaban, Weidenkaff, and Murakami; new opportunities of photonics by Fompeyrine, and Zuniga-Perez; multiferroic materials including novel phenomena by Ramesh, Spaldin, Mertig, Lorenz, Srinivasan, and Prellier; and concepts for topol. oxide electronics by Kawasaki, Pentcheva, and Gegenwart. Finally, Miletto Granozio presents the European action 'towards oxide-based electronics' which develops an oxide electronics roadmap with emphasis on future nonvolatile memories and the required technologies. In summary, we do hope that this oxide roadmap appears as an interesting up-to-date snapshot on one of the most exciting and active areas of solid state physics, materials science, and chem., which even after many years of very successful development shows in short intervals novel insights and achievements.
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    5. 5
      Gonçalves, G.; Barquinha, P.; Pereira, L.; Franco, N.; Alves, E.; Martins, R.; Fortunato, E. High Mobility a-IGO Films Produced at Room Temperature and Their Application in TFTs. Electrochem. Solid-State Lett. 2010, 13, H20– H22,  DOI: 10.1149/1.3257613
      There is no corresponding record for this reference.
    6. 6
      Huang, W.-L.; Hsu, M.-H.; Chang, S.-P.; Chang, S.-J.; Chiou, Y.-Z. Indium Gallium Oxide Thin Film Transistor for Two-Stage UV Sensor Application. ECS J. Solid State Sci. Technol. 2019, 8, Q3140– Q3143,  DOI: 10.1149/2.0251907jss
      6
      Indium gallium oxide thin film transistor for two-stage UV sensor application
      Huang, Wei-Lun; Hsu, Ming-Hung; Chang, Sheng-Po; Chang, Shoou-Jinn; Chiou, Yu-Zung
      ECS Journal of Solid State Science and Technology (2019), 8 (7), Q3140-Q3143CODEN: EJSSBG; ISSN:2162-8769. (Electrochemical Society)
      In this work, indium gallium oxide (IGO) thin film transistor (TFT) was fabricated by radio-frequency (RF) sputtering. The transmittance of the TFT shows larger than 80% cross the visible light region. As a wide bandgap and high transparency semiconducting material, IGO is a potential candidate for UV-detection applications.Measured in the dark, the IGO TFT exhibits a threshold voltage of 0.9 V, mobility of 2.66 cm2/Vs, on-off ratio of 1.21×106, subthreshold swing of 0.41 V/dec. The TFT was then employed to detect UV light and the sensing properties are investigated. The IGO phototransistor has a high responsivity of 5.012 A/W and a rejection ratio of 1.65×105. The above results reveal that IGO phototransistor is a brilliant multi-functional device, which can serve as either a switch component or a UV sensor.
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    7. 7
      Kim, Y. G.; Kim, T.; Avis, C.; Lee, S.-H.; Jang, J. Stable and High-Performance Indium Oxide Thin-Film Transistor by Ga Doping. IEEE Trans. Electron Devices 2016, 63, 1078– 1084,  DOI: 10.1109/TED.2016.2518703
      7
      Stable and high-performance indium oxide thin-film transistor by Ga doping
      Kim, Youn Goo; Kim, Taehun; Avis, Christophe; Lee, Seung-Hun; Jang, Jin
      IEEE Transactions on Electron Devices (2016), 63 (3), 1078-1084CODEN: IETDAI; ISSN:0018-9383. (Institute of Electrical and Electronics Engineers)
      Research on a replacement of amorphous silicon for a thin-film transistor (TFT) and large area electronics has been driven by costly vacuum processed indium-gallium-zinc oxide (IGZO). Even though widely studied, the performances still require improvement, and a wide no. of other materials have been tested. While indium-zinc oxide, IGZO, indium-zinc-tin oxide (ZTO), and ZTO have been widely investigated, gallium-doped indium oxide (IGO) has not been under highlight. Here, we report the use of simple and cost effective spin-coated IGO TFT using spin-coated AlOx gate dielec. We achieved high mobility over 50 cm2/Vs and high stability. The thin films are studied by transmission electron microscopy, X-ray diffraction, XPS, Raman spectra, at. force microscopy, and field-effect measurements. Analyses reveal the strong dependence between crystallinity, mobility, and stability. All TFTs show excellent operation, with champion characteristics for the 10% Ga-doped InOx, revealing a mobility of 52.6 cm2/Vs, on/off ratios of 108, and VTH variation of <0.1 V during 1 h of stress measurement.
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    8. 8
      von Wenckstern, H.; Splith, D.; Werner, A.; Müller, S.; Lorenz, M.; Grundmann, M. Properties of Schottky Barrier Diodes on (InxGa1-x)2O3 for 0.01 ≤ x ≤ 0.85 Determined by a Combinatorial Approach. ACS Comb. Sci. 2015, 17, 710– 715,  DOI: 10.1021/acscombsci.5b00084
      8
      Properties of Schottky Barrier Diodes on (InxGa1-x)2O3 for 0.01 ≤ x ≤ 0.85 Determined by a Combinatorial Approach
      von Wenckstern, H.; Splith, D.; Werner, A.; Mueller, S.; Lorenz, M.; Grundmann, M.
      ACS Combinatorial Science (2015), 17 (12), 710-715CODEN: ACSCCC; ISSN:2156-8944. (American Chemical Society)
      We investigated properties of an (InxGa1-x)2O3 thin film with laterally varying cation compn. that was realized by a large-area offset pulsed laser deposition approach. Within a two inch diam. thin film, the compn. varies between 0.01 ≤ x ≤ 0.85, and three crystallog. phases (cubic, hexagonal, and monoclinic) were identified. We obsd. a correlation between characteristic parameters of Schottky barrier diodes fabricated on the thin film and its chem. and structural material properties. The highest Schottky barriers and rectification of the diodes were found for low indium contents. The thermal stability of the diodes is also best for Ga-rich parts of the sample. Conversely, the series resistance is lowest for large In content. Overall, the (InxGa1-x)2O3 alloy is well-suited for potential applications such as solar-blind photodetectors with a tunable absorption edge.
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    9. 9
      Kranert, C.; Lenzner, J.; Jenderka, M.; Lorenz, M.; von Wenckstern, H.; Schmidt-Grund, R.; Grundmann, M. Lattice Parameters and Raman-active Phonon Modes of (InxGa1-x)2O3 for x < 0.4. J. Appl. Phys. 2014, 116, 013505,  DOI: 10.1063/1.4886895
      9
      Lattice parameters and Raman-active phonon modes of (InxGa1-x)2O3 for x < 0.4
      Kranert, Christian; Lenzner, Joerg; Jenderka, Marcus; Lorenz, Michael; von Wenckstern, Holger; Schmidt-Grund, Ruediger; Grundmann, Marius
      Journal of Applied Physics (Melville, NY, United States) (2014), 116 (1), 013505/1-013505/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      We present X-ray diffraction and Raman spectroscopy investigations of (InxGa1-x)2O3 thin films and bulk-like ceramics in dependence of their compn. The thin films grown by pulsed laser deposition have a continuous lateral compn. spread allowing the detn. of phonon mode properties and lattice parameters with high sensitivity to the compn. from a single 2-in. wafer. In the regime of low indium concn., the phonon energies depend linearly on the compn. and show a good agreement between both sample types. We detd. the slopes of these dependencies for eight different Raman modes. While the lattice parameters of the ceramics follow Vegard's rule, deviations are obsd. for the thin films. Further, we found indications of the high-pressure phase InGaO3 II in the thin films above a crit. indium concn., its value depending on the type of substrate. (c) 2014 American Institute of Physics.
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    10. 10
      Shannon, R. D.; Prewitt, C. T. Synthesis and Structure of Phases in the In2O3-Ga2O3 System. J. Inorg. Nucl. Chem. 1968, 30, 1389– 1398,  DOI: 10.1016/0022-1902(68)80277-5
      10
      Synthesis and structure of phases in the indium sesquioxide-gallium sesquioxide system
      Shannon, R. D.; Prewitt, C. T.
      Journal of Inorganic and Nuclear Chemistry (1968), 30 (6), 1389-98CODEN: JINCAO; ISSN:0022-1902.
      Compns. Ga2-xInxO3 (x = 0.0-1.0) having the β-Ga2O3 structure were prepd. in the powder form and as single crystals by several techniques and characterized by x-ray and resistivity measurements. The compn. InGaO3 transforms at high pressures to a new phase, InGaO3 II. InGaO3 II has a simple hexagonal oxide structure which is the 1st structure reported contg. Ga3+ in 5-fold coordination. The cell parameters are: a 3.310 ± 0.002, c 12.039 ± 0.002 A.; Z = 2; ρ(calcd.) = 6.756; and the space group is P63/mmc. Interat. distances obtained from least-sqs. refinement results are In-O(1)[×2], 3.010 A.; In-O(2)[×6], 2.174 A.; Ga-O(1)[×3], 1.911 A.; and Ga-O(2)[×2], 1.973 A. InGaO3 II is isotypic with hexagonal YAlO3, and its structure is related to that of YMnO3, which contains 5 coordinated Mn3+. 20 references.
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    11. 11
      Kneiß, M.; Hassa, A.; Splith, D.; Sturm, C.; von Wenckstern, H.; Lorenz, M.; Grundmann, M. Epitaxial Stabilization of Single Phase κ-(InxGa1-x)2O3 Thin Films up to x = 0.28 on c-Sapphire and κ-Ga2O3(001) Templates by Tin-Assisted VCCS-PLD. APL Mater. 2019, 7, 101102,  DOI: 10.1063/1.5120578
      11
      Epitaxial stabilization of single phase κ (InxGa1-x)2O3 thin films up to x = 0.28 on c-sapphire and κ -Ga2O3(001) templates by tin-assisted VCCS-PLD
      Kneiss, M.; Hassa, A.; Splith, D.; Sturm, C.; von Wenckstern, H.; Lorenz, M.; Grundmann, M.
      APL Materials (2019), 7 (10), 101102/1-101102/10CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      High-quality (InxGa1-x)2O3 thin films in the orthorhombic κ -phase were grown by pulsed-laser deposition (PLD) on c-sapphire substrates as well as PLD-grown κ -Ga2O3 thin film templates. We varied the In-content 0 = x = 0.38 of the layers using a single, elliptically segmented, and tin-doped (In0.4Ga0.6)2O3/Ga2O3 target, employing the vertical continuous compn. spread (VCCS) PLD-technique. A stoichiometric transfer of In and Ga from the target to the thin films has been confirmed, suggesting that the formation of volatile Ga2O and In2O suboxides is not a limiting factor in the tin-assisted growth mode. For all x, the thin films crystd. predominantly in the κ -modification as demonstrated by XRD 2θ -ω scans. However, for x > 0.28, phase sepn. of the cubic bixbyite and the κ -phase occurred. The κ -Ga2O3 template increased the cryst. quality of the κ -(InxGa1-x)2O3 thin film layers remarkably. Epitaxial, but relaxed growth with three in-plane rotational domains has been found for all thin films by XRD κ -scans or reciprocal space map measurements. Smooth surface morphologies (Rq < 3 nm) for all phase pure thin films were evidenced by at. force microscopy measurements, making them suitable for multilayer heterostructures. The compn.-dependent in- and out-of plane lattice consts. follow a linear behavior according to Vegard's law. A linear relationship can also be confirmed for the optical bandgaps that demonstrate the feasibility of bandgap engineering in the energy range of 4.1-4.9 eV. The results suggest κ -(InxGa1-x)2O3 as a promising material for heterostructure device applications or photodetectors. (c) 2019 American Institute of Physics.
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      Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207– 8215,  DOI: 10.1063/1.1564060
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      Hybrid functionals based on a screened Coulomb potential
      Heyd, Jochen; Scuseria, Gustavo E.; Ernzerhof, Matthias
      Journal of Chemical Physics (2003), 118 (18), 8207-8215CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      Hybrid d. functionals are very successful in describing a wide range of mol. properties accurately. In large mols. and solids, however, calcg. the exact (Hartree-Fock) exchange is computationally expensive, esp. for systems with metallic characteristics. In the present work, we develop a new hybrid d. functional based on a screened Coulomb potential for the exchange interaction which circumvents this bottleneck. The results obtained for structural and thermodn. properties of mols. are comparable in quality to the most widely used hybrid functionals. In addn., we present results of periodic boundary condition calcns. for both semiconducting and metallic single wall carbon nanotubes. Using a screened Coulomb potential for Hartree-Fock exchange enables fast and accurate hybrid calcns., even of usually difficult metallic systems. The high accuracy of the new screened Coulomb potential hybrid, combined with its computational advantages, makes it widely applicable to large mols. and periodic systems.
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      Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953– 17979,  DOI: 10.1103/PhysRevB.50.17953
      13
      Projector augmented-wave method
      Blochl
      Physical review. B, Condensed matter (1994), 50 (24), 17953-17979 ISSN:0163-1829.
      There is no expanded citation for this reference.
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      Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-energy Calculations using a Plane-wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169– 11186,  DOI: 10.1103/PhysRevB.54.11169
      14
      Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
      Kresse, G.; Furthmueller, J.
      Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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      Blöchl, P. E.; Jepsen, O.; Andersen, O. K. Improved Tetrahedron Method for Brillouin-zone Integrations. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 16223– 16233,  DOI: 10.1103/PhysRevB.49.16223
      15
      Improved tetrahedron method for Brillouin-zone integrations
      Blochl, Peter E.; Jepsen, O.; Andersen, O. K.
      Physical Review B: Condensed Matter and Materials Physics (1994), 49 (23), 16223-33CODEN: PRBMDO; ISSN:0163-1829.
      Several improvements of the tetrahedron method for Brillouin-zone integrations are presented. (1) A translational grid of k points and tetrahedra is suggested that renders the results for insulators identical to those obtained with special-point methods with the same no. of k points. (2) A simple correction formula goes beyond the linear approxn. of matrix elements within the tetrahedra and also improves the results for metals significantly. For a required accuracy this reduces the no. of k points by orders of magnitude. (3) Irreducible k points and tetrahedra are selected by a fully automated procedure, requiring as input only the space-group operations. (4) The integration is formulated as a weighted sum over irreducible k points with integration wts. calcd. using the tetrahedron method once for a given band structure. This allows an efficient use of the tetrahedron method also in plane-wave-based electronic-structure methods.
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    16. 16
      Onuma, T.; Saito, S.; Sasaki, K.; Goto, K.; Masui, T.; Yamaguchi, T.; Honda, T.; Kuramata, A.; Higashiwaki, M. Temperature-Dependent Exciton Resonance Energies and their Correlation with IR-active Optical Phonon Modes in β-Ga2O3 Single Crystals. Appl. Phys. Lett. 2016, 108, 101904,  DOI: 10.1063/1.4943175
      16
      Temperature-dependent exciton resonance energies and their correlation with IR-active optical phonon modes in β-Ga2O3 single crystals
      Onuma, T.; Saito, S.; Sasaki, K.; Goto, K.; Masui, T.; Yamaguchi, T.; Honda, T.; Kuramata, A.; Higashiwaki, M.
      Applied Physics Letters (2016), 108 (10), 101904/1-101904/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Temp.-dependent exciton resonance energies Eexciton in β-Ga2O3 single crystals are studied by using polarized reflectance measurement. The Eexciton values exhibit large energy changes at 179-268 meV from 5 to 300 K. The IR-active Au and Bu optical phonon modes are selectively obsd. in the IR ellipsometry spectra by reflecting the polarization selection rules. The LO phonon energies can be divided into 3 ranges: ℏωLO = 35-48, 70-73, and 88-99 meV. The broadening parameters, which are obtained from the reflectance measurements, correspond to the lower 2 ranges of ℏωLO at low temp. and 75 meV >150 K. The large Eexciton changes with temp. in β-Ga2O3 are originated from the exciton-LO-phonon interaction. (c) 2016 American Institute of Physics.
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    17. 17
      Ingebrigtsen, M. E.; Varley, J. B.; Kuznetsov, A. Y.; Svensson, B. G.; Alfieri, G.; Mihaila, A.; Badstübner, U.; Vines, L. Iron and Intrinsic Deep Level States in Ga2O3. Appl. Phys. Lett. 2018, 112, 042104,  DOI: 10.1063/1.5020134
      17
      Iron and intrinsic deep level states in Ga2O3
      Ingebrigtsen, M. E.; Varley, J. B.; Kuznetsov, A. Yu.; Svensson, B. G.; Alfieri, G.; Mihaila, A.; Badstubner, U.; Vines, L.
      Applied Physics Letters (2018), 112 (4), 042104/1-042104/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Using a combination of deep level transient spectroscopy, secondary ion mass spectrometry, proton irradn., and hybrid functional calcns., we identify two similar deep levels that are assocd. with Fe impurities and intrinsic defects in bulk crystals and mol. beam epitaxy and hydride vapor phase epitaxi-grown epilayers of β-Ga2O3. First, our results indicate that FeGa, and not an intrinsic defect, acts as the deep acceptor responsible for the often dominating E2 level at ∼0.78 eV below the conduction band min. Second, by provoking addnl. intrinsic defect generation via proton irradn., we identified the emergence of a new level, labeled as E2*, having the ionization energy very close to that of E2, but exhibiting an order of magnitude larger capture cross section. Importantly, the properties of E2* are found to be consistent with its intrinsic origin. As such, contradictory opinions of a long standing literature debate on either extrinsic or intrinsic origin of the deep acceptor in question converge accounting for possible contributions from E2 and E2* in different exptl. conditions. (c) 2018 American Institute of Physics.
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    18. 18
      Geller, S. Crystal Structure of β-Ga2O3. J. Chem. Phys. 1960, 33, 676– 684,  DOI: 10.1063/1.1731237
      18
      Crystal structure of β-Ga2O3
      Geller, S.
      Journal of Chemical Physics (1960), 33 (), 676-84CODEN: JCPSA6; ISSN:0021-9606.
      The monoclinic crystal has cell dimensions a = 12.23, b = 3.04, c = 5.80 A., β = 103.7°. There are 4 mols. per unit cell. The most probable space group is C32h-C2/m; the atoms are in 5 sets of special positions 4i: (000, 1/2 1/2 0) ± (x0z). There are 2 kinds of co.ovrddot.ordination for Ga+++ in this structure, tetrahedral and octahedral. Av. interionic distances are: tetrahedral Ga-O; 1.83 A.; octahedral Ga-O, 2.00 A.; tetrahedron edge O-O, 3.02 A.; and octahedron edge O-O, 2.84 A. Because of the reduced co.ovrddot.ordination of half of the metal ions, the d. of β-Ga2O3 is lower than that of α-Ga2O3. The av. Ga-O distances in the structure account for the fact that although the Ga+++ is substantially larger than the Al+++, its quant. preference for tetrahedrally co.ovrddot.ordinated sites when substituted for Fe+++ in the Fe garnets is nearly the same as that of the Al+++ ion. The magnetic aspects of the β-Ga2O3 structure are discussed, and a possible Fe2O3 isomorph may be expected to be at least antiferromagnetic with a Neel temp. of about 700°K.
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    19. 19
      Prozheeva, V.; Hölldobler, R.; von Wenckstern, H.; Grundmann, M.; Tuomisto, F. Effects of Alloy Composition and Si-doping on Vacancy Defect Formation in (InxGa1-x)2O3 Thin Films. J. Appl. Phys. 2018, 123, 125705,  DOI: 10.1063/1.5022245
      19
      Effects of alloy composition and Si-doping on vacancy defect formation in (InxGa1-x)2O3 thin films
      Prozheeva, V.; Holldobler, R.; von Wenckstern, H.; Grundmann, M.; Tuomisto, F.
      Journal of Applied Physics (Melville, NY, United States) (2018), 123 (12), 125705/1-125705/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Various nominally undoped and Si-doped (InxGa1-x)2O3 thin films were grown by pulsed laser deposition in a continuous compn. spread mode on c-plane α-sapphire and (100)-oriented MgO substrates. Positron annihilation spectroscopy in the Doppler broadening mode was used as the primary characterization technique in order to investigate the effect of alloy compn. and dopant atoms on the formation of vacancy-type defects. In the undoped samples, we observe a Ga2O3-like trend for low indium concns. changing to In2O3-like behavior along with the increase in the indium fraction. Increasing indium concn. is found to suppress defect formation in the undoped samples at [In] > 70 at. %. Si doping leads to positron satn. trapping in VIn-like defects, suggesting a vacancy concn. of at least mid-1018 cm-3 independent of the indium content. This is a solid soln. study. (c) 2018 American Institute of Physics.
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    20. 20
      Schmidt-Grund, R.; Kranert, C.; Bontgen, T.; von Wenckstern, H.; Krauß, H.; Grundmann, M. Dielectric Function in the NIR-VUV Spectral Range of (InxGa1-x)2O3 Thin Films. J. Appl. Phys. 2014, 116, 053510,  DOI: 10.1063/1.4891521
      20
      Dielectric function in the NIR-VUV spectral range of (InxGa1-x)2O3 thin films
      Schmidt-Grund, R.; Kranert, C.; Boentgen, T.; von Wenckstern, H.; Krauss, H.; Grundmann, M.
      Journal of Applied Physics (Melville, NY, United States) (2014), 116 (5), 053510/1-053510/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      We detd. the dielec. function of the alloy system (InxGa1-x)2O3 by spectroscopic ellipsometry in the wide spectral range from 0.5 eV to 8.5 eV and for In contents ranging from x = 0.02 to x = 0.61. The predicted optical transitions for binary, monoclinic β-Ga2O3, and cubic bcc-In2O3 are well reflected by the change of the dielec. functions' lineshape as a function of the In content. In an intermediate compn. range with phase-sepd. material (x ≈ 0.3...0.4), the lineshape differs considerably, which we assign to the presence of the high-pressure rhombohedral InGaO3-II phase, which we also observe in Raman expts. in this range. By model anal. of the dielec. function, we derived spectra of the refractive index and the absorption coeff. and energy parameters of electronic band-band transitions. We discuss the sub-band gap absorption tail in relation to the influence of the In 4d orbitals on the valence bands. The data presented here provide a basis for a deeper understanding of the electronic properties of this technol. important material system and may be useful for device engineering. (c) 2014 American Institute of Physics.
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    21. 21
      von Wenckstern, H.; Kneiß, M.; Hassa, A.; Storm, P.; Splith, D.; Grundmann, M. A Review of the Segmented-Target Approach to Combinatorial Material Synthesis by Pulsed-Laser Deposition. Phys. Status Solidi B 2020, 257, 1900626,  DOI: 10.1002/pssb.201900626
      21
      A Review of the Segmented-Target Approach to Combinatorial Material Synthesis by Pulsed-Laser Deposition
      von Wenckstern, Holger; Kneiss, Max; Hassa, Anna; Storm, Philipp; Splith, Daniel; Grundmann, Marius
      Physica Status Solidi B: Basic Solid State Physics (2020), 257 (7), 1900626CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Combinatorial material synthesis has led to a significant acceleration in the optimization of multinary compds. and a more efficient usage of source and substrate materials. Various growth methods, including phys. vapor deposition, can be adopted to realize material libraries. Herein, two approaches to combinatorial material synthesis based on ablation of segmented targets during pulsed-laser deposition are reviewed. For these two processes, either laterally or radially segmented targets are utilized and allow the creation of lateral and vertical compn. spreads, resp. Radially segmented targets can addnl. be used to synthesize a discrete binary material library. Both approaches are introduced by calcg. the expected material distribution with a simple geometric plasma expansion model. Then, exptl. detd. elemental distributions and growth rates are compared to predictions and it is demonstrated that differences between calcd. and exptl. data contain vital information on the influence of, for example, thermodn. processes on the growth mechanism.
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    22. 22
      Vines, L.; Bhoodoo, C.; von Wenckstern, H.; Grundmann, M. Electrical Conductivity of In2O3 and Ga2O3 after Low Temperature Ion Irradiation; Implications for Intrinsic Defect Formation and Charge Neutrality Level. J. Phys.: Condens. Matter 2018, 30, 025502,  DOI: 10.1088/1361-648X/aa9e2a
      22
      Electrical conductivity of In2O3 and Ga2O3 after low temperature ion irradiation; implications for instrinsic defect formation and charge neutrality level
      Vines, L.; Bhoodoo, C.; von Wenckstern, H.; Grundmann, M.
      Journal of Physics: Condensed Matter (2018), 30 (2), 025502/1-025502/6CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
      The evolution of sheet resistance of n-type In2O3 and Ga2O3 exposed to bombardment with MeV 12C and 28Si ions at 35 K is studied in situ. While the sheet resistance of Ga2O3 increased by more than 8-fold as a result of ion irradn., In2O3 showed a more complex defect evolution and became more conductive when irradiated at the highest doses. Heating up to room temp. reduced the sheet resistivity somewhat, but Ga2O3 remained highly resistive, while In2O3 showed a lower resistance than as deposited samples. Thermal admittance spectroscopy and deep level transient spectroscopy did not reveal new defect levels for irradn. up to 2 × 1012 cm-2. A model where larger defect complexes preferentially produce donor like defects in In2O3 is proposed, and may reveal a microscopic view of a charge neutrality level within the conduction band, as previously proposed.
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    23. 23
      King, P. D. C.; Veal, T. D.; Fuchs, F.; Wang, C. Y.; Payne, D. J.; Bourlange, A.; Zhang, H.; Bell, G. R.; Cimalla, V.; Ambacher, O.; Egdell, R. G.; Bechstedt, F.; McConville, C. F. Band Gap, Electronic Structure, and Surface Electron Accumulation of Cubic and Rhombohedral In2O3. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 205211,  DOI: 10.1103/PhysRevB.79.205211
      23
      Band gap, electronic structure, and surface electron accumulation of cubic and rhombohedral In2O3
      King, P. D. C.; Veal, T. D.; Fuchs, F.; Wang, Ch. Y.; Payne, D. J.; Bourlange, A.; Zhang, H.; Bell, G. R.; Cimalla, V.; Ambacher, O.; Egdell, R. G.; Bechstedt, F.; McConville, C. F.
      Physical Review B: Condensed Matter and Materials Physics (2009), 79 (20), 205211/1-205211/10CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The bulk and surface electronic structure of In2O3 has proved controversial, prompting the current combined exptl. and theor. investigation. The band gap of single-cryst. In2O3 is detd. as 2.93±0.15 and 3.02±0.15 eV for the cubic bixbyite and rhombohedral polymorphs, resp. The valence-band d. of states is investigated from x-ray photoemission spectroscopy measurements and d.-functional theory calcns. These show excellent agreement, supporting the absence of any significant indirect nature of the In2O3 band gap. Clear exptl. evidence for an s-d coupling between In 4d and O 2s derived states is also obsd. Electron accumulation, recently reported at the (001) surface of bixbyite material, is also shown to be present at the bixbyite (111) surface and the (0001) surface of rhombohedral In2O3.
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    24. 24
      Swallow, J. E. N.; Varley, J. B.; Jones, L. A. H.; Gibbon, J. T.; Piper, L. F. J.; Dhanak, V. R.; Veal, T. D. Transition from Electron Accumulation to Depletion at β-Ga2O3 Surfaces: The Role of Hydrogen and the Charge Neutrality Level. APL Mater. 2019, 7, 022528,  DOI: 10.1063/1.5054091
      24
      Transition from electron accumulation to depletion at β-Ga2O3 surfaces: The role of hydrogen and the charge neutrality level
      Swallow, J. E. N.; Varley, J. B.; Jones, L. A. H.; Gibbon, J. T.; Piper, L. F. J.; Dhanak, V. R.; Veal, T. D.
      APL Materials (2019), 7 (2), 022528/1-022528/8CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      The surface electronic properties of bulk-grown β-Ga2O3 (201) single crystals are investigated. The band gap is found using optical transmission to be 4.68 eV. High-resoln. x-ray photoemission coupled with hybrid d. functional theory calcn. of the valence band d. of states provides insights into the surface band bending. Importantly, the std. linear extrapolation method for detg. the surface valence band max. (VBM) binding energy is found to underestimate the sepn. from the Fermi level by ∼0.5 eV. According to our interpretation, most reports of surface electron depletion and upward band bending based on photoemission spectroscopy actually provide evidence of surface electron accumulation. For uncleaned surfaces, the surface VBM to Fermi level sepn. is found to be 4.95 ± 0.10 eV, corresponding to downward band bending of ∼0.24 eV and an electron accumulation layer with a sheet d. of ∼5 × 1012 cm-2. Uncleaned surfaces possess hydrogen termination which acts as surface donors, creating electron accumulation and downward band bending at the surface. In situ cleaning by thermal annealing removes H from the surface, resulting in a ∼0.5 eV shift of the surface VBM and formation of a surface electron depletion layer with upward band bending of ∼0.26 eV due to native acceptor surface states. These results are discussed in the context of the charge neutrality level, calcd. bulk interstitial hydrogen transition levels, and related previous exptl. findings. (c) 2019 American Institute of Physics.
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    25. 25
      Nagata, T.; Hoga, T.; Yamashita, A.; Asahi, T.; Yagyu, S.; Chikyow, T. Valence Band Modification of a (GaxIn1-x)2O3 Solid Solution System Fabricated by Combinatorial Synthesis. ACS Comb. Sci. 2020, 22, 433439,  DOI: 10.1021/acscombsci.0c00033
      There is no corresponding record for this reference.
    26. 26
      King, P. D. C.; Veal, T. D.; Payne, D. J.; Bourlange, A.; Egdell, R. G.; McConville, C. F. Surface Electron Accumulation and the Charge Neutrality Level in In2O3. Phys. Rev. Lett. 2008, 101, 116808,  DOI: 10.1103/PhysRevLett.101.116808
      26
      Surface electron accumulation and the charge neutrality level in In2O3
      King, P. D. C.; Veal, T. D.; Payne, D. J.; Bourlange, A.; Egdell, R. G.; McConville, C. F.
      Physical Review Letters (2008), 101 (11), 116808/1-116808/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      High-resoln. x-ray photoemission spectroscopy, IR reflectivity and Hall effect measurements, combined with surface space-charge calcns., are used to show that electron accumulation occurs at the surface of undoped single-cryst. In2O3. From a combination of measurements performed on undoped and heavily Sn-doped samples, the charge neutrality level is shown to lie ∼0.4 eV above the conduction band min. in In2O3, explaining the electron accumulation at the surface of undoped material, the propensity for n-type cond., and the ease of n-type doping in In2O3, and hence its use as a transparent conducting oxide material.
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    27. 27
      Lovejoy, T. C.; Chen, R.; Zheng, X.; Villora, E. G.; Shimamura, K.; Yoshikawa, H.; Yamashita, Y.; Ueda, S.; Kobayashi, K.; Dunham, S. T.; Ohuchi, F. S.; Olmstead, M. A. Band Bending and Surface Defects in β-Ga2O3. Appl. Phys. Lett. 2012, 100, 181602,  DOI: 10.1063/1.4711014
      27
      Band bending and surface defects in β-Ga2O3
      Lovejoy, T. C.; Chen, Renyu; Zheng, X.; Villora, E. G.; Shimamura, K.; Yoshikawa, H.; Yamashita, Y.; Ueda, S.; Kobayashi, K.; Dunham, S. T.; Ohuchi, F. S.; Olmstead, M. A.
      Applied Physics Letters (2012), 100 (18), 181602/1-181602/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Surface band bending and surface defects on the UV-transparent conducting oxide β-Ga2O3 (100) are studied with hard x-ray photoemission spectroscopy and scanning tunneling microscopy. Highly doped β-Ga2O3 shows flat bands near the surface, while the bands on nominally undoped (but still n-type), air-cleaved β-Ga2O3 are bent upwards by > 0.5 eV. Neg. charged surface defects are obsd. on vacuum annealed β-Ga2O3, which also shows upward band bending. D. functional calcns. show O vacancies are not likely to be ionized in the bulk, but could be activated by surface band bending. The large band bending may also hinder formation of ohmic contacts. (c) 2012 American Institute of Physics.
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    28. 28
      Navarro-Quezada, A.; Alamé, S.; Esser, N.; Furthmüller, J.; Bechstedt, F.; Galazka, Z.; Skuridina, D.; Vogt, P. Near Valence-band Electronic Properties of Semiconducting β-Ga2O3 (100) Single Crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 195306,  DOI: 10.1103/PhysRevB.92.195306
      28
      Near valence-band electronic properties of semiconducting β-Ga2O3 (100) single crystals
      Navarro-Quezada, A.; Alame, S.; Esser, N.; Furthmueller, J.; Bechstedt, F.; Galazka, Z.; Skuridina, D.; Vogt, P.
      Physical Review B: Condensed Matter and Materials Physics (2015), 92 (19), 195306/1-195306/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      β-Ga2O3 is a transparent wide-band-gap semiconductor that has attracted considerable interest in recent years due to its suitable elec. cond. and transparency in the UV spectral region. In this work we investigate the electronic properties of the near valence-band-edge region for semiconducting β-Ga2O3 (100) bulk single crystals using core-level photoelectron spectroscopy and ab initio theory within the framework of d. functional theory and the GW approach. We find good agreement between the exptl. results and the theor. calcns. This is explained by the hybridization of the Ga 3d and O 2s states, similar as for In2O3.
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    29. 29
      King, P. D. C.; Veal, T. D.; Schleife, A.; Zúñiga-Pérez, J.; Martel, B.; Jefferson, P. H.; Fuchs, F.; Muñoz-Sanjosé, V.; Bechstedt, F.; McConville, C. F. Valence-band Electronic Structure of CdO, ZnO, and MgO from X-ray Photoemission Spectroscopy and Quasi-particle-corrected Density-functional Theory Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 205205,  DOI: 10.1103/PhysRevB.79.205205
      29
      Valence-band electronic structure of CdO, ZnO, and MgO from x-ray photoemission spectroscopy and quasi-particle-corrected density-functional theory calculations
      King, P. D. C.; Veal, T. D.; Schleife, A.; Zuniga-Perez, J.; Martel, B.; Jefferson, P. H.; Fuchs, F.; Munoz-Sanjose, V.; Bechstedt, F.; McConville, C. F.
      Physical Review B: Condensed Matter and Materials Physics (2009), 79 (20), 205205/1-205205/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The valence-band d. of states of single-cryst. rock-salt CdO(001), wurtzite c-plane ZnO, and rock- salt MgO(001) are investigated by high-resoln. x-ray photoemission spectroscopy. A classic two-peak structure is obsd. in the VB-DOS due to the anion 2p-dominated valence bands. Good agreement is found between the exptl. results and quasi-particle-cor. d.-functional theory calcns. Occupied shallow semicore d levels are obsd. in CdO and ZnO. While these exhibit similar spectral features to the calcns., they occur at slightly higher binding energies, detd. as 8.8 eV and 7.3 eV below the valence band max. in CdO and ZnO, resp. The implications of these on the electronic structure are discussed.
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    30. 30
      Moses, P. G.; Van de Walle, C. G. Band Bowing and Band Alignment in InGaN Alloys. Appl. Phys. Lett. 2010, 96, 021908,  DOI: 10.1063/1.3291055
      30
      Band bowing and band alignment in InGaN alloys
      Moses, Poul Georg; Van de Walle, Chris G.
      Applied Physics Letters (2010), 96 (2), 021908/1-021908/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We use d. functional theory calcns. with the HSE06 hybrid exchange-correlation functional to investigate InGaN alloys and accurately det. band gaps and band alignments. We find a strong band-gap bowing at low In content. Band positions on an abs. energy scale are detd. from surface calcns. The resulting GaN/InN valence-band offset is 0.62 eV. The dependence of InGaN valence-band alignment on In content is found to be almost linear. Based on the values of band gaps and band alignments, we conclude that InGaN fulfills the requirements for a photoelectrochem. electrode for In contents up to 50%. (c) 2010 American Institute of Physics.
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    31. 31
      Peelaers, H.; Steiauf, D.; Varley, J. B.; Janotti, A.; Van de Walle, C. G. (InxGa1-x)2O3 Alloys for Transparent Electronics. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 085206,  DOI: 10.1103/PhysRevB.92.085206
      31
      (InxGa1-x)2O3 alloys for transparent electronics
      Peelaers, Hartwin; Steiauf, Daniel; Varley, Joel B.; Janotti, Anderson; Van de Walle, Chris G.
      Physical Review B: Condensed Matter and Materials Physics (2015), 92 (8), 085206/1-085206/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      (InxGa1-x)2O3 alloys show promise as transparent conducting oxides. Using hybrid d. functional calcns., band gaps, formation enthalpies, and structural parameters are detd. for monoclinic and bixbyite crystal structures. In the monoclinic phase the band gap exhibits a linear dependence on alloy concn., whereas in the bixbyite phase a large band-gap bowing occurs. The calcd. formation enthalpies show that the monoclinic structure is favorable for In compns. up to 50% and bixbyite for larger compns. This is caused by In strongly preferring sixfold oxygen coordination. The formation enthalpy of the 50:50 monoclinic alloy is much lower than the formation enthalpy of the 50:50 bixbyite alloy and also lower than most monoclinic alloys with lower In concn.; these trends are explained in terms of local strain. Consequences for expt. and applications are discussed.
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    32. 32
      Oshima, T.; Fujita, S. Properties of Ga2O3-based (InxGa1-x)2O3 Alloy Thin Films Grown by Molecular Beam Epitaxy. Phys. Status Solidi C 2008, 5, 3113– 3115,  DOI: 10.1002/pssc.200779297
      32
      Properties of Ga2O3-based (InxGa1-x)2O3 alloy thin films grown by molecular beam epitaxy
      Oshima, Takayoshi; Fujita, Shizuo
      Physica Status Solidi C: Current Topics in Solid State Physics (2008), 5 (9), 3113-3115CODEN: PSSCGL; ISSN:1862-6351. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A series of Ga2O3-based (InxGa1-x)2O3 alloy thin films were grown on c-plane Al2O3 substrates with a thin Ga2O3 buffer layer by plasma-assisted MBE. At growth temps. of 700° and higher, even with a slight inclusion of In2O3 to Ga2O3, for example, the film of (In0.08Ga0.92)2O3, exhibited a rough surface and degraded transmission spectrum resulting from phase sepn. of In2O3. Due to low temp. growth at 600°, however, the phase sepn. was suppressed for the In compn. ≤35%, which was confirmed by x-ray diffraction measurement, and the films exhibited high transmittance over 85% with sharp absorption edges. The bandgap could be tuned from 5.0-4.0 eV. The results encourage the application of (InxGa1-x)2O3 thin films in short-wavelength optical devices.
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    33. 33
      Regoutz, A.; Egdell, R.; Morgan, D.; Palgrave, R.; Téllez, H.; Skinner, S.; Payne, D.; Watson, G.; Scanlon, D. Electronic and Surface Properties of Ga-doped In2O3 Ceramics. Appl. Surf. Sci. 2015, 349, 970– 982,  DOI: 10.1016/j.apsusc.2015.04.106
      33
      Electronic and surface properties of Ga-doped In2O3 ceramics
      Regoutz, A.; Egdell, R. G.; Morgan, D. J.; Palgrave, R. G.; Tellez, H.; Skinner, S. J.; Payne, D. J.; Watson, G. W.; Scanlon, D. O.
      Applied Surface Science (2015), 349 (), 970-982CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      The limit of soly. of Ga2O3 in the cubic bixbyite In2O3 phase was established by x-ray diffraction and Raman spectroscopy to correspond to replacement of around 6% of In cations by Ga for samples prepd. at 1250°. D. functional theory calcns. suggest that Ga substitution should lead to widening of the bulk bandgap, as expected from the much larger gap of Ga2O3 as compared to In2O3. However both diffuse reflectance spectroscopy and valence band x-ray photoemission reveal an apparent narrowing of the gap with Ga doping. It is tentatively concluded that this anomaly arises from introduction of Ga+ surface lone pair states at the top of the valence band and structure at the top of the valence band in Ga-segregated samples is assigned to these lone pair states. In addn. photoemission reveals a broadening of the valence band edge. Core x-ray photoemission spectra and low energy ion scattering spectroscopy both reveal pronounced segregation of Ga to the ceramic surface, which may be linked to both relief of strain in the bulk and the preferential occupation of surface sites by lone pair cations. Surprisingly Ga segregation is not accompanied by the development of chem. shifted structure in Ga 2p core XPS assocd. with Ga+. However expts. on ion bombarded Ga2O3, where a shoulder at the top edge of the valence band spectra provide a clear signature of Ga+ at the surface, show that the chem. shift between Ga+ and Ga3+ is too small to be resolved in Ga 2p core level spectra. Thus the failure to observe chem. shifted structure assocd. with Ga+ is not inconsistent with the proposal that band gap narrowing is assocd. with lone pair states at surfaces and interfaces.
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    34. 34
      von Wenckstern, H.; Splith, D.; Purfürst, M.; Zhang, Z.; Kranert, C.; Müller, S.; Lorenz, M.; Grundmann, M. Structural and Optical Properties of (In,Ga)2O3 Thin Films and Characteristics of Schottky Contacts Thereon. Semicond. Sci. Technol. 2015, 30, 024005,  DOI: 10.1088/0268-1242/30/2/024005
      34
      Structural and optical properties of (In, Ga)2O3 thin films and characteristics of Schottky contacts thereon
      von Wenckstern, H.; Splith, D.; Purfuerst, M.; Zhang, Z.; Kranert, Ch.; Mueller, S.; Lorenz, M.; Grundmann, M.
      Semiconductor Science and Technology (2015), 30 (2), 24005/1-24005/7, 7 pp.CODEN: SSTEET; ISSN:0268-1242. (IOP Publishing Ltd.)
      We report on structural and optical properties of a (InxGa1-x)2O3 thin film having a monotonic lateral variation of the indium content x (0 ≤ x ≤ 0.9). The growth condition for each In content is similar allowing precise detn. of the dependence of material properties on x. For low In content (x < 0.15) the thin film has monoclinic crystal structure; for highest In contents (x > 0.8) the cubic bixbyite phase is predominant. For intermediate alloying we observe addnl. the rhombohedral InGaO3(II) crystallog. phase. The optical band-gap decreases systematically with increasing indium content and has a linear dependency on x for parts of the sample having the monoclinic phase, only. Further, properties of Pt Schottky diodes are reported for monoclinic (InxGa1-x)2O3 and photo response measurements for x < 0.1.
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    35. 35
      Yang, F.; Ma, J.; Luan, C.; Kong, L. Structural and Optical Properties of Ga2(1-x)In2xO3 films Prepared on α-Al2O3 (0001) by MOCVD. Appl. Surf. Sci. 2009, 255, 4401– 4404,  DOI: 10.1016/j.apsusc.2008.10.129
      35
      Structural and optical properties of Ga2(1-x)In2xO3 films prepared on α-Al2O3 (0001) by MOCVD
      Yang, Fan; Ma, Jin; Luan, Caina; Kong, Lingyi
      Applied Surface Science (2009), 255 (8), 4401-4404CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Ga2(1-x)In2xO3 thin films with different indium content x [In/(Ga + In) at. ratio] were prepd. on α-Al2O3 (0 0 0 1) substrates by the metal org. chem. vapor deposition (MOCVD). The structural and optical properties of the Ga2(1-x)In2xO3 films were investigated in detail. Microstructure anal. revealed that the film deposited with compn. x = 0.2 was polycryst. structure and the sample prepd. with x up to 0.8 exhibited single cryst. structure of In2O3. The optical band gap of the films varied with increasing Ga content from 3.72 to 4.58 eV. The av. transmittance for the films in the visible range was over 90%.
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    36. 36
      Zhang, Z.; von Wenckstern, H.; Lenzner, J.; Lorenz, M.; Grundmann, M. Visible-blind and Solar-blind Ultraviolet Photodiodes Based on (InxGa1-x)2O3. Appl. Phys. Lett. 2016, 108, 123503,  DOI: 10.1063/1.4944860
      36
      Visible-blind and solar-blind ultraviolet photodiodes based on (InxGa1-x)2O3
      Zhang, Zhipeng; von Wenckstern, Holger; Lenzner, Joerg; Lorenz, Michael; Grundmann, Marius
      Applied Physics Letters (2016), 108 (12), 123503/1-123503/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      UV and deep-UV selective photodiodes from visible-blind to solar-blind were realized based on a Si-doped (InxGa1-x)2O3 thin film with a monotonic lateral variation of 0.0035 < x < 0.83. Such layer was deposited by employing a continuous compn. spread approach relying on the ablation of a single segmented target in pulsed-laser deposition. The photo response signal is provided from a metal-semiconductor-metal structure upon backside illumination. The absorption onset was tuned from 4.83 to 3.22 eV for increasing x. Higher responsivities were obsd. for photodiodes fabricated from indium-rich part of the sample, for which an internal gain mechanism could be identified. (c) 2016 American Institute of Physics.
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    37. 37
      Michel, J.; Splith, D.; Rombach, J.; Papadogianni, A.; Berthold, T.; Krischok, S.; Grundmann, M.; Bierwagen, O.; von Wenckstern, H.; Himmerlich, M. Processing Strategies for High-Performance Schottky Contacts on n-Type Oxide Semiconductors: Insights from In2O3. ACS Appl. Mater. Interfaces 2019, 11, 27073– 27087,  DOI: 10.1021/acsami.9b06455
      37
      Processing strategies for high-performance Schottky contacts on n-type oxide semiconductors: insights from In2O3
      Michel, Jonas; Splith, Daniel; Rombach, Julius; Papadogianni, Alexandra; Berthold, Theresa; Krischok, Stefan; Grundmann, Marius; Bierwagen, Oliver; von Wenckstern, Holger; Himmerlich, Marcel
      ACS Applied Materials & Interfaces (2019), 11 (30), 27073-27087CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Prepn. of rectifying Schottky contacts on n-type oxide semiconductors, such as indium oxide (In2O3), is often challenged by the presence of a distinct surface electron accumulation layer. Here, the authors investigated the material properties and elec. transport characteristics of platinum contact/indium oxide heterojunctions to define routines for the prepn. of high-performance Schottky diodes on n-type oxide semiconductors. Combining the evaluation of different Pt deposition methods, such as electron-beam evapn. and (reactive) sputtering in an (O and) Ar atm., with oxygen plasma interface treatments, the authors identify key parameters to obtain Schottky-type contacts with high electronic barrier height and high rectification ratio. Different photoelectron spectroscopy approaches are compared to characterize the chem. properties of the contact layers and the interface region toward In2O3, to analyze charge transfer and plasma oxidn. processes as well as to evaluate the precision and limits of different methodologies to det. heterointerface energy barriers. An oxygen-plasma-induced passivation of the semiconductor surface, which induces electron depletion and generates an intrinsic interface energy barrier, is found to be not sufficient to generate rectifying platinum contacts. The dissoln. of the functional interface oxide layer within the Pt film results in an energy barrier of ∼0.5 eV, which is too low for an In2O3 electron concn. of ∼1018 cm-3. A reactive sputter process in an Ar and O atm. is required to fabricate rectifying contacts that are composed of platinum oxide (PtOx). Combining oxygen plasma interface oxidn. of the semiconductor surface with reactive sputtering of PtOx layers results in the generation of a high Schottky barrier of ∼0.9 eV and a rectification ratio of up to 106. An addnl. oxygen plasma treatment after contact deposition further reduced the reverse leakage current, likely by eliminating a surface conduction path between the coplanar Ohmic and Schottky contacts. We conclude that processes that allow us to increase the oxygen content in the interface and contact region are essential for fabrication of device-quality-rectifying contacts on various oxide semiconductors.
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    38. 38
      Rombach, J.; Papadogianni, A.; Mischo, M.; Cimalla, V.; Kirste, L.; Ambacher, O.; Berthold, T.; Krischok, S.; Himmerlich, M.; Selve, S.; Bierwagen, O. The Role of Surface Electron Accumulation and Bulk Doping for Gas-Sensing Explored with Single-Crystalline In2O3 Thin Films. Sens. Actuators, B 2016, 236, 909– 916,  DOI: 10.1016/j.snb.2016.03.079
      38
      The role of surface electron accumulation and bulk doping for gas-sensing explored with single-crystalline In2O3 thin films
      Rombach, Julius; Papadogianni, Alexandra; Mischo, Markus; Cimalla, Volker; Kirste, Lutz; Ambacher, Oliver; Berthold, Theresa; Krischok, Stefan; Himmerlich, Marcel; Selve, Soeren; Bierwagen, Oliver
      Sensors and Actuators, B: Chemical (2016), 236 (), 909-916CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      Single cryst. and textured In2O3 thin films with (1 1 1) surface orientation, grown by plasma-assisted mol. beam epitaxy, were used as a model system to study the role of bulk and surface electron accumulation layer conductance for ozone sensing. Both conductance contributions, which add to the total film conductance, were systematically varied. The resulting ozone sensitivity was detd. by total conductance measurements in synthetic air with defined ozone concn. using UV irradn. instead of heating to regenerate the In2O3 surface. Depletion of the surface electron accumulation by an oxygen plasma treatment, confirmed by XPS, rendered the films ozone insensitive. The ozone response of films with an accumulation layer was increased by thickness redn. or by designing the bulk of the film semi-insulating using deep acceptor doping by Mg. Our results of using electron accumulation layers for gas sensing and bulk doping by deep acceptors to increase sensitivity can be generalized to other gas sensing materials. The use of single cryst. films allows selecting the most sensitive crystallog. surface orientation and may have further advantages over polycryst. films, such as increased stability and sensing speed.
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    39. 39
      Veal, T. D.; Jefferson, P. H.; Piper, L. F. J.; McConville, C. F.; Joyce, T. B.; Chalker, P. R.; Considine, L.; Lu, H.; Schaff, W. J. Transition from Electron Accumulation to Depletion at InGaN Surfaces. Appl. Phys. Lett. 2006, 89, 202110,  DOI: 10.1063/1.2387976
      39
      Transition from electron accumulation to depletion at InGaN surfaces
      Veal, T. D.; Jefferson, P. H.; Piper, L. F. J.; McConville, C. F.; Joyce, T. B.; Chalker, P. R.; Considine, L.; Lu, Hai; Schaff, W. J.
      Applied Physics Letters (2006), 89 (20), 202110/1-202110/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The compn. dependence of the Fermi-level pinning at the oxidized (0001) surfaces of n-type InxGa1-xN films (0 ≤ x ≤ 1) was studied using x-ray photoemission spectroscopy. The surface Fermi-level position varies from high above the conduction band min. (CBM) at InN surfaces to significantly below the CBM at GaN surfaces, with the transition from electron accumulation to depletion occurring at approx. x = 0.3. The results are consistent with the compn. dependence of the band edges with respect to the charge neutrality level.
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    40. 40
      Kajiyama, K.; Mizushima, Y.; Sakata, S. Schottky Barrier Height of n-InxGa1-xAs Diodes. Appl. Phys. Lett. 1973, 23, 458,  DOI: 10.1063/1.1654957
      40
      Schottky barrier height of n-indium gallium arsenide (n-InxGa1-xAs) diodes
      Kajiyama, K.; Mizushima, Y.; Sakata, S.
      Applied Physics Letters (1973), 23 (8), 458-9CODEN: APPLAB; ISSN:0003-6951.
      The barrier heights, ΦB, of Au/n-InxGa1-xAs diodes were measured by the capacitance-voltage and satn. current methods. The compn. dependence of the barrier height is ΦB (eV) = 0.95 - 1.90x + 0.90x2. A low barrier height with a relatively wide band gap is obtained in this system.
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    41. 41
      Lüth, H. Research on III-V Semiconductor Interfaces: Its Impact on Technology and Devices. Physica Status Solidi (a) 2001, 187, 33– 44,  DOI: 10.1002/1521-396X(200109)187:1<33::AID-PSSA33>3.0.CO;2-9
      There is no corresponding record for this reference.
    42. 42
      Wei, S.-H.; Zunger, A. Role of Metal d States in II-VI Semiconductors. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 8958,  DOI: 10.1103/PhysRevB.37.8958
      42
      Role of metal d states in II-VI semiconductors
      Wei, S. H.; Zunger, Alex
      Physical Review B: Condensed Matter and Materials Physics (1988), 37 (15), 8958-81CODEN: PRBMDO; ISSN:0163-1829.
      All-electron band-structure calcns. and photoemission expts. on II-VI semiconductors both exhibit a metal d subband inside the main valence band. It has nevertheless been customary in pseudopotential and tight-binding approaches to neglect the metal d band by choosing Hamiltonian parameters which place this band inside the chem. inert at. cores. By using all-electron self-consistent electronic-structure techniques (which treat the outermost d electrons in the same way as other valence electrons) and by comparing the results to those obtained by methods which remove the d band from the valence spectrum, the effects on valence properties were studied. For II-VI semiconductors p-d repulsion and hybridization (1) lower the band gaps, (2) reduce the cohesive energy, (3) increase the equil. lattice parameters, (4) reduce the spin-orbit splitting, (5) alter the sign of the crystal-field splitting, (6) increase the valence-band offset between common-anion II-VI semiconductors, and (7) modify the charge distributions of various II-VI systems and their alloys. P-d repulsion is also responsible for the occurrence of deep Cu acceptor levels in II-VI semiconductors (compared with shallow acceptors of Zn in III-V), for the anomalously small band gaps in chalcopyrites, and for the neg. exchange splitting in ferromagnetic MnTe.
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    43. 43
      Varley, J. B.; Weber, J. R.; Janotti, A.; Van de Walle, C. G. Oxygen Vacancies and Donor Impurities in β-Ga2O3. Appl. Phys. Lett. 2010, 97, 142106,  DOI: 10.1063/1.3499306
      43
      Oxygen vacancies and donor impurities in β-Ga2O3
      Varley, J. B.; Weber, J. R.; Janotti, A.; Van de Walle, C. G.
      Applied Physics Letters (2010), 97 (14), 142106/1-142106/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Using hybrid functionals we have investigated the role of O vacancies and various impurities in the elec. and optical properties of the transparent conducting oxide β-Ga2O3. We find that O vacancies are deep donors, and thus cannot explain the unintentional n-type cond. Instead, we attribute the cond. to common background impurities such as Si and H. Monoat. H has low formation energies and acts as a shallow donor in both interstitial and substitutional configurations. We also explore other dopants, where substitutional forms of Si, Ge, Sn, F, and Cl are shown to behave as shallow donors. (c) 2010 American Institute of Physics.
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    44. 44
      Karazhanov, S. Z.; Ravindran, P.; Vajeeston, P.; Ulyashin, A.; Finstad, T. G.; Fjellvag, H. Phase Stability, Electronic Structure, and Optical Properties of Indium Oxide Polytypes. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 075129,  DOI: 10.1103/PhysRevB.76.075129
      44
      Phase stability, electronic structure, and optical properties of indium oxide polytypes
      Karazhanov, S. Zh.; Ravindran, P.; Vajeeston, P.; Ulyashin, A.; Finstad, T. G.; Fjellvag, H.
      Physical Review B: Condensed Matter and Materials Physics (2007), 76 (7), 075129/1-075129/13CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Structural phase stability, electronic structure, optical properties, and high-pressure behavior of polytypes of In2O3 in three space group symmetries I213, Ia3, and R3 were studied by 1st-principles d.-functional calcns. From structural optimization based on total energy calcns., lattice and positional parameters were established, which are in good agreement with the corresponding exptl. data except for I213, where the symmetry anal. for optimized structure indicates that it arrived at the Ia3 phase. In2O3 of space group symmetry Ia3 is found to undergo a pressure-induced phase transition to the R3 phase at pressures around 3.8 GPa. From the anal. of band structure coming out from the calcns. within the local d. and generalized gradient approxns., In2O3 of space group symmetry I213 and R3 are indirect band gap semiconductors, while the other phase of space group Ia3 is having direct band gap. The calcd. carrier effective masses for all these three phases are compared with available exptl. and theor. values. From charge-d. and electron localization function anal., these phases have dominant ionic bonding with noticeable covalent interaction between In and O. The magnitudes of the absorption and reflection coeffs. for In2O3 with space groups Ia3 and R3 are small in the energy range 0-5 eV, indicating that these phases can be regarded and classified as transparent.
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    45. 45
      Mryasov, O. N.; Freeman, A. J. Electronic Band Structure of Indium Tin Oxide and Criteria for Transparent Conducting Behavior. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 64, 233111,  DOI: 10.1103/PhysRevB.64.233111
      45
      Electronic band structure of indium tin oxide and criteria for transparent conducting behavior
      Mryasov, O. N.; Freeman, A. J.
      Physical Review B: Condensed Matter and Materials Physics (2001), 64 (23), 233111/1-233111/3CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      Indium-based transparent conductors, notably indium tin oxide (ITO), have a wide range of applications due to a unique combination of visible light transparency and modest cond. A fundamental understanding of such an unusual combination of properties is strongly motivated by the great demand for materials with improved transparent conducting properties. Here we formulate conditions for transparent conducting behavior on the basis of the local d. full-potential linear muffin-tin orbital electronic band structure calcns. for Sn-doped In2O3 and available exptl. data. We conclude that the position, dispersion, and character of the lowest conduction band are the key characteristics of the band structure responsible for its electro-optical properties. Further, we find that this lowest band is split with Sn doping due to the strong hybridization with dopant s-type states and this splitting contributes to both the decrease of the plasma frequency and the mobility of the carriers.
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    46. 46
      Furthmüller, J.; Bechstedt, F. Quasiparticle Bands and Spectra of Ga2O3 Polymorphs. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 115204,  DOI: 10.1103/PhysRevB.93.115204
      There is no corresponding record for this reference.
    47. 47
      Hajnal, Z.; Miró, J.; Kiss, G.; Réti, F.; Deák, P.; Herndon, R. C.; Kuperberg, J. M. Role of Oxygen Vacancy Defect States in the n-type Conduction of β-Ga2O3. J. Appl. Phys. 1999, 86, 3792,  DOI: 10.1063/1.371289
      47
      Role of oxygen vacancy defect states in the n-type conduction of β-Ga2O3
      Hajnal, Zoltan; Miro, Jozsef; Kiss, Gabor; Reti, Ferenc; Deak, Peter; Herndon, Roy C.; Kuperberg, J. Michael
      Journal of Applied Physics (1999), 86 (7), 3792-3796CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Based on semiempirical quantum-chem. calcns., the electronic band structure of β-Ga2O3 is presented and the formation and properties of oxygen vacancies are analyzed. The equil. geometries and formation energies of neutral and doubly ionized vacancies were calcd. Using the calcd. donor level positions of the vacancies, the high temp. n-type conduction is explained. The vacancy concn. is obtained by fitting to the exptl. resistivity and electron mobility.
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    48. 48
      Swallow, J. E. N. Influence of Polymorphism on the Electronic Structure of Ga2O3. Chem. Mater. 2020, 32, 8460– 8470,  DOI: 10.1021/acs.chemmater.0c02465
      48
      Influence of Polymorphism on the Electronic Structure of Ga2O3
      Swallow, Jack E. N.; Vorwerk, Christian; Mazzolini, Piero; Vogt, Patrick; Bierwagen, Oliver; Karg, Alexander; Eickhoff, Martin; Schormann, Jorg; Wagner, Markus R.; Roberts, Joseph W.; Chalker, Paul R.; Smiles, Matthew J.; Murgatroyd, Philip; Razek, Sara A.; Lebens-Higgins, Zachary W.; Piper, Louis F. J.; Jones, Leanne A. H.; Thakur, Pardeep K.; Lee, Tien-Lin; Varley, Joel B.; Furthmuller, Jurgen; Draxl, Claudia; Veal, Tim D.; Regoutz, Anna
      Chemistry of Materials (2020), 32 (19), 8460-8470CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The search for new wide-band-gap materials is intensifying to satisfy the need for more advanced and energy-efficient power electronic devices. Ga2O3 has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main β-phase. Here, three polymorphs of Ga2O3, α, β, and ε, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theor. approaches to gain insights into their structure-electronic structure relationships. Valence and conduction electronic structure as well as semicore and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron d. functional theory and many-body perturbation theory, provides detailed understanding of the spectroscopic results. The calcd. spectra provide very accurate descriptions of all exptl. spectra and addnl. illuminate the origin of obsd. spectral features. This work provides a strong basis for the exploration of the Ga2O3 polymorphs as materials at the heart of future electronic device generations.
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    49. 49
      Scofield, J. Theoretical photoionization cross sections from 1 to 1500 keV, 1973.
      There is no corresponding record for this reference.
    50. 50
      Persson, C.; Zunger, A. s-d Coupling in Zinc-blende Semiconductors. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 073205,  DOI: 10.1103/PhysRevB.68.073205
      50
      s-d coupling in zinc-blende semiconductors
      Persson, Clas; Zunger, Alex
      Physical Review B: Condensed Matter and Materials Physics (2003), 68 (7), 073205/1-073205/4CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      Most zinc blende semiconductors have a single anion-like s state near the bottom of the valence band, found in d.-of-states (DOS) calcns., and seen in photoemission. Here, we discuss the case where two s-like peaks appear, due to strong s-d coupling. Indeed, away from the k = 0 Brillouin zone center, cation d states and anion s states can couple in zinc blende symmetry. Depending on the energy difference ΔEsd = Esanion - Edcation, this interaction can lead to either a single or two s-like peaks in the DOS and photoemission. We find four types of behaviors: (i) in GaP, GaAs, InP, and InAs, ΔEsd is large, giving rise to a single cation d peak well below the single anion s peak; (ii) similarly, in CdS, CdSe, ZnS, ZnSe, and ZnTe, we see also a single s peak, but now the cation d is above the anion s. In both (i) and (ii) the s-d coupling is very weak. For (iii) in GaN and InN, the local d. approxn. (LDA) predicts two s-like peaks bracketing below and above the cation d-like state. Correcting the too low binding energies of LDA by LDA + SIC (self-interaction correction) still leaves the two s-like peaks. The occurrence of two s-like peaks represents the fingerprint of strong s-d coupling. And (iv) in CdTe, LDA predicts a single s-like peak just as in case (ii) above. However, LDA + SIC correction shifts down the cation d state closer to the anion s band, enhancing the s-d coupling, and leading to the appearance of two s-like peaks. Case (iv) is a remarkable situation where LDA errors cause not only quant. energetic errors, but actually leads to a qual. effect of a DOS peak that exists in LDA + SIC but is missing in LDA. We predict that the double-s peak should be obsd. in photoemission for GaN, InN, and CdTe.
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    51. 51
      Fuchs, F.; Bechstedt, F. Indium-oxide Polymorphs from First Principles: Quasiparticle Electronic States. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 155107,  DOI: 10.1103/PhysRevB.77.155107
      51
      Indium-oxide polymorphs from first principles: Quasiparticle electronic states
      Fuchs, F.; Bechstedt, F.
      Physical Review B: Condensed Matter and Materials Physics (2008), 77 (15), 155107/1-155107/10CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The electronic structure of In2O3 polymorphs is calcd. from first principles using d. functional theory (DFT) and many-body perturbation theory (MBPT). DFT calcns. with a local exchange-correlation (XC) functional give the relaxed at. coordinates of the two stable polymorphs. Their electronic structure, i.e., the band structure and d. of states, is studied within MBPT. The quasiparticle equation is solved in two steps. As the zeroth approxn. for the XC self-energy the nonlocal potential resulting from a HSE03 hybrid functional is used. In the sense of a self-consistent procedure G0W0 quasiparticle corrections are computed on top. The calcd. direct quasiparticle gaps at Γ amt. to 3.3 eV (rhombohedral) and 3.1 eV (cubic). The rhombohedral polymorph is found to exhibit a near degeneracy of the valence-band maxima at the Γ point and on the Γ-L line, while the valence-band max. of the cubic polymorph occurs near Γ. Interconduction band transitions are identified as possible origin of conflicting exptl. reports, claiming a much larger difference between the direct and indirect gap. The results for gaps, d-band positions, and d. of states are compared with available exptl. data.
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    52. 52
      Fuchs, F.; Furthmüller, J.; Bechstedt, F.; Shishkin, M.; Kresse, G. Quasiparticle Band Structure based on a Generalized Kohn-Sham Scheme. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 115109,  DOI: 10.1103/PhysRevB.76.115109
      52
      Quasiparticle band structure based on a generalized Kohn-Sham scheme
      Fuchs, F.; Furthmuller, J.; Bechstedt, F.; Shishkin, M.; Kresse, G.
      Physical Review B: Condensed Matter and Materials Physics (2007), 76 (11), 115109/1-115109/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We present a comparative full-potential study of generalized Kohn-Sham (gKS) schemes with explicit focus on their suitability as starting point for the soln. of the quasiparticle equation. We compare G0W0 quasiparticle band structures calcd. upon local-d. approxn. (LDA), screened-exchange, HSE03, PBE0, and Hartree-Fock functionals for exchange and correlation (XC) for Si, InN, and ZnO. Furthermore, the HSE03 functional is studied and compared to the generalized gradient approxn. (GGA) for 15 nonmetallic materials for its use as a starting point in the calcn. of quasiparticle excitation energies. For this case, the effects of self-consistency in the GW self-energy are also analyzed. It is shown that the use of a gKS scheme as a starting point for a perturbative quasiparticle correction can improve upon the deficiencies found for LDA or GGA starting points for compds. with shallow d bands. For these solids, the order of the valence and conduction bands is often inverted using local or semilocal approxns. for XC, which makes perturbative G0W0 calcns. unreliable. The use of a gKS starting point allows for the calcn. of fairly accurate band gaps even in these difficult cases, and generally single-shot G0W0 calcns. following calcns. using the HSE03 functional are very close to expt.
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    53. 53
      Ley, L.; Pollak, R. A.; McFeely, F. R.; Kowalczyk, S. P.; Shirley, D. A. Total Valence-band Densities of States of III-V and II-VI Compounds from X-ray Photoemission Spectroscopy. Phys. Rev. B 1974, 9, 600– 621,  DOI: 10.1103/PhysRevB.9.600
      53
      Total valence-band densities of states of [Groups] III-V and II-VI compounds from x-ray photoemission spectroscopy
      Ley, L.; Pollak, R. A.; McFeely, F. R.; Kowalczyk, S. P.; Shirley, D. A.
      Physical Review B: Solid State (1974), 9 (2), 600-21CODEN: PLRBAQ; ISSN:0556-2805.
      A comprehensive survey of the total valence-band x-ray-photoemission spectra of 14 semiconductors is reported. The x-ray photoelectron spectra of cubic GaP, GaAs, GaSb, InP, InAs, InSb, ZnS, ZnSe, ZnTe, CdTe, and HgTe, and of hexagonal ZnO, CdS, and CdSe were obtained from freshly cleaved single crystals, in the 0-50-eV binding-energy range, by using monochromatized Al Kα (1486.6 eV) radiation. The binding energies of the outermost d shells are reported. They were detd. relative both to the top of the valence bands (EBV) and to the Fermi level of a thin layer of Au that was vapor deposited after each run (EBF). These data also yielded accurate measures of sample charging. The Fermi level fell near the center of the gap for 6 samples, near the top for 2, and near the bottom for 3. Evidence for an apparent increase in core d-level spin-orbit splitting over free-atom values was interpreted as a possible spreading of a Γ7 and a Γ8 level from the upper (d3/2) Γ8 level by a tetrahedral crystal field. The s, p valence-band spectra showed 3 main peaks, with considerable structure on the least-bound peak. A discussion is given of the validity of comparing the valence-band (VB) spectrum I'(E) with VB d. of states, including cross-section modulation, final-state modulation, and relaxation effects. Characteristic binding energies of spectra features in I'(E) are tabulated. In addn., the energies of the characteristic symmetry points L3, X5, W2, Σ1min., W1, X3(L1), X1, L1, and Γ1 are given for the 11 cubic compds. These are compared with uv photoemission spectroscopy results where available and with theor. band-structure results where available. The energies calcd. by using the relativistic-orthogonalized-plane-wave approach with Xαβ exchange agree very well with expt., on the whole. In particular, they predict the important ionicity gap X3-X1 quite accurately. The ds. of states calcd. by using the empirical-pseudopotential method provided a useful basis for relating features in I'(E) to energies of the characteristic symmetry points. Band-structure calcns. in combination with x-ray-photoemission spectra appears to provide a very powerful approach to establishing the total valence-band structure of semiconductors.
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    54. 54
      Erhart, P.; Klein, A.; Egdell, R. G.; Albe, K. Band Structure of Indium Oxide: Indirect Versus Direct Band Gap. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 153205,  DOI: 10.1103/PhysRevB.75.153205
      54
      Band structure of indium oxide: Indirect versus direct band gap
      Erhart, Paul; Klein, Andreas; Egdell, Russell G.; Albe, Karsten
      Physical Review B: Condensed Matter and Materials Physics (2007), 75 (15), 153205/1-153205/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The nature of the band gap of In oxide is still a matter of debate. Based on optical measurements the presence of an indirect band gap was suggested, which is 0.9 to 1.1 eV smaller than the direct band gap at the Γ point. This could be caused by strong mixing of O 2p and In 4d orbitals off Γ. The authors have performed extensive d. functional theory calcns. using the LDA+U and the GGA+U methods to elucidate the contribution of the In 4d states and the effect of spin-orbit coupling on the valence band structure. Although an indirect band gap is obtained, the energy difference between the overall valence band max. and the highest occupied level at the Γ point is <50 meV. The exptl. observation cannot be related to the electronic structure of the defect free bulk material.
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    55. 55
      Wei, S.-H.; Nie, X.; Batyrev, I. G.; Zhang, S. B. Breakdown of the Band-gap-common-cation Rule: The Origin of the Small Band Gap of InN. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 165209,  DOI: 10.1103/PhysRevB.67.165209
      55
      Breakdown of the band-gap-common-cation rule: the origin of the small band gap of InN
      Wei, Su-Huai; Nie, Xiliang; Batyrev, Iskander G.; Zhang, S. B.
      Physical Review B: Condensed Matter and Materials Physics (2003), 67 (16), 165209/1-165209/4CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      It is well accepted that the band gap of a semiconductor compd. increases as the at. no. decreases. However, recent measurements of the small band gap of InN (Eg ∼ 0.9 eV) suggest that this rule may not hold for the common-cation In compds. Using a band-structure method that includes band-gap correction, we systematically study the chem. trends of the band-gap variation in III-V semiconductors. The calcd. InN band gap is 0.85 ± 0.1 eV, much smaller than previous exptl. value of ∼1.9 eV. The InN band-gap anomaly is explained in terms of at.-orbital energies and the band-gap deformation potentials.
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    56. 56
      Wei, S.-H.; Zunger, A. Predicted Band-gap Pressure Coefficients of all Diamond and Zinc-blende Semiconductors: Chemical Trends. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60, 5404,  DOI: 10.1103/PhysRevB.60.5404
      56
      Predicted band-gap pressure coefficients of all diamond and zinc-blende semiconductors: Chemical trends
      Wei, Su-Huai; Zunger, Alex
      Physical Review B: Condensed Matter and Materials Physics (1999), 60 (8), 5404-5411CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      We have studied systematically the chem. trends of the band-gap pressure coeffs. of all Group IVA, IIIA-VA, and IIB-VIA semiconductors using first-principles band-structure method. We have also calcd. the individual "abs." deformation potentials of the valence-band max. (VBM) and conduction-band min. (CBM). We find that (1) the vol. deformation potentials of the Γ6c CBM are usually large and always neg., while (2) the vol. deformation potentials of the Γ8v VBM state are usually small and neg. for compds. contg. occupied valence d state but pos. for compds. without occupied valence d orbitals. Regarding the chem. trends of the band-gap pressure coeffs., we find that (3) apΓ-Γ decreases as the ionicity increases (e.g., from Ge → GaAs → ZnSe), (4) apΓ-Γ increases significantly as anion at. no. increases (e.g., from GaN → GaP → GaAs → GaSb), (5) apΓ-Γ decreases slightly as cation at. no. increases (e.g., from AlAs → GaAs → InAs), (6) the variation of apΓ-L are relatively small and follow similar trends as apΓ-Γ, and (7) the magnitude of apΓ-X are small and usually neg., but are sometimes slightly pos. for compds. contg. first-row elements. Our calcd. chem. trends are explained in terms of the energy levels of the at. valence orbitals and coupling between these orbital. In light of the above, we suggest that "empirical rule" of the pressure coeffs. should be modified.
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    57. 57
      Li, Y.-H.; Gong, X. G.; Wei, S.-H. Ab Initio All-Electron Calculation of Absolute Volume Deformation Potentials of IV-IV, III-V, and II-VI Semiconductors: The Chemical Trends. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 245206,  DOI: 10.1103/PhysRevB.73.245206
      57
      Ab initio all-electron calculation of absolute volume deformation potentials of IV-IV, III-V, and II-VI semiconductors: The chemical trends
      Li, Yong-Hua; Gong, X. G.; Wei, Su-Huai
      Physical Review B: Condensed Matter and Materials Physics (2006), 73 (24), 245206/1-245206/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We calc. systematically the abs. vol. deformation potential (AVDP) of the Γ8v valence band max. (VBM) and the Γ6c conduction band min. (CBM) states for all group IV, III-V, and II-VI semiconductors. Unlike previous calcns. that involve various assumptions, the AVDPs are calcd. using a recently developed approach that is independent of the selection of the ref. energy levels. We find that although the vol. deformation potentials of the CBM state are usually large and always neg., those of the VBM state are usually small and always pos. The AVDP of the VBM state decreases as the p-d coupling increases, e.g., in the II-VI compds. The AVDP of CBM decreases as the ionicity increases. Our calcd. chem. trends of the AVDPs are explained in terms of the AO energy levels and coupling between these orbitals.
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    58. 58
      Mann, J. B.; Meek, T. L.; Allen, L. C. Configuration Energies of the Main Group Elements. J. Am. Chem. Soc. 2000, 122, 2780– 2783,  DOI: 10.1021/ja992866e
      58
      Configuration Energies of the Main Group Elements
      Mann, Joseph B.; Meek, Terry L.; Allen, Leland C.
      Journal of the American Chemical Society (2000), 122 (12), 2780-2783CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Configuration energies (CEs), formerly called spectroscopic electronegativities, are an attempt to quantum mech. define, and extend, the important chem. concept of electronegativity. In a previous paper (J. Am. Chem. Soc. 1989, 111, 9003) we reported high-resoln. exptl. values obtained from the National Institutes of Science and Technol. spectroscopic energy level tables using the formula CE = (nεs + mεp)/(n + m); n and m are the no. of s and p electrons and εs and εp are their multiplet-averaged one-electron energies, for the 34 s and p-block atoms H→Xe. This CE definition is a direct extension of N. Bohr's introduction of electron configurations to quantum mech. rationalize the periodic table (hence its designation as configuration energy). Here we give exptl. nos. for the remaining 8 sixth row representative atoms plus Zn, Cd, and Hg. In addn., we have carried out high accuracy numerical Dirac-Hartree-Fock solns. for all 45 atoms. Results from these calcns. closely parallel the exptl. values and enable us to est. some of the at. multiplet levels for which no exptl. data exist. CE leads to nos. which are interpretable as an "electron attracting power" in the same manner as the traditional scales of Pauling and Allred & Rochow. They are also strongly correlated with at. energy level spacings, therefore providing an addnl. interpretability compatible with energy level data and the MO diagrams that dominate much of contemporary chem. Likewise, CEs are able to rationalize the origin of the metalloid band (diagonal line sepg. metals from nonmetals) in the periodic table and the new detn. of sixth row CEs permit designation of bismuth and polonium as metalloids, clarifying their previous uncertain classification between metal and metalloid.
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    59. 59
      Fischer, C. F. Average-energy-of-configuration Hartree-Fock Results for the Atoms Helium to Radon. At. Data Nucl. Data Tables 1973, 12, 87– 99,  DOI: 10.1016/0092-640X(73)90014-4
      59
      Average-energy-of-configuration Hartree-Fock results for the atoms helium to radon
      Fischer, Charlotte Froese
      Atomic Data and Nuclear Data Tables (1973), 12 (1), 87-99CODEN: ADNDAT; ISSN:0092-640X.
      Table I of the paper [Atomic Data, 1972, 4, 301] is actually for the lowest term of the configuration rather than for the av.-energy-of-the-configuration, as the paper implies. For configurations contg. 4f electrons the results of that paper correspond to an incorrect energy expression. When the energy of the lowest term is also the av. energy as for complete groups or complete groups plus (or minus) one electron, all atomic parameters are correct except for the total energy which is too high by the amt. q(q - 1) [2/95 - 2/195]F2(4f,4f), where q is the no. of 4f electrons. In all other cases, the radial functions were detd. from an energy expression too low by the same amt. The at. properties are reasonably accurate but the energy is too low. Table I for the av.-energy-of-the-configuration is given.
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    60. 60
      Herman, F.; Skillman, S. Atomic Structure Calculations; Prentice-Hall: Englewood Cliffs, NJ, 1963.
      There is no corresponding record for this reference.
    61. 61
      Vurgaftman, I.; Meyer, J. R.; Ram-Mohan, L. R. Band Parameters for III-V Compound Semiconductors and their Alloys. J. Appl. Phys. 2001, 89, 5815– 5875,  DOI: 10.1063/1.1368156
      61
      Band parameters for III-V compound semiconductors and their alloys
      Vurgaftman, I.; Meyer, J. R.; Ram-Mohan, L. R.
      Journal of Applied Physics (2001), 89 (11, Pt. 1), 5815-5875CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      A review with 1001 refs. We present a comprehensive, up-to-date compilation of band parameters for the technol. important III-V zinc blende and wurtzite compd. semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calcns., we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temp. and alloy-compn. dependences where available. Heterostructure band offsets are also given, on an abs. scale that allows any material to be aligned relative to any other.
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    62. 62
      Wu, J.; Walukiewicz, W.; Shan, W.; Yu, K. M.; Ager, J. W.; Li, S. X.; Haller, E. E.; Lu, H.; Schaff, W. J. Temperature Dependence of the Fundamental Band Gap of InN. J. Appl. Phys. 2003, 94, 4457– 4460,  DOI: 10.1063/1.1605815
      62
      Temperature dependence of the fundamental band gap of InN
      Wu, J.; Walukiewicz, W.; Shan, W.; Yu, K. M.; Ager, J. W.; Li, S. X.; Haller, E. E.; Lu, Hai; Schaff, William J.
      Journal of Applied Physics (2003), 94 (7), 4457-4460CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      The fundamental band gap of InN films grown by MBE were measured by transmission and photoluminescence spectroscopy as a function of temp. The band edge absorption energy and its temp. dependence depend on the doping level. The band gap variation and Varshni parameters of InN are compared with other Group III nitrides. The energy of the photoluminescence peak is affected by the emission from localized states and cannot be used to det. the band gap energy. Based on the results obtained on two samples with distinctly different electron concns., the effect of degenerate doping on the optical properties of InN is discussed.
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    63. 63
      Tippins, H. H. Optical Absorption and Photoconductivity in the Band Edge of β-Ga2O3. Phys. Rev. 1965, 140, A316– A319,  DOI: 10.1103/PhysRev.140.A316
      63
      Optical absorption and photoconductivity in the band edge of β-Ga2O3
      Tippins, H. H.
      Physical Review (1965), 140 (1A), 316-19CODEN: PHRVAO; ISSN:0031-899X.
      Optical absorption and photocond. were observed in the uv in single crystals of nominally pure β-Ga2O3. At room temp. a steep absorption edge, characteristic of a band-to-band transition, is observed at 2700 A. The edge is shifted approx. 100 A. toward shorter wavelengths when the temp. is reduced to 77°K. Photocond. begins coincident with the absorption edge at 77°K., but could not be detected at room temp. A model is proposed in which the absorption arises as a result of excitation of an electron from the O 2p band to the Ga 4s band. Calcns. using this model and the Born-Haber cycle are in good agreement with the observed band gap of 4.7 ev. The much smaller band gap of β-Ga2O3 as compared with sapphire is due to the reduced coordination no. of the ions involved in the transition.
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    64. 64
      Varley, J. B.; Schleife, A. Bethe-Salpeter Calculation of Optical-absorption Spectra of In2O3 and Ga2O3. Semicond. Sci. Technol. 2015, 30, 024010,  DOI: 10.1088/0268-1242/30/2/024010
      64
      Bethe-Salpeter calculation of optical-absorption spectra of In2O3 and Ga2O3
      Varley, Joel B.; Schleife, Andre
      Semiconductor Science and Technology (2015), 30 (2), 24010/1-24010/5, 5 pp.CODEN: SSTEET; ISSN:0268-1242. (IOP Publishing Ltd.)
      Transparent conducting oxides keep attracting strong scientific interest not only due to their promising potential for 'transparent electronics' applications but also due to their intriguing optical absorption characteristics. Materials such as In2O3 and Ga2O3 have complicated unit cells and, consequently, are interesting systems for studying the physics of excitons and anisotropy of optical absorption. Since currently no exptl. data is available, for instance, for their dielec. functions across a large photon-energy range, we employ modern first-principles computational approaches based on many-body perturbation theory to provide theor.-spectroscopy results. Using the Bethe-Salpeter framework, we compute dielec. functions and we compare to spectra computed without excitonic effects. We find that the electron-hole interaction strongly modifies the spectra and we discuss the anisotropy of optical absorption that we find for Ga2O3 in relation to existing theor. and exptl. data.
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    65. 65
      Wei, S.-H.; Zunger, A. Calculated Natural Band Offsets of all II-VI and III-V Semiconductors: Chemical Trends and the Role of Cation d Orbitals. Appl. Phys. Lett. 1998, 72, 2011,  DOI: 10.1063/1.121249
      65
      Calculated natural band offsets of all II-VI and III-V semiconductors: Chemical trends and the role of cation d orbitals
      Wei, Su-Huai; Zunger, Alex
      Applied Physics Letters (1998), 72 (16), 2011-2013CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Using first-principles all-electron band structure method, we have systematically calcd. the natural band offsets ΔEv between all II-VI and sep. between III-V semiconductor compds. Fundamental regularities are uncovered: for common-cation systems ΔEv decreases when the cation at. no. increases, while for common-anion systems ΔEv decreases when the anion at. no. increases. We find that coupling between anion p and cation d states plays a decisive role in detg. the abs. position of the valence band max. and thus the obsd. chem. trends.
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    66. 66
      Wei, S.-H.; Zunger, A. Role of d Orbitals in Valence-Band Offsets of Common-Anion Semiconductors. Phys. Rev. Lett. 1987, 59, 144,  DOI: 10.1103/PhysRevLett.59.144
      66
      Role of d orbitals in valence-band offsets of common-anion semiconductors
      Wei, Su Huai; Zunger, Alex
      Physical Review Letters (1987), 59 (1), 144-7CODEN: PRLTAO; ISSN:0031-9007.
      All-electron first-principles electronic structure calcns. of core levels were made on common-anion semiconductors AlAs-GaAs and CdTe-HgTe and contrary to previous expectations, the valence-band offsets are decided primarily by intrinsic bulk effects and the interface charge transfer has but a small effect on these quantities. The failure of previous models results primarily from the omission of cation d orbitals.
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    67. 67
      Van de Walle, C. G.; Martin, R. M. Absolute Deformation Potentials: Formulation and Ab initio Calculations for Semiconductors. Phys. Rev. Lett. 1989, 62, 2028– 2031,  DOI: 10.1103/PhysRevLett.62.2028
      67
      "Absolute" deformation potentials: formulation and ab initio calculations for semiconductors
      Van de Walle, Chris G.; Martin, Richard M.
      Physical Review Letters (1989), 62 (17), 2028-31CODEN: PRLTAO; ISSN:0031-9007.
      The subjects of this paper are the proper inclusion of long-range electrostatic terms in the theory of electronic deformation potentials, a way to include these terms by using supercells in ab initio d.-functional methods, and calcns. for selected semiconductors. The connection with the heterojunction problem is described. The results are compared with previous model theories and with expt.
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    68. 68
      Cardona, M.; Christensen, N. E. Acoustic Deformation Potentials and Heterostructure Band Offsets in Semiconductors. Phys. Rev. B: Condens. Matter Mater. Phys. 1987, 35, 6182– 6194,  DOI: 10.1103/PhysRevB.35.6182
      68
      Acoustic deformation potentials and heterostructure band offsets in semiconductors
      Cardona, Manuel; Christensen, Niels E.
      Physical Review B: Condensed Matter and Materials Physics (1987), 35 (12), 6182-94CODEN: PRBMDO; ISSN:0163-1829.
      The abs. hydrostatic deformation potentials calcd. by J. A. Verges, et al., (1982) for tetrahedral semiconductors with the linear muffin-tin-orbital method must be screened by the dielec. response of the material before using them to calc. electron-phonon interaction. This screening can be estd. by using the midpoint of an av. dielec. gap evaluated at special (Baldereschi) points of the band structure. This dielec. midgap energy (DME) was related to the charge-neutrality point (Tersoff, J., 1984) to evaluate band offsets in heterojunctions and Schottky-barrier heights. Band offsets obtained with this method for several heterojunctions are tabulated, and compared with existing exptl. results and other theor. calcns. The DME's are tabulated, and compared with those based on charge-neutrality points.
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    69. 69
      Li, Y.-H.; Gong, X. G.; Wei, S.-H. Ab initio All-electron Calculation of Absolute Volume Deformation Potentials of IV-IV, III-V, and II-VI Semiconductors: The Chemical Trends. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 245206,  DOI: 10.1103/PhysRevB.73.245206
      69
      Ab initio all-electron calculation of absolute volume deformation potentials of IV-IV, III-V, and II-VI semiconductors: The chemical trends
      Li, Yong-Hua; Gong, X. G.; Wei, Su-Huai
      Physical Review B: Condensed Matter and Materials Physics (2006), 73 (24), 245206/1-245206/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We calc. systematically the abs. vol. deformation potential (AVDP) of the Γ8v valence band max. (VBM) and the Γ6c conduction band min. (CBM) states for all group IV, III-V, and II-VI semiconductors. Unlike previous calcns. that involve various assumptions, the AVDPs are calcd. using a recently developed approach that is independent of the selection of the ref. energy levels. We find that although the vol. deformation potentials of the CBM state are usually large and always neg., those of the VBM state are usually small and always pos. The AVDP of the VBM state decreases as the p-d coupling increases, e.g., in the II-VI compds. The AVDP of CBM decreases as the ionicity increases. Our calcd. chem. trends of the AVDPs are explained in terms of the AO energy levels and coupling between these orbitals.
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    70. 70
      Varley, J. B.; Samanta, A.; Lordi, V. Descriptor-Based Approach for the Prediction of Cation Vacancy Formation Energies and Transition Levels. J. Phys. Chem. Lett. 2017, 8, 5059– 5063,  DOI: 10.1021/acs.jpclett.7b02333
      70
      Descriptor-based approach for the prediction of cation vacancy formation energies and transition levels
      Varley, Joel B.; Samanta, Amit; Lordi, Vincenzo
      Journal of Physical Chemistry Letters (2017), 8 (20), 5059-5063CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Point defects largely det. the obsd. optical and elec. properties of a given material, yet the characterization and identification of defects has remained a slow and tedious process, both exptl. and theor. We demonstrate a computationally-cheap model that can reliably predict the formation energies of cation vacancies as well as the location of their electronic states in a large set of II-VI and III-V materials using only parameters obtained from the bulk primitive unit cell (2-4 atoms). We apply our model to ordered alloys within the CdZnSeTe, CdZnS, and ZnMgO systems and predict the positions of cation vacancy charge-state transition levels with a mean abs. error of < 0.2 eV compared to the explicitly calcd. values, showing useful accuracy without the need for the expensive and large-scale calcns. typically required. This suggests the properties of other point defects may also be predicted with useful accuracy from only bulk-derived properties.
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    71. 71
      Schleife, A.; Fuchs, F.; Rödl, C.; Furthmüller, J.; Bechstedt, F. Branch-point Energies and Band Discontinuities of III-nitrides and III-/II-oxides from Quasiparticle Band-structure Calculations. Appl. Phys. Lett. 2009, 94, 012104,  DOI: 10.1063/1.3059569
      71
      Branch-point energies and band discontinuities of III-nitrides and III-/II-oxides from quasiparticle band-structure calculations
      Schleife, A.; Fuchs, F.; Roedl, C.; Furthmueller, J.; Bechstedt, F.
      Applied Physics Letters (2009), 94 (1), 012104/1-012104/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Using quasiparticle band structures based on modern electronic-structure theory, we calc. the branch-point energies for zinc blende (GaN, InN), rocksalt (MgO, CdO), wurtzite (AlN, GaN, InN, ZnO), and rhombohedral crystals (In2O3). For InN, CdO, ZnO, and also In2O3 the branch-point energies are located within the lowest conduction band. These predictions are in agreement with observations of surface electron accumulation (InN, CdO) or conducting behavior of the oxides (ZnO, In2O3). The results are used to predict natural band offsets for the materials investigated. (c) 2009 American Institute of Physics.
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    72. 72
      Shapera, E. P.; Schleife, A. Database-Driven Materials Selection for Semiconductor Heterojunction Design. Advanced Theory and Simulations 2018, 1, 1800075,  DOI: 10.1002/adts.201800075
      There is no corresponding record for this reference.
    73. 73
      Walsh, A.; Catlow, C. R. A.; Zhang, K. H. L.; Egdell, R. G. Control of the Band-gap States of Metal Oxides by the Application of Epitaxial Strain: The Case of Indium Oxide. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 161202(R)  DOI: 10.1103/PhysRevB.83.161202
      73
      Control of the band-gap states of metal oxides by the application of epitaxial strain. The case of indium oxide
      Walsh, Aron; Catlow, C. Richard A.; Zhang, K. H. L.; Egdell, Russell G.
      Physical Review B: Condensed Matter and Materials Physics (2011), 83 (16), 161202/1-161202/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We demonstrate that metal oxides exhibit the same relationship between lattice strain and electronic band gap as nonpolar semiconductors. Epitaxial growth of ultrathin [111]-oriented single-crystal In2O3 films on a mismatched YSZ substrate reveals a net band-gap decrease, which is dissipated as the film thickness is increased and the epitaxial strain is relieved. Calcn. of the band-gap deformation of In2O3, using a hybrid d. functional, confirms that, while the uniaxial lattice contraction along [111] results in a band-gap increase due to a raise of the conduction band, the lattice expansion in the (111) plane caused by the substrate mismatch compensates, resulting in a net band-gap decrease. These results have direct implications for tuning the band gaps and transport properties of oxides for application in optoelectronic devices.
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    • XPS core levels and associated analysis, optical transmission and absorption spectra, XPS semicore levels, DFT of semicore levels accounting for SOC, DFT calculated band gap deformation potentials (PDF)


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