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Fast and Efficient Sub-Band Gap Photodetection in Al:ZnO/Si Heterojunction by Enhanced Photoexcited Hole Transport via Interfacial Defect States
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ACS Photonics

Cite this: ACS Photonics 2025, 12, 1, 62–70
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https://doi.org/10.1021/acsphotonics.4c01057
Published December 18, 2024

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Abstract

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This study focuses on sub-band gap photodetection in n+-Al:ZnO/n-Si isotype heterojunction-based photodiodes via interfacial defects induced by metal oxide films. Through comparative studies on the photoresponse of Schottky junction-based photodiodes with the modified electronic band structure by controlling the structural and electrical properties of Al:ZnO films, as well as the Si substrate’s doping level, we investigate the underlying mechanisms of interfacial defect states for sub-band gap photodetection in Si. Our analysis suggests that these interfacial defects not only act as additional sources for photoexcited carrier generations but also serve as pathways for photogenerated holes in the Si valence band, enabling their flow into the Al:ZnO film and improving the operating speed. Time-resolved photocurrent measurements under near-infrared illumination illustrate an enhancement in photocurrent with lower oxygen partial pressures (0 mTorr) attributed to alterations in the energy band structure caused by interfacial defect states. Significantly, the Al:ZnO/Si photodiode fabricated under optimized conditions exhibits a photoresponse of 2.48 mA/W at 1310 nm with fast rise/fall times of 5.5/5.25 μs at 1 kHz and a 3 dB bandwidth of approximately 150 kHz, without introducing additional bulk trap states in Si. In light of these findings, the combination of simple fabrication and excellent switching speed of interfacial defect-mediated Si photodiodes has the potential to significantly impact the technologies of Si photonics and advanced Si-based photoelectric devices.

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1. Introduction

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The growing demand for near-infrared (NIR) photodetection, with applications in biomedical imaging, gas sensing, energy conversion, and optical communication, has attracted significant interest in developing cost-efficient NIR photodiodes. (1−4) In particular, recent advances in integrated silicon (Si) photonics, predominantly operating within telecommunication wavelengths, have intensified the focus on NIR photodetection at room temperature within the Si platform. As the most mature semiconductor, Si has traditionally been a robust photodetection platform due to its remarkable quantum efficiency under light illumination with wavelengths below 1100 nm. However, its inherent limitation, an indirect band gap of 1.12 eV, restricts its applicability for NIR photodetection below this band gap energy.
To broaden the spectral range for sub-band gap photodetection, researchers have explored various strategies for material integration on Si, including germanium (Ge), (5,6) indium selenide (In2Se3), (7) III–V compound semiconductors (GaAs, InAs), (8,9) and silicon–germanium (SiGe). (10) While these approaches have effectively demonstrated efficient photodetection at telecommunication wavelengths, they often encounter practical challenges like incompatibility with complementary metal oxide semiconductor (CMOS) technology, limited bandwidth, and high manufacturing costs. (11) To expand Si photodetection into the NIR spectrum, considerable efforts have been directed toward manipulating intrinsic properties, such as absorption and doping. (12) Recent breakthroughs include hyperdoped Si by ion-implanting other materials such as metals and chalcogens (Te, Se, and S), (13−19) demonstrating high photoresponsivity across an extended spectral range, even reaching into the mid-infrared (MIR) spectrum. (20,21) Although introducing deep-level trap states in Si’s inherent band gap enhances photoresponse performance, it can result in slow response speeds (∼ms). This is because trap energy states impede electron transport, thus limiting their use in high-speed optoelectronic devices.
An alternative method to achieve sub-band gap photoresponse in Si via defect states involves NIR photodetection assisted by surface defect states. (12) Introducing impurities to the Si surface through foreign atoms or molecules can modify the surface electronic states in Si, enabling carrier generation under sub-band gap illumination. Such surface defect states can be simply induced by depositing films on the Si surface, commonly termed interfacial defect states. Isotype heterojunction-based Schottky photodiodes have recently highlighted the potential of NIR photodetection in Si by inducing interfacial defect states by depositing n-type metal oxide films on n-type Si substrates. (22−26) While numerous studies on Schottky-based photodetection using metals and metal oxides, known for their fast photoresponse, have been reported, a comprehensive analysis of their operational speed has not yet been provided. (4,27,28)
Therefore, this paper investigates the characteristics of Si photodiodes incorporating aluminum-doped zinc oxide (Al:ZnO) films for NIR photodetection, proposing Schottky-based photodetection through interfacial defect states. A Schottky junction is constructed at the Al:ZnO/Si isotype heterojunction for the generation of electron–hole pairs (EHP) in the depletion region of the n-type Si substrate, as illustrated in Figure 1. Using structural analyses (AFM, XRD, and XPS) and examining the impact of the Si substrate’s doping level and the electrical properties of Al:ZnO films on photodetection, we emphasize the underlying mechanisms by which interfacial defects act as sources for generating photoexcited electron–hole pairs and establish pathways for channeling photogenerated carriers (holes) from Si into Al:ZnO, thereby facilitating the movement of photoexcited holes and enhancing the switching speed (∼μs). Additionally, the photoresponse characteristics of Si photodiodes were measured at various NIR wavelengths, including the two main telecommunication wavelengths (1310 and 1550 nm). Specifically, at 1310 nm and room temperature under zero-bias conditions, the photodiode achieved a photoresponse of 2.48 mA/W when paired with Al:ZnO films deposited at a low oxygen partial pressure (0 mTorr). Furthermore, we quantify the frequency and temporal response of Al:ZnO/Si Schottky photodiodes, revealing rapid rise/fall times of 5.5/5.25 μs at 1 kHz without degradation while preserving a steady photoresponse at high frequencies with a 3 dB bandwidth of approximately 150 kHz. Since photodiodes for optical communication wavelengths require fast switching speeds to process large amounts of data, our research can provide a unique approach to achieving high-speed photodetection in the telecommunication wavelength.

Figure 1

Figure 1. Schematic diagram of the 50 nm thick Al:ZnO/n-type Si planar photodiode. Interfacial defects establish pathways for channeling photogenerated carriers (holes) in the depletion region of the n-type Si.

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2. Materials and Methods

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2.1. Sample Preparation

A monocrystalline n-type (100) silicon substrate, polished on both sides with a thickness of 500 μm, was chosen for the experiment. Each sample underwent a thorough cleaning with acetone and methanol. To ensure an optimal surface condition, the native oxide layer was effectively removed using a buffered oxide etcher (BOE) solution for 15 s. Al:ZnO films were centrally deposited via pulsed laser deposition (PLD) to fabricate the Al:ZnO/Si photodiodes. For the device’s cathode, a 10 nm thick Ti layer and a 120 nm thick Au layer were deposited at the substrate’s corner through e-beam evaporation. The Al:ZnO films were deposited on Si substrates using a KrF excimer laser (Coherent) operating at a wavelength of 248 nm for source material ablation. 3 wt % aluminum-doped ZnO target with a purity of 99.99% (Toshima) was purchased. During the Al:ZnO film deposition, the substrate temperature was set at 165 °C. The laser beam’s repetition rate and energy density were set to 5 Hz and 2.9J/cm2. To provide a better description of the sample preparation, schematic fabrication procedures and an optical microscope image are presented in Figure 2.

Figure 2

Figure 2. Schematic fabrication procedures of the 50 nm thick Al:ZnO/n-type Si planar photodiode and its optical microscope image.

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2.2. Sample Characterization

Surface imaging and roughness analyses were performed using atomic force microscopy (AFM, Park Systems XE-100). Structural analyses of the samples were carried out with high-resolution X-ray diffraction (HRXRD, Rigaku ATX-G) employing CuKα (λ = 1.5406 Å) and X-ray photoelectron spectroscopy (XPS, ULVAC-PHI PHI 5000 VersaProbe). For electrical characterization, a semiconductor parameter analyzer (Keithley 4200) was used to measure current–voltage (I–V) characteristics, while the ultrafast I–V module (Keithley 4225-PMU) was employed for current–time (I–t) characteristic measurements. All of the evaluations were conducted at room temperature. Optical power monitoring was executed by using a hand-held optical power meter and a wand-style photodiode power sensor (Newport 918D-ST-IR). Transmission data were collected using a UV–visible spectrophotometer (Shimadzu UV-3600), spanning 875–1575 nm wavelengths.

3. Results and Discussion

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As depicted in Figure 1, we constructed planar photodiodes with a 50 nm thick Al:ZnO film on an n-type Si substrate. We prepared three photodiodes with each Al:ZnO film deposited at different oxygen partial pressures: 0, 10, and 50 mTorr. We used commercially available phosphorus (P)-doped n-type Si substrates with a resistivity range of 10–30 Ω·cm, and the substrate’s resistivity was determined to be 18.6 Ω·cm by a Hall measurement.
Figure 3a illustrates the surface morphology of the Al:ZnO film, as captured by AFM. The Al:ZnO film exhibits a notably smooth surface, with root-mean-squared (RMS) roughness values of 0.49, 0.19, and 0.25 nm for films deposited at oxygen partial pressures of 0, 10, and 50 mTorr, respectively. These RMS values reveal that the surface at 0 mTorr is comparatively rougher than at higher pressures, with a slight increase in roughness as oxygen partial pressure rises. This trend can be expected by the high kinetic energy of particles in the plasma plume at 0 mTorr due to less scattering with oxygen atoms in the chamber, which leads to increase RMS roughness. (29−31) As shown in Figure 3b, HRXRD measurements indicated that the intensity of XRD peaks from the (002) plane decreased as the partial oxygen pressure decreased. Concurrently, the (002) peak shifted to lower angles, transitioning from 34.4 to 33.9°. This observation suggests that Al:ZnO films deposited at lower oxygen partial pressures experienced significant lattice strain and a deterioration in film crystallinity, likely attributed to the presence of defect states at the interface between Al:ZnO and Si.

Figure 3

Figure 3. (a) Atomic force microscope images of the Al:ZnO thin film under different oxygen partial pressures (Poxygen): 0 mTorr (top), 10 mTorr (middle), and 50 mTorr (bottom). (b) High-resolution X-ray diffraction patterns under different oxygen partial pressures (Poxygen): 0 (blue), 10 (cyan), and 50 mTorr (purple). X-ray photoelectron spectroscopy measurements were performed at various oxygen partial pressures (Poxygen): (c) 0 mTorr, (d) 10, and (e) 50 mTorr. Circles represent the experimental data points, and the fitting curves (solid lines) are the sum of the individual components (O1, O2, and O3) obtained by deconvoluting the spectra. Red lines represent the baseline for each spectrum. O1, the peak at the lowest binding energy, corresponds to O2– ions in the wurtzite ZnO lattice. O2, the peak at the intermediate binding energy, is related to oxygen vacancies within the oxygen-deficient regions of the ZnO matrix, with a more pronounced O2 peak signifying a higher concentration of defect states in Al:ZnO. O3, the peak at the highest binding energy, is associated with loosely bound oxygen species on the ZnO surface.

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To investigate the relation between oxygen partial pressure and defects further, XPS was conducted, as shown in Figure 3c–e. The O 1s peaks were deconvoluted using near-Gaussian subpeaks into three different peaks (O1, O2, and O3). The lower binding energy peak (O1, 531.0 ± 0.1 eV) is attributed to the O2– ions from the wurtzite ZnO lattice, while the intermediate binding energy peak (O2, 531.6 ± 0.3 eV) is correlated with oxygen vacancies in the oxygen-deficient region within the ZnO matrix. The higher binding energy peak (O3, 532.8 ± 0.2 eV) originates from loosely bound oxygen on the surface of ZnO. It is noted that the relative ratio of the O2 peak increased as the oxygen partial pressure decreased. (32−36) Thus, structural analyses with XRD and XPS demonstrate that a decrease in the oxygen partial pressure during the deposition of Al:ZnO films can generate oxygen vacancy defects.
Figure 4a displays the dark I–V characteristics of Al:ZnO/Si photodiodes, confirming that a Schottky contact was successfully established between the Al:ZnO films and the n-type Si substrate regardless of the oxygen partial pressure during Al:ZnO deposition. Interestingly, when the oxygen partial pressure decreased, both the forward and reverse currents of the photodiode increased. This result suggests the presence of interfacial trap states because such defect states can provide additional pathways for carriers to tunnel through the barrier in both forward and reverse bias conditions. Time-resolved photocurrent (I–t) measurements for all photodiodes under NIR light illumination at a wavelength of 1310 nm are presented in Figure 4b. The incident laser operated at a frequency of 1 kHz, with a pulse width of 500 μs, and an intensity of 9.61 mW. As the oxygen partial pressure increased, we observed a significant reduction in photocurrent and a corresponding rise in response time. To show this tendency, photocurrent values at 10 and 50 mTorr are magnified by a factor of 3. Additionally, to illustrate the behavior of the fabricated photodiodes as a function of oxygen partial pressure, we tabulated the parameters at a wavelength of 1310 nm and an intensity of 9.61 mW, as shown in Table 1.

Figure 4

Figure 4. (a) Dark I–V characteristics and (b) time-resolved photocurrent (I–t) graphs of the photodiodes with different oxygen partial pressures (Poxygen): 0 (blue), 10 (cyan), and 50 mTorr (purple). (c) Energy band structure of the Al:ZnO/Si Schottky junction is illustrated under both dark conditions (cases I and II) and illuminated conditions (cases III and IV), respectively. Ec, Ev, and Ef represent the energy of the conduction band, the valence band, and the Fermi level, respectively. Eg_Al: ZnO and Eg_Si denote the Al:ZnO and the Si band gap. Blue and red circles indicate the holes and electrons, respectively, under illuminated conditions. In the oxygen-deficient condition, numerous interfacial defect energy states, denoted as “Et_interface” are generated on the surface of the Al:ZnO/Si Schottky junction (case I). As light illuminates the junction, photogenerated electrons move toward the Ti/Au electrode, while for the charge-neutral condition, holes move to the Al:ZnO using Et_interface (case III). On the other hand, in the oxygen-sufficient condition, a “no defect” condition is established (case II). Since hole transport is limited, photoexcited electron–hole pairs cannot contribute to photocurrent (case IV).

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Table 1. Parameters of Fabricated Photodiodes with Varying Oxygen Partial Pressure at a Wavelength of 1310 nm and Intensity of 9.61 mW
oxygen partial pressure [mTorr]dark current [μA]photocurrent [μA]responsivity [mA/W]detectivity [× 109 Jones]rise/fall time [μs]
00.1223.992.486.185.5/5.25
100.100.880.0820.2371/210
500.050.520.0490.1962/225
This observation can be explained by the role of interfacial defect states, as depicted in Figure 4c, which illustrates the energy band structure of the Al:ZnO/Si Schottky junction under two distinct scenarios: one with defect states in both the Si bulk and at the interface and the other with defect states solely in the Si bulk. As the dark I–V curve shows, the Al:ZnO/Si isotype heterojunction forms a Schottky barrier (ϕSB). This barrier can be calculated as 0.3 eV using the equation ϕSB = χAl: ZnO–χSi, where χAl: ZnO (4.35 eV) and χSi (4.05 eV) are the electron affinity of Al:ZnO and Si, respectively. (37−39) Additionally, the position of the Fermi energy relative to the conduction band minimum in Al:ZnO was determined based on references discussing the relationship between the carrier density of Al:ZnO and its work function. (40,41) We deduced that the Fermi energy level of Al:ZnO lies within the conduction band based on the electron concentration of our Al:ZnO film (1 × 1021 cm–3), which was determined through ellipsometry analysis using the Lorentz–Drude oscillator model. (42) In Figure 4c, Et_bulk represents all types of Si bulk defect energy states, which include intrinsic defect states caused by crystalline defects and extrinsic defects resulting from dopants. Meanwhile, Et_interface denotes interfacial defect energy states induced by the Al:ZnO film.
When NIR sub-band gap light is incident upon the device (from the left in the band structure illustration), electrons from the valence band are excited to available bulk defect states and the interfacial defect states. The internal electric field, resulting from the upward band-bending in the depletion region (W), forces these electrons into the conduction band, a phenomenon known as trap-assisted tunneling. (43,44) To maintain net charge neutrality in the depletion region, the interfacial defect trap states provide pathways for holes in the Si valence band to inject holes into Al:ZnO. In contrast, without defects at the interface, photogenerated holes and electrons in the depletion region will recombine under zero-bias conditions.
In an oxygen-deficient atmosphere, the number of oxygen vacancies increases. These vacancies can create defect energy states below the conduction band edge, (36) which is consistent with the XPS results, as shown in Figure 3c–e. Therefore, these defect states act as pathways for channeling photogenerated carriers. Thus, the Si Schottky photodiode with an Al:ZnO film deposited at the low oxygen partial pressure exhibits an abundance of interfacial trap states. These states serve as both carrier paths and additional sources of carrier emission, leading to a high NIR photoresponse. In comparison, very low photocurrent in the Si photodiode with Al:ZnO film deposited at oxygen partial pressures of 10 and 50 mTorr may indicate the presence of a few surface defects, even when Al:ZnO films are deposited at a high partial oxygen pressure. In conclusion, Al:ZnO film deposition on Si can efficiently extract photoexcited carriers in a Schottky junction by inducing interfacial defects, achieving a significant photoresponse without introducing additional defect states into the Si substrate.
We further investigated the role of interfacial defects by fabricating photodiodes with 50 nm thick Al:ZnO films deposited at oxygen partial pressure of 0 mTorr on Si substrates with various resistivity values. The commercially available resistivities were 0.1–0.5, 1–10, and 10–30 Ω·cm for P-doped Si substrate and 0.01–0.02 for antimony (Sb)-doped Si substrate. Hall measurement confirmed the substrate’s resistivity to be 0.01, 0.19, 9.83, and 18.6 Ω·cm. The dark I–V characteristics of all Al:ZnO/Si photodiodes were characterized, as shown in Figure 5a. Notably, a distinct transition from Schottky to Ohmic contact was observed as the resistivity of Si substrate decreased. This transition was attributed to variations in the thickness of the depletion region, dependent on the doping level of n-type Si. Using the equation Wd=2εSiΦB/(qnSi), where εSi is the dielectric constant of Si and nSi is the carrier density of Si, we estimated the depletion width (W) to be 0.942, 0.720, 0.108, and 0.011 μm for Si substrate with the resistivity of 18.6, 9.83, 0.19, and 0.01 Ω·cm, respectively. The carrier density of the Si substrate was calculated based on the resistivity of Si. (45) As shown in Figure 5b, the heavily doped Si (P-doped, 0.19 Ω·cm) photodiode exhibits a reduced depletion width (WHD), leading to a sufficiently narrow barrier that allows for direct electron tunneling through the interface. Consequently, considerable alterations in the electronic band structure of the Al:ZnO/Si junction arise from changes in the Si doping levels.

Figure 5

Figure 5. (a) Dark I–V characteristics of the photodiodes with Si substrate resistivity values of 0.01, 0.19, 9.83, and 18.6 Ω·cm. (b) Energy band structure of the Al:ZnO/Si Schottky junction for the lightly doped n-type Si substrate (I) and the heavily doped n+-type Si substrate (II). ϕSB denotes the Schottky barrier of the photodiode. WLD and Vbi_LD represent the depletion width and the built-in potential of the lightly doped Si-based photodiode, respectively, while WHD and Vbi_HD denote the depletion width and the built-in potential of the heavily doped Si-based photodiode, respectively.

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With insights into the modification in the electronic band structure caused by the change in the resistivity of the Si substrate, we carried out I–t measurements on all samples (Figure 6a) under NIR light with a wavelength of 1310 nm, allowing us to gain insights into the origin and mechanism of the photocurrent. We observed that as the substrate resistivity decreased, the photoresponse of the photodiodes correspondingly diminished. Notably, the Si substrate with a resistivity of 0.01 Ω·cm showed almost no photoresponse. This result implies that narrowing the depletion region limits the available defect states that act as sources for photoexcited carriers, resulting in the absence of photocurrent under Ohmic contact conditions. A notable observation was the 2.5-fold reduction in photocurrent when the depletion width shrank by nearly 9 times, as we changed the resistivity of the Si substrate from 18.6 to 0.19 Ω·cm. This result suggests that interfacial defects can also act as sources for photoemission. Even with a significant reduction in bulk defects attributed to the narrowed depletion width, interfacial defects persisted in both samples, given that the Al:ZnO films were consistently deposited under the same conditions. In Figure 6b, we compare the rise/fall time of photodiodes with Si resistivities of 9.83, 18.6, and 0.19 Ω·cm, as well as a commercial germanium photodiode (Ge PD, Thorlabs Inc.). The rise/fall time of the Ge PD is measured to be around 2 μs at zero-bias condition, and the photodiodes with Si resistivity of 9.83 and 18.6 Ω·cm exhibit approximately 5.5/5.25 μs. This demonstrates that the Al:ZnO film photodiode with an appropriate substrate performs similarly to the commercial Ge PD. Likewise, it shows comparable performance to previous studies on Si-based sub-band gap photodetectors utilizing the hot carrier effect. (46) In contrast, the photodiode with Si resistivity of 0.19 Ω·cm displayed rise and fall times of 48.0 and 35.0 μs, respectively, despite the decrease in the series resistivity of the photodiode. We anticipate that the delayed temporal response in the 0.19 Ω·cm Si-based photodiodes is due to increased extrinsic defect densities from heavy doping, similar to the slow operating speeds observed in hyperdoped Si-based photodiodes. (47)

Figure 6

Figure 6. (a) Time-resolved photocurrent (I–t) graphs of photodiodes with Si substrate resistivity values of 0.01, 0.19, 9.83, and 18.6 Ω·cm. Photocurrent was measured at a wavelength of 1310 nm, a frequency of 1 kHz, and an input light intensity of 9.61 mW. (b) Rise and fall times of photodiodes with Si substrate resistivity values of 0.01, 0.19, 9.83, and 18.6 Ω·cm and a commercial Ge PD (gray dotted line). Rise and fall times are calculated using the data within the magenta dashed lines at 0.1 and 0.9.

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Lastly, we fabricated Si photodiodes with Al:ZnO films of varying thickness (20, 50, 100, and 150 nm) on an 18.6 Ω·cm Si substrate, with the Al:ZnO films deposited at a 0 mTorr oxygen partial pressure. In Figure 7a, the dark I–V characteristics reveal that Al:ZnO/Si photodiodes form distinct Schottky contacts regardless of the film thickness. Notably, as the film thickness increases, there is a gradual increase in forward bias, which can be attributed to the enhanced Al:ZnO film conductance. Figure 7b illustrates the photoresponsivity of all samples at three different NIR wavelengths (904, 1310, and 1550 nm). Intriguingly, photoresponse at 904 nm increases with Al:ZnO thickness, while responses at 1310 and 1550 nm decrease as Al:ZnO thickness increases. This phenomenon can be attributed to the thickness-dependent transmission of the Al:ZnO film within the NIR spectrum.

Figure 7

Figure 7. (a) Dark I–V characteristics and (b) photoresponsivity at wavelengths of 904 (violet), 1310 (pink), and 1550 nm (olive) of the photodiodes with varying Al:ZnO film thicknesses of 20, 50, 100, and 150 nm. (c) Transmission spectrum of Al:ZnO films deposited on a quartz substrate with thicknesses of 20, 50, and 150 nm, respectively.

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Figure 7c presents the transmission spectrum of Al:ZnO films deposited on a quartz substrate with a thickness identical to that deposited on the Si substrate for the photodiode. As heavily doped Al:ZnO behaves like a metallic film, it reflects light above the plasma wavelength, intensifying with increased thickness. In comparison, the transmission of the Al:ZnO film at the wavelength of 904 nm experiences a reduction of less than 10% as the thickness increases from 20 to 150 nm, while the transmission at 1550 nm diminishes by 40%. Consequently, the responsivity of the photodiode at 1310 and 1550 nm decreases correspondingly with the increasing Al:ZnO thickness. Despite a slight drop in transmission, the incremental rise in photoresponsivity at 904 nm can be attributed to improved conductance with increased Al:ZnO thickness, as shown in the I–V characteristics.
Based on the photoresponsivity and temporal response, we determined the optimal condition of the Al:ZnO/Si photodiode operating at a wavelength of 1310 nm. A star-shaped symbol in Figure 7b marks this condition. The best resistivity of the Si substrate is 18.6 Ω·cm, with the optimal 50 nm thick Al:ZnO film. It can be confirmed that the optimal thickness is 50 nm from Table 2 (varying AZO thickness), which tabulates the parameters at a wavelength of 1310 nm and an intensity of 9.61 mW.
Table 2. Parameters of Fabricated Photodiodes with Varying AZO Thickness at a Wavelength of 1310 nm and Intensity of 9.61 mW
AZO thickness [nm]dark current [μA]photocurrent [μA]responsivity [mA/W]detectivity [× 109 Jones]rise/fall time [μs]
200.2118.531.913.588.75/12
500.1223.992.486.185.5/5.25
1000.1818.221.883.805.5/4.75
1500.2316.491.693.055.25/4.5
We quantified the frequency response under zero-bias conditions, as shown in Figure 8. The responsivity of the 50 nm thick Al:ZnO-based Si photodiode at low-frequency operation is approximately 2.48 mA/W and exhibits a 3 dB bandwidth of approximately 150 kHz. The inset of Figure 8 presents the relationship between the on/off current and laser intensity. It is noted that the on–off current at low-frequency operation exhibits a linear response concerning the laser intensity, verifying that the photoexcited carrier generation in the devices is not the result of nonlinear transitions such as carrier multiplication and two-photon absorption. We also calculated specific detectivity (D*) by the following equation:
D*=RASn=RA2qIdark
In this equation, R represents the responsivity at the specific wavelength; A denotes the area of the photodiode; Sn is the noise spectral density (NSD) of the shot noise, which mainly contributes to dark current; (47−52) q is the charge of the electron; and Idark is the dark current of the device. Based on this equation, the peak detectivity of the device at a wavelength of 1310 nm is calculated by 6.1834 × 109 cm·Hz1/2/W (Jones).

Figure 8

Figure 8. Frequency response of the photodiode at a wavelength of 1310 nm under zero-bias conditions. 3 dB bandwidth (f3dB) is indicated by the magenta dotted lines at 150 kHz. The inset of the figure shows the relationship between the on–off current and laser intensity.

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4. Conclusions

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We report sub-band gap photodetection in Al:ZnO/Si Schottky junction-based photodiode resulting from efficient transport of photoexcited carriers in extrinsic Si (P-doped) via interfacial defects induced by Al:ZnO film. Through the analytical comparison study, we determined the optimal conditions for the photodiode to achieve high responsivity and fast switching speed. To compare this work with relevant studies, such as hyperdoped black silicon, 2D materials, and hot carrier devices, we tabulated the performance metrics, including operation wavelength, bias voltage, responsivity, detectivity, and rise/fall time, as shown in Table 3. This comparison highlights that our Al:ZnO/Si photodiodes achieve fast switching speeds through a simple Schottky junction structure, using interfacial defects as channels for photogenerated carriers. These characteristics indicate their potential for application in high-speed optoelectronic devices. Furthermore, a theoretical model for defect states would guide the design of materials to develop improved interface defect-mediated Si photodiodes. Fundamentally, Al:ZnO/Si photodiodes operate with low energy consumption due to zero biasing; therefore, these photodiodes would be advantageous for energy-efficient optical interconnects in on-chip Si photonics in the telecommunication regime.
Table 3. Comparison of Relevant Works
devicewavelength [nm]bias voltage [V]responsivity [mA/W]detectivity [Jones]rise/fall time [μs]ref
N hyperdoped black Silicon1310–105.3 3400 (21)
Ar hyperdoped black Silicon1310–129751.14 × 1010  (17)
Te hyperdoped black Silicon1550–256.82.54 × 109  (15)
TiN/Ge160000.07396.51 × 105∼100,000 (6)
Au/n-Si131001.6 7.5/8 (46)
MoS2 film15501 (VDS)∼ 1.1∼ 1.7 × 105  (48)
AgNPs/MoS2 film15501 (VDS)∼ 3.9∼ 5.8 × 105 
n+-ZnO/n-Si300–940–1.52001.12 × 101264 (26)
300–9400121.8 × 1011 
Au/TiO2–x/p-Si125001   (28)
TiN/TiO2–x/p-Si1250–0.454  
n+-Al:ZnO/n-Si131002.486.1834 × 1095.5/5.25this work

Data Availability

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The data supporting this study’s findings are available from the corresponding author upon reasonable request.

Author Information

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  • Corresponding Authors
  • Author
    • Geunpil Kim - Nanophotonics Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea;  School of Electrical Engineering, Korea University, Seoul 02841, Republic of Korea;  Orcidhttps://orcid.org/0000-0003-1126-2914
  • Author Contributions

    G.K.: Conceptualization, Formal analysis, Investigation, Methodology, Visualization, Writing-original draft. B.C.L.: Funding acquisition, Writing-review and editing. J.K.: Conceptualization, Funding acquisition, Project administration, Supervision, Validation, Writing-review and editing.

  • Funding

    This work was partially supported by the Institute of Information and Communications Technology Planning and Evaluation (IITP) grant funded by the Korean government (MSIT) (No. RS-2023-00223082, Development of Quantum Technology for High-Precision Gravity Sensing) and by the R&D Program (2E32541) funded by the Korea Institute of Science and Technology (KIST). This study was also partially supported by the Nanomedical Devices Development Project of the NNFC in 2023 (No. 1711160154).

  • Notes
    The authors declare no competing financial interest.

References

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This article references 52 other publications.

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    A review. In recent years, near-IR (NIR) spectroscopy has gained wide acceptance in different fields by virtue of its advantages over other anal. techniques, the most salient of which is its ability to record spectra for solid and liq. samples with no prior manipulation. Also, developments in instrumentation resulted in the manuf. of spectrophotometers capable of quickly providing spectra that are flexible enough for use in different situations; thus, portable equipment can record spectra on site or even at prodn. lines. This article discusses the features of NIR spectroscopy that have driven forward its dramatic development in a wide range of anal. fields in the last few years.
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    A nanogenerator, as a self-powered system, can operate without an external power supply for energy harvesting, signal processing, and active sensing. Near-IR (NIR) photothermal triggered pyroelec. nanogenerators based on pn-junctions are demonstrated in a p-Si/n-ZnO nanowire (NW) heterostructure for self-powered NIR photosensing. The pyroelec.-polarization potential (pyro-potential) induced within wurtzite ZnO NWs couples with the built-in elec. field of the pn-junction. At the moment of turning on or off the NIR illumination, external current flow is induced by the time-varying internal elec. field of the pn-heterostructure, which enables a bias-free operation of the photodetectors (PDs). The NIR PD exhibits a high on/off photocurrent ratio up to 107 and a fast photoresponse component with a rise time of 15 μs and a fall time of 21 μs. This work provides an unconventional strategy to achieve active NIR sensing, which may find promising applications in biol. imaging, optoelectronic communications, and optothermal detections.
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    Internal photoemission (IPE) is a promising phenomenon for sub-bandgap photodetection at near-IR wavelengths using large bandgap semiconductor materials. To improve the photon-to-electron conversion efficiency in Si-based sub-bandgap Schottky barrier photodetectors (SBPDs), previous studies have mainly focused on subwavelength-scale nanostructures to enhance the elec. fields and optical absorption. Here, in a different way from the previous approaches, the authors theor. and exptl. demonstrate a rigorous quantum efficiency anal. framework that can quant. explain the hot carrier transport processes. Guided by the design principles from the hot carrier loss mechanism anal., the authors exptl. demonstrate patternless thin-film SBPDs that can surpass the performance of conventional nanostructured SBPDs, exceeding the external quantum efficiency of 10-3. The authors' work shows that optical absorption and hot carrier generation are responsible for only a part of the entire IPE process and other transport mechanisms should be carefully considered in a wholistic manner, indicating the importance of quant. quantum efficiency anal.
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    The past decade has seen rapid progress in research into high-performance Ge-on-Si photodetectors. Owing to their excellent optoelectronic properties, which include high responsivity from visible to near-IR wavelengths, high bandwidths and compatibility with silicon complementary metal-oxide-semiconductor circuits, these devices can be monolithically integrated with silicon-based read-out circuits for applications such as high-performance photonic data links and IR imaging at low cost and low power consumption. This Review summarizes the major developments in Ge-on-Si photodetectors, including epitaxial growth and strain engineering, free-space and waveguide-integrated devices, as well as recent progress in Ge-on-Si avalanche photodetectors.
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    Due to recent breakthroughs, silicon photonics is now the most active discipline within the field of integrated optics and, at the same time, a present reality with commercial products available on the market. Silicon photodiodes are excellent detectors at visible wavelengths, but the development of high-performance photodetectors on silicon CMOS platforms at wavelengths of interest for telecommunications has remained an imperative but unaccomplished task so far. In recent years, however, a number of near-infrared all-silicon photodetectors have been proposed and demonstrated for optical interconnect and power-monitoring applications. In this paper, a review of the state of the art is presented. Devices based on mid-bandgap absorption, surface-state absorption, internal photoemission absorption and two-photon absorption are reported, their working principles elucidated and their performance discussed and compared.
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    Nature communications (2014), 5 (), 3011 ISSN:.
    Room-temperature infrared sub-band gap photoresponse in silicon is of interest for telecommunications, imaging and solid-state energy conversion. Attempts to induce infrared response in silicon largely centred on combining the modification of its electronic structure via controlled defect formation (for example, vacancies and dislocations) with waveguide coupling, or integration with foreign materials. Impurity-mediated sub-band gap photoresponse in silicon is an alternative to these methods but it has only been studied at low temperature. Here we demonstrate impurity-mediated room-temperature sub-band gap photoresponse in single-crystal silicon-based planar photodiodes. A rapid and repeatable laser-based hyperdoping method incorporates supersaturated gold dopant concentrations on the order of 10(20) cm(-3) into a single-crystal surface layer ~150 nm thin. We demonstrate room-temperature silicon spectral response extending to wavelengths as long as 2,200 nm, with response increasing monotonically with supersaturated gold dopant concentration. This hyperdoping approach offers a possible path to tunable, broadband infrared imaging using silicon at room temperature.
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    Optics express (2020), 28 (4), 5239-5247 ISSN:.
    Femtosecond laser hyperdoped silicon, also known as the black silicon (BS), has a large number of defects and damages, which results in unstable and undesirable optical and electronic properties in photonics platform and optoelectronic integrated circuits (OEICs). We propose a novel method that elevates the substrate temperature during the femtosecond laser irradiation and fabricates tellurium (Te) hyperdoped BS photodiodes with high responsivity and low dark current. At 700 K, uniform microstructures with single crystalline were formed in the hyperdoped layer. The velocity of cooling and resolidification is considered as an important role in the formation of a high-quality crystal after irradiation by the femtosecond laser. Because of the high crystallinity and the Te hyperdoping, a photodiode made from BS processed at 700 K has a maximum responsivity of 120.6 A/W at 1120 nm, which is far beyond the previously reported Te-doped silicon photodetectors. In particular, the responsivity of the BS photodiode at 1300 nm and 1550 nm is 43.9 mA/W and 56.8 mA/W with low noise, respectively, which is valuable for optical communication and interconnection. Our result proves that hyperdoping at a high substrate temperature has great potential for femtosecond-laser-induced semiconductor modification, especially for the fabrication of photodetectors in the silicon-based photonic integration circuits.
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    Scientific reports (2017), 7 (), 43688 ISSN:.
    The optoelectronic applications of Si are restricted to the visible and near-infrared spectral range due to its 1.12 eV-indirect band gap. Sub-band gap light detection in Si, for instance, has been a long-standing scientific challenge for many decades since most photons with sub-band gap energies pass through Si unabsorbed. This fundamental shortcoming, however, can be overcome by introducing non-equilibrium deep-level dopant concentrations into Si, which results in the formation of an impurity band allowing for strong sub-band gap absorption. Here, we present steady-state room-temperature short-wavelength infrared p-n photodiodes from single-crystalline Si hyperdoped with Se concentrations as high as 9 × 10(20) cm(-3), which are introduced by a robust and reliable non-equilibrium processing consisting of ion implantation followed by millisecond-range flash lamp annealing. We provide a detailed description of the material properties, working principle and performance of the photodiodes as well as the main features in the studied wavelength region. This work fundamentally contributes to establish the short-wavelength infrared detection by hyperdoped Si in the forefront of the state-of-the-art of short-IR Si photonics.
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    The effects of deposition conditions on the phys. and elec. performance of the n-ZnO/p-Si heterojunction were systematically investigated. ZnO films were deposited on the Si and glass substrates using d.c. magnetron sputtering with various ambients and substrate temps. The results showed that increasing the O2 content and substrate temp. during the deposition process could improve the crystallinity and stoichiometry of the ZnO film, resulting in a lower carrier concn. and higher resistivity. The elec. properties of the n-ZnO/p-Si heterojunctions were also affected by the deposition parameters. For the junctions fabricated in the pure Ar ambient, the sample deposited at room temp. (RT) showed Ohmic behavior, while the one deposited at 300° exhibited poor rectifying behavior. On the other hand, the junctions fabricated in the O2/Ar ambient possessed ideal rectifying behaviors. The different carrier transport mechanisms for the heterojunctions under forward and reverse bias were systematically studied using a high temp. current-voltage (I-V) measurement. The recombination-tunneling current showed temp. insensitive performance while the space-charge limited current (SCLC) changed with the measurement temp.
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    Applied Physics Letters (2015), 107 (15), 152102/1-152102/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    In this paper, we have demonstrated the dual role of boron doping in enhancing the device performance parameters as well as the device stability in low temps. (200°C) sol-gel processed ZnO thin film transistors (TFTs). Our studies suggest that boron is able to act as a carrier generator and oxygen vacancy suppressor simultaneously. Boron-doped ZnO TFTs with 8 mol.% of boron concn. demonstrated field-effect mobility value of 1.2 cm2 V-1 s-1 and threshold voltage of 6.2 V, resp. Further, these devices showed lower shift in threshold voltage during the hysteresis and bias stress measurements as compared to undoped ZnO TFTs. (c) 2015 American Institute of Physics.
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    ACS Applied Materials & Interfaces (2016), 8 (12), 7862-7868CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The intriguing properties of zinc oxide-based semiconductors are being extensively studied as they are attractive alternatives to current silicon-based semiconductors for applications in transparent and flexible electronics. Although they have promising properties, significant improvements on performance and elec. reliability of ZnO-based thin film transistors (TFTs) should be achieved before they can be applied widely in practical applications. This work demonstrates a rational and elegant design of TFT, composed of poly cryst. ZnO:H/ZnO bilayer structure without using other metal elements for doping. The field-effect mobility and gate bias stability of the bilayer structured devices have been improved. In this device structure, the hydrogenated ultrathin ZnO:H active layer (∼3 nm) could provide suitable carrier concn. and decrease the interface trap d., while thick pure-ZnO layer could control channel conductance. Based on this novel structure, a high field-effect mobility of 42.6 cm2 V-1 s-1, a high on/off current ratio of 108 and a small subthreshold swing of 0.13 V dec-1 have been achieved. Addnl., the bias stress stability of the bilayer structured devices is enhanced compared to the simple single channel layer ZnO device. These results suggest that the bilayer ZnO:H/ZnO TFTs have a great potential for low-cost thin-film electronics.
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    Nanoscale research letters (2012), 7 (1), 470 ISSN:.
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  1. Zhi‐Xiang Yin, Hao Chen, Sheng‐Feng Yin, Dan Zhang, Xin‐Gui Tang, Vellaisamy A L Roy, Qi‐Jun Sun. Recent Progress on Heterojunction‐Based Memristors and Artificial Synapses for Low‐Power Neural Morphological Computing. Small 2025, 21 (17) https://doi.org/10.1002/smll.202412851
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  • Abstract

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    Figure 1

    Figure 1. Schematic diagram of the 50 nm thick Al:ZnO/n-type Si planar photodiode. Interfacial defects establish pathways for channeling photogenerated carriers (holes) in the depletion region of the n-type Si.

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    Figure 2

    Figure 2. Schematic fabrication procedures of the 50 nm thick Al:ZnO/n-type Si planar photodiode and its optical microscope image.

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    Figure 3

    Figure 3. (a) Atomic force microscope images of the Al:ZnO thin film under different oxygen partial pressures (Poxygen): 0 mTorr (top), 10 mTorr (middle), and 50 mTorr (bottom). (b) High-resolution X-ray diffraction patterns under different oxygen partial pressures (Poxygen): 0 (blue), 10 (cyan), and 50 mTorr (purple). X-ray photoelectron spectroscopy measurements were performed at various oxygen partial pressures (Poxygen): (c) 0 mTorr, (d) 10, and (e) 50 mTorr. Circles represent the experimental data points, and the fitting curves (solid lines) are the sum of the individual components (O1, O2, and O3) obtained by deconvoluting the spectra. Red lines represent the baseline for each spectrum. O1, the peak at the lowest binding energy, corresponds to O2– ions in the wurtzite ZnO lattice. O2, the peak at the intermediate binding energy, is related to oxygen vacancies within the oxygen-deficient regions of the ZnO matrix, with a more pronounced O2 peak signifying a higher concentration of defect states in Al:ZnO. O3, the peak at the highest binding energy, is associated with loosely bound oxygen species on the ZnO surface.

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    Figure 4

    Figure 4. (a) Dark I–V characteristics and (b) time-resolved photocurrent (I–t) graphs of the photodiodes with different oxygen partial pressures (Poxygen): 0 (blue), 10 (cyan), and 50 mTorr (purple). (c) Energy band structure of the Al:ZnO/Si Schottky junction is illustrated under both dark conditions (cases I and II) and illuminated conditions (cases III and IV), respectively. Ec, Ev, and Ef represent the energy of the conduction band, the valence band, and the Fermi level, respectively. Eg_Al: ZnO and Eg_Si denote the Al:ZnO and the Si band gap. Blue and red circles indicate the holes and electrons, respectively, under illuminated conditions. In the oxygen-deficient condition, numerous interfacial defect energy states, denoted as “Et_interface” are generated on the surface of the Al:ZnO/Si Schottky junction (case I). As light illuminates the junction, photogenerated electrons move toward the Ti/Au electrode, while for the charge-neutral condition, holes move to the Al:ZnO using Et_interface (case III). On the other hand, in the oxygen-sufficient condition, a “no defect” condition is established (case II). Since hole transport is limited, photoexcited electron–hole pairs cannot contribute to photocurrent (case IV).

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    Figure 5

    Figure 5. (a) Dark I–V characteristics of the photodiodes with Si substrate resistivity values of 0.01, 0.19, 9.83, and 18.6 Ω·cm. (b) Energy band structure of the Al:ZnO/Si Schottky junction for the lightly doped n-type Si substrate (I) and the heavily doped n+-type Si substrate (II). ϕSB denotes the Schottky barrier of the photodiode. WLD and Vbi_LD represent the depletion width and the built-in potential of the lightly doped Si-based photodiode, respectively, while WHD and Vbi_HD denote the depletion width and the built-in potential of the heavily doped Si-based photodiode, respectively.

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    Figure 6

    Figure 6. (a) Time-resolved photocurrent (I–t) graphs of photodiodes with Si substrate resistivity values of 0.01, 0.19, 9.83, and 18.6 Ω·cm. Photocurrent was measured at a wavelength of 1310 nm, a frequency of 1 kHz, and an input light intensity of 9.61 mW. (b) Rise and fall times of photodiodes with Si substrate resistivity values of 0.01, 0.19, 9.83, and 18.6 Ω·cm and a commercial Ge PD (gray dotted line). Rise and fall times are calculated using the data within the magenta dashed lines at 0.1 and 0.9.

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    Figure 7

    Figure 7. (a) Dark I–V characteristics and (b) photoresponsivity at wavelengths of 904 (violet), 1310 (pink), and 1550 nm (olive) of the photodiodes with varying Al:ZnO film thicknesses of 20, 50, 100, and 150 nm. (c) Transmission spectrum of Al:ZnO films deposited on a quartz substrate with thicknesses of 20, 50, and 150 nm, respectively.

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    Figure 8

    Figure 8. Frequency response of the photodiode at a wavelength of 1310 nm under zero-bias conditions. 3 dB bandwidth (f3dB) is indicated by the magenta dotted lines at 150 kHz. The inset of the figure shows the relationship between the on–off current and laser intensity.

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    10. 10
      Chen, H.; Galili, M.; Verheyen, P.; De Heyn, P.; Lepage, G.; De Coster, J.; Balakrishnan, S.; Absil, P.; Oxenlowe, L.; Van Campenhout, J. 100-Gbps RZ Data Reception in 67-GHz Si-Contacted Germanium Waveguide p-i-n Photodetectors. Journal of Lightwave Technology 2017, 35 (4), 722– 726,  DOI: 10.1109/JLT.2016.2593942
      There is no corresponding record for this reference.
    11. 11
      Brouckaert, J.; Roelkens, G.; Van Thourhout, D.; Baets, R. Thin-Film III–V Photodetectors Integrated on Silicon-on-Insulator Photonic ICs. Journal of Lightwave Technology 2007, 25 (4), 1053– 1060,  DOI: 10.1109/JLT.2007.891172
      There is no corresponding record for this reference.
    12. 12
      Casalino, M.; Coppola, G.; Iodice, M.; Rendina, I.; Sirleto, L. Near-Infrared Sub-Bandgap All-Silicon Photodetectors: State of the Art and Perspectives. Sensors 2010, 10 (12), 10571– 10600,  DOI: 10.3390/s101210571
      12
      Near-infrared sub-bandgap all-silicon photodetectors: state of the art and perspectives
      Casalino Maurizio; Coppola Giuseppe; Iodice Mario; Rendina Ivo; Sirleto Luigi
      Sensors (Basel, Switzerland) (2010), 10 (12), 10571-600 ISSN:.
      Due to recent breakthroughs, silicon photonics is now the most active discipline within the field of integrated optics and, at the same time, a present reality with commercial products available on the market. Silicon photodiodes are excellent detectors at visible wavelengths, but the development of high-performance photodetectors on silicon CMOS platforms at wavelengths of interest for telecommunications has remained an imperative but unaccomplished task so far. In recent years, however, a number of near-infrared all-silicon photodetectors have been proposed and demonstrated for optical interconnect and power-monitoring applications. In this paper, a review of the state of the art is presented. Devices based on mid-bandgap absorption, surface-state absorption, internal photoemission absorption and two-photon absorption are reported, their working principles elucidated and their performance discussed and compared.
    13. 13
      Zhao, Z.; Zhang, Z.; Jing, J.; Gao, R.; Liao, Z.; Zhang, W.; Liu, G.; Wang, Y.; Wang, K.; Xue, C. Black silicon for near-infrared and ultraviolet photodetection: A review. APL Mater. 2023, 11 (2), 021107  DOI: 10.1063/5.0133770
      There is no corresponding record for this reference.
    14. 14
      Mailoa, J. P.; Akey, A. J.; Simmons, C. B.; Hutchinson, D.; Mathews, J.; Sullivan, J. T.; Recht, D.; Winkler, M. T.; Williams, J. S.; Warrender, J. M. Room-temperature sub-band gap optoelectronic response of hyperdoped silicon. Nat. Commun. 2014, 5 (1), 3011,  DOI: 10.1038/ncomms4011
      14
      Room-temperature sub-band gap optoelectronic response of hyperdoped silicon
      Mailoa Jonathan P; Akey Austin J; Simmons Christie B; Sullivan Joseph T; Buonassisi Tonio; Hutchinson David; Persans Peter D; Mathews Jay; Warrender Jeffrey M; Recht Daniel; Aziz Michael J; Winkler Mark T; Williams James S
      Nature communications (2014), 5 (), 3011 ISSN:.
      Room-temperature infrared sub-band gap photoresponse in silicon is of interest for telecommunications, imaging and solid-state energy conversion. Attempts to induce infrared response in silicon largely centred on combining the modification of its electronic structure via controlled defect formation (for example, vacancies and dislocations) with waveguide coupling, or integration with foreign materials. Impurity-mediated sub-band gap photoresponse in silicon is an alternative to these methods but it has only been studied at low temperature. Here we demonstrate impurity-mediated room-temperature sub-band gap photoresponse in single-crystal silicon-based planar photodiodes. A rapid and repeatable laser-based hyperdoping method incorporates supersaturated gold dopant concentrations on the order of 10(20) cm(-3) into a single-crystal surface layer ~150 nm thin. We demonstrate room-temperature silicon spectral response extending to wavelengths as long as 2,200 nm, with response increasing monotonically with supersaturated gold dopant concentration. This hyperdoping approach offers a possible path to tunable, broadband infrared imaging using silicon at room temperature.
    15. 15
      Jia, Z.; Wu, Q.; Jin, X.; Huang, S.; Li, J.; Yang, M.; Huang, H.; Yao, J.; Xu, J. Highly responsive tellurium-hyperdoped black silicon photodiode with single-crystalline and uniform surface microstructure. Opt. Express 2020, 28 (4), 5239,  DOI: 10.1364/OE.385887
      15
      Highly responsive tellurium-hyperdoped black silicon photodiode with single-crystalline and uniform surface microstructure
      Jia Zixi; Wu Qiang; Jin Xiaorong; Huang Song; Li Jinze; Yang Ming; Huang Hui; Yao Jianghong; Xu Jingjun
      Optics express (2020), 28 (4), 5239-5247 ISSN:.
      Femtosecond laser hyperdoped silicon, also known as the black silicon (BS), has a large number of defects and damages, which results in unstable and undesirable optical and electronic properties in photonics platform and optoelectronic integrated circuits (OEICs). We propose a novel method that elevates the substrate temperature during the femtosecond laser irradiation and fabricates tellurium (Te) hyperdoped BS photodiodes with high responsivity and low dark current. At 700 K, uniform microstructures with single crystalline were formed in the hyperdoped layer. The velocity of cooling and resolidification is considered as an important role in the formation of a high-quality crystal after irradiation by the femtosecond laser. Because of the high crystallinity and the Te hyperdoping, a photodiode made from BS processed at 700 K has a maximum responsivity of 120.6 A/W at 1120 nm, which is far beyond the previously reported Te-doped silicon photodetectors. In particular, the responsivity of the BS photodiode at 1300 nm and 1550 nm is 43.9 mA/W and 56.8 mA/W with low noise, respectively, which is valuable for optical communication and interconnection. Our result proves that hyperdoping at a high substrate temperature has great potential for femtosecond-laser-induced semiconductor modification, especially for the fabrication of photodetectors in the silicon-based photonic integration circuits.
    16. 16
      Fu, J.; Yang, D.; Yu, X. Hyperdoped Crystalline Silicon for Infrared Photodetectors by Pulsed Laser Melting: A Review. Phys. Status Solidi (a) 2022, 219 (14), 2100772  DOI: 10.1002/pssa.202100772
      There is no corresponding record for this reference.
    17. 17
      Li, C.; Zhao, J.-H.; Liu, X.-H.; Ren, Z.-Y.; Yang, Y.; Chen, Z.-G.; Chen, Q.-D.; Sun, H.-B. Record-Breaking-High-Responsivity Silicon Photodetector at Infrared 1.31 and 1.55 μm by Argon Doping Technique. IEEE Trans. Electron Devices 2023, 70 (5), 2364– 2369,  DOI: 10.1109/TED.2023.3261823
      There is no corresponding record for this reference.
    18. 18
      Berencén, Y.; Prucnal, S.; Liu, F.; Skorupa, I.; Hübner, R.; Rebohle, L.; Zhou, S.; Schneider, H.; Helm, M.; Skorupa, W. Room-temperature short-wavelength infrared Si photodetector. Sci. Rep. 2017, 7 (1), 43688,  DOI: 10.1038/srep43688
      18
      Room-temperature short-wavelength infrared Si photodetector
      Berencen Yonder; Prucnal Slawomir; Liu Fang; Skorupa Ilona; Hubner Rene; Rebohle Lars; Zhou Shengqiang; Schneider Harald; Helm Manfred; Skorupa Wolfgang; Helm Manfred
      Scientific reports (2017), 7 (), 43688 ISSN:.
      The optoelectronic applications of Si are restricted to the visible and near-infrared spectral range due to its 1.12 eV-indirect band gap. Sub-band gap light detection in Si, for instance, has been a long-standing scientific challenge for many decades since most photons with sub-band gap energies pass through Si unabsorbed. This fundamental shortcoming, however, can be overcome by introducing non-equilibrium deep-level dopant concentrations into Si, which results in the formation of an impurity band allowing for strong sub-band gap absorption. Here, we present steady-state room-temperature short-wavelength infrared p-n photodiodes from single-crystalline Si hyperdoped with Se concentrations as high as 9 × 10(20) cm(-3), which are introduced by a robust and reliable non-equilibrium processing consisting of ion implantation followed by millisecond-range flash lamp annealing. We provide a detailed description of the material properties, working principle and performance of the photodiodes as well as the main features in the studied wavelength region. This work fundamentally contributes to establish the short-wavelength infrared detection by hyperdoped Si in the forefront of the state-of-the-art of short-IR Si photonics.
    19. 19
      Qiu, X.; Yu, X.; Yuan, S.; Gao, Y.; Liu, X.; Xu, Y.; Yang, D. Trap Assisted Bulk Silicon Photodetector with High Photoconductive Gain, Low Noise, and Fast Response by Ag Hyperdoping. Adv. Opt. Mater. 2018, 6 (3), 1700638  DOI: 10.1002/adom.201700638
      There is no corresponding record for this reference.
    20. 20
      Wang, M.; García-Hemme, E.; Berencén, Y.; Hübner, R.; Xie, Y.; Rebohle, L.; Xu, C.; Schneider, H.; Helm, M.; Zhou, S. Silicon-Based Intermediate-Band Infrared Photodetector Realized by Te Hyperdoping. Adv. Opt. Mater. 2021, 9 (4), 2001546  DOI: 10.1002/adom.202001546
      There is no corresponding record for this reference.
    21. 21
      Li, C.-H.; Wang, X.-P.; Zhao, J.-H.; Zhang, D.-Z.; Yu, X.-Y.; Li, X.-B.; Feng, J.; Chen, Q.-D.; Ruan, S.-P.; Sun, H.-B. Black silicon IR photodiode supersaturated with nitrogen by femtosecond laser irradiation. IEEE Sensors Journal 2018, 18 (9), 3595– 3601,  DOI: 10.1109/JSEN.2018.2812730
      There is no corresponding record for this reference.
    22. 22
      Li, X.; Deng, Z.; Li, J.; Li, Y.; Guo, L.; Jiang, Y.; Ma, Z.; Wang, L.; Du, C.; Wang, Y. Hybrid nano-scale Au with ITO structure for a high-performance near-infrared silicon-based photodetector with ultralow dark current. Photonics Research 2020, 8 (11), 1662– 1670,  DOI: 10.1364/PRJ.398450
      There is no corresponding record for this reference.
    23. 23
      Kim, H.-S.; Kumar, M. D.; Patel, M.; Kim, J. High-performing ITO/CuO/n-Si photodetector with ultrafast photoresponse. Sensors and Actuators A: Physical 2016, 252, 35– 41,  DOI: 10.1016/j.sna.2016.11.014
      There is no corresponding record for this reference.
    24. 24
      Akgul, G.; Akgul, F. A.; Mulazimoglu, E.; Unalan, H. E.; Turan, R. Fabrication and characterization of copper oxide-silicon nanowire heterojunction photodiodes. J. Phys. D: Appl. Phys. 2014, 47 (6), 065106  DOI: 10.1088/0022-3727/47/6/065106
      There is no corresponding record for this reference.
    25. 25
      Kim, J.; Krayer, L. J.; Garrett, J. L.; Munday, J. N. Interfacial Defect-Mediated Near-Infrared Silicon Photodetection with Metal Oxides. ACS Appl. Mater. Interfaces 2019, 11 (50), 47516– 47524,  DOI: 10.1021/acsami.9b14953
      There is no corresponding record for this reference.
    26. 26
      Mondal, S.; Halder, S.; Basak, D. Ultrafast and Ultrabroadband UV–vis-NIR Photosensitivity under Reverse and Self-Bias Conditions by n+-ZnO/n-Si Isotype Heterojunction with > 1 kHz Bandwidth. ACS Applied Electronic Materials 2023, 5 (2), 1212– 1223,  DOI: 10.1021/acsaelm.2c01668
      There is no corresponding record for this reference.
    27. 27
      Lv, P.; Zhang, X.; Zhang, X.; Deng, W.; Jie, J. High-Sensitivity and Fast-Response Graphene/Crystalline Silicon Schottky Junction-Based Near-IR Photodetectors. IEEE Electron Device Lett. 2013, 34 (10), 1337– 1339,  DOI: 10.1109/LED.2013.2275169
      27
      High-sensitivity and fast-response graphene/crystalline silicon Schottky junction-based near-IR photodetectors
      Lv, Peng; Zhang, Xiujuan; Zhang, Xiwei; Deng, Wei; Jie, Jiansheng
      IEEE Electron Device Letters (2013), 34 (10), 1337-1339CODEN: EDLEDZ; ISSN:0741-3106. (Institute of Electrical and Electronics Engineers)
      Schottky junction near-IR photodetectors were constructed by combing monolayer graphene (MLG) film and bulk silicon. Notably, the device could operate at zero external voltage bias because of the strong photovoltaic behavior of the MLG/Si Schottky junction, giving rise to high responsivity and detectivity of 29 mAW-1 and 3.9 × 1011 cmHz1/2W-1, resp., at room temp. Time response measurement revealed a high response speed of 100 μs, which allowed the device following a fast varied light with frequency up to 2100 Hz. In addn., the device showed great potential for low light detection with intensity <1 nWcm-2 at 10 K.
    28. 28
      Güsken, N. A.; Lauri, A.; Li, Y.; Matsui, T.; Doiron, B.; Bower, R.; Regoutz, A.; Mihai, A.; Petrov, P. K.; Oulton, R. F. TiO2–x-Enhanced IR Hot Carrier Based Photodetection in Metal Thin Film–Si Junctions. ACS Photonics 2019, 6 (4), 953– 960,  DOI: 10.1021/acsphotonics.8b01639
      There is no corresponding record for this reference.
    29. 29
      Haško, D.; Bruncko, J. AFM surface analysis of ZnO layers prepared by pulsed laser deposition at different oxygen pressures. Vacuum 2009, 84 (1), 166– 169,  DOI: 10.1016/j.vacuum.2009.05.009
      There is no corresponding record for this reference.
    30. 30
      Zhang, Z.; Zhou, F.; Wei, X.; Liu, M.; Sun, G.; Chen, C.; Xue, C.; Zhuang, H.; Man, B. Effects of oxygen pressures on pulsed laser deposition of ZnO films. Physica E: Low-dimensional Systems and Nanostructures 2007, 39 (2), 253– 257,  DOI: 10.1016/j.physe.2007.05.028
      There is no corresponding record for this reference.
    31. 31
      Kumar, V.; Ntwaeaborwa, O.; Swart, H. Effect of oxygen partial pressure during pulsed laser deposition on the emission of Eu doped ZnO thin films. Physica B: Condensed Matter 2020, 576, 411713  DOI: 10.1016/j.physb.2019.411713
      There is no corresponding record for this reference.
    32. 32
      Chen, T.; Liu, S.-Y.; Xie, Q.; Detavernier, C.; Van Meirhaeghe, R.; Qu, X.-P. The effects of deposition temperature and ambient on the physical and electrical performance of DC-sputtered n-ZnO/p-Si heterojunction. Appl. Phys. A: Mater. Sci. Process. 2010, 98, 357– 365,  DOI: 10.1007/s00339-009-5386-9
      32
      The effects of deposition temperature and ambient on the physical and electrical performance of DC-sputtered n-ZnO/p-Si heterojunction
      Chen, Tao; Liu, Shu-Yi; Xie, Qi; Detavernier, Christophe; Meirhaeghe, R. L.; Qu, Xin-Ping
      Applied Physics A: Materials Science & Processing (2010), 98 (2), 357-365CODEN: APAMFC; ISSN:0947-8396. (Springer)
      The effects of deposition conditions on the phys. and elec. performance of the n-ZnO/p-Si heterojunction were systematically investigated. ZnO films were deposited on the Si and glass substrates using d.c. magnetron sputtering with various ambients and substrate temps. The results showed that increasing the O2 content and substrate temp. during the deposition process could improve the crystallinity and stoichiometry of the ZnO film, resulting in a lower carrier concn. and higher resistivity. The elec. properties of the n-ZnO/p-Si heterojunctions were also affected by the deposition parameters. For the junctions fabricated in the pure Ar ambient, the sample deposited at room temp. (RT) showed Ohmic behavior, while the one deposited at 300° exhibited poor rectifying behavior. On the other hand, the junctions fabricated in the O2/Ar ambient possessed ideal rectifying behaviors. The different carrier transport mechanisms for the heterojunctions under forward and reverse bias were systematically studied using a high temp. current-voltage (I-V) measurement. The recombination-tunneling current showed temp. insensitive performance while the space-charge limited current (SCLC) changed with the measurement temp.
    33. 33
      Gandla, S.; Gollu, S. R.; Sharma, R.; Sarangi, V.; Gupta, D. Dual role of boron in improving electrical performance and device stability of low temperature solution processed ZnO thin film transistors. Appl. Phys. Lett. 2015, 107 (15), 152102,  DOI: 10.1063/1.4933304
      33
      Dual role of boron in improving electrical performance and device stability of low temperature solution processed ZnO thin film transistors
      Gandla, Srinivas; Gollu, Sankara Rao; Sharma, Ramakant; Sarangi, Venkateshwarlu; Gupta, Dipti
      Applied Physics Letters (2015), 107 (15), 152102/1-152102/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      In this paper, we have demonstrated the dual role of boron doping in enhancing the device performance parameters as well as the device stability in low temps. (200°C) sol-gel processed ZnO thin film transistors (TFTs). Our studies suggest that boron is able to act as a carrier generator and oxygen vacancy suppressor simultaneously. Boron-doped ZnO TFTs with 8 mol.% of boron concn. demonstrated field-effect mobility value of 1.2 cm2 V-1 s-1 and threshold voltage of 6.2 V, resp. Further, these devices showed lower shift in threshold voltage during the hysteresis and bias stress measurements as compared to undoped ZnO TFTs. (c) 2015 American Institute of Physics.
    34. 34
      Abliz, A.; Huang, C.-W.; Wang, J.; Xu, L.; Liao, L.; Xiao, X.; Wu, W.-W.; Fan, Z.; Jiang, C.; Li, J. Rational design of ZnO:H/ZnO bilayer structure for high-performance thin-film transistors. ACS Appl. Mater. Interfaces 2016, 8 (12), 7862– 7868,  DOI: 10.1021/acsami.5b10778
      34
      Rational Design of ZnO:H/ZnO Bilayer Structure for High-Performance Thin-Film Transistors
      Abliz, Ablat; Huang, Chun-Wei; Wang, Jingli; Xu, Lei; Liao, Lei; Xiao, Xiangheng; Wu, Wen-Wei; Fan, Zhiyong; Jiang, Changzhong; Li, Jinchai; Guo, Shishang; Liu, Chuansheng; Guo, Tailiang
      ACS Applied Materials & Interfaces (2016), 8 (12), 7862-7868CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The intriguing properties of zinc oxide-based semiconductors are being extensively studied as they are attractive alternatives to current silicon-based semiconductors for applications in transparent and flexible electronics. Although they have promising properties, significant improvements on performance and elec. reliability of ZnO-based thin film transistors (TFTs) should be achieved before they can be applied widely in practical applications. This work demonstrates a rational and elegant design of TFT, composed of poly cryst. ZnO:H/ZnO bilayer structure without using other metal elements for doping. The field-effect mobility and gate bias stability of the bilayer structured devices have been improved. In this device structure, the hydrogenated ultrathin ZnO:H active layer (∼3 nm) could provide suitable carrier concn. and decrease the interface trap d., while thick pure-ZnO layer could control channel conductance. Based on this novel structure, a high field-effect mobility of 42.6 cm2 V-1 s-1, a high on/off current ratio of 108 and a small subthreshold swing of 0.13 V dec-1 have been achieved. Addnl., the bias stress stability of the bilayer structured devices is enhanced compared to the simple single channel layer ZnO device. These results suggest that the bilayer ZnO:H/ZnO TFTs have a great potential for low-cost thin-film electronics.
    35. 35
      Tu, Y.; Chen, S.; Li, X.; Gorbaciova, J.; Gillin, W. P.; Krause, S.; Briscoe, J. Control of oxygen vacancies in ZnO nanorods by annealing and their influence on ZnO/PEDOT: PSS diode behaviour. Journal of Materials Chemistry C 2018, 6 (7), 1815– 1821,  DOI: 10.1039/C7TC04284A
      There is no corresponding record for this reference.
    36. 36
      Vempati, S.; Mitra, J.; Dawson, P. One-step synthesis of ZnO nanosheets: a blue-white fluorophore. Nanoscale Res. Lett. 2012, 7, 470,  DOI: 10.1186/1556-276X-7-470
      36
      One-step synthesis of ZnO nanosheets: a blue-white fluorophore
      Vempati Sesha; Mitra Joy; Dawson Paul
      Nanoscale research letters (2012), 7 (1), 470 ISSN:.
      Zinc oxide is synthesised at low temperature (80°C) in nanosheet geometry using a substrate-free, single-step, wet-chemical method and is found to act as a blue-white fluorophore. Investigation by atomic force microscopy, electron microscopy, and X-ray diffraction confirms zinc oxide material of nanosheet morphology where the individual nanosheets are polycrystalline in nature with the crystalline structure being of wurtzite character. Raman spectroscopy indicates the presence of various defects, while photoluminescence measurements show intense green (centre wavelength approximately 515 nm) blue (approximately 450 nm), and less dominant red (approximately 640 nm) emissions due to a variety of vacancy and interstitial defects, mostly associated with surfaces or grain boundaries. The resulting colour coordinate on the CIE-1931 standard is (0.23, 0.33), demonstrating potential for use as a blue-white fluorescent coating in conjunction with ultraviolet emitting LEDs. Although the defects are often treated as draw-backs of ZnO, here we demonstrate useful broadband visible fluorescence properties in as-prepared ZnO.
    37. 37
      Tong, C.; Kumar, M.; Yun, J.-H.; Kim, J.; Kim, S. J. High-Quality ITO/Al-ZnO/n-Si Heterostructures with Junction Engineering for Improved Photovoltaic Performance. Applied Sciences 2020, 10 (15), 5285,  DOI: 10.3390/app10155285
      There is no corresponding record for this reference.
    38. 38
      Swami, S. K.; Khan, J. I.; Dutta, V.; Lee, J.; Laquai, F.; Chaturvedi, N. Spray-Deposited Aluminum-Doped Zinc Oxide as an Efficient Electron Transport Layer for Inverted Organic Solar Cells. ACS Applied Energy Materials 2023, 6 (5), 2906– 2913,  DOI: 10.1021/acsaem.2c03858
      There is no corresponding record for this reference.
    39. 39
      Lee, E.; Park, J.; Yim, M.; Kim, Y.; Yoon, G. Characteristics of piezoelectric ZnO/AlN–stacked flexible nanogenerators for energy harvesting applications. Appl. Phys. Lett. 2015, 106 (2), 023901  DOI: 10.1063/1.4904270
      There is no corresponding record for this reference.
    40. 40
      Jia, J.; Takasaki, A.; Oka, N.; Shigesato, Y. Experimental observation on the Fermi level shift in polycrystalline Al-doped ZnO films. J. Appl. Phys. 2012, 112 (1), 013718  DOI: 10.1063/1.4733969
      There is no corresponding record for this reference.
    41. 41
      Kim, T.; Choo, D. C.; No, Y.; Choi, W.; Choi, E. H. High work function of Al-doped zinc-oxide thin films as transparent conductive anodes in organic light-emitting devices. Appl. Surf. Sci. 2006, 253 (4), 1917– 1920,  DOI: 10.1016/j.apsusc.2006.03.032
      There is no corresponding record for this reference.
    42. 42
      Kim, J.; Carnemolla, E. G.; DeVault, C.; Shaltout, A. M.; Faccio, D.; Shalaev, V. M.; Kildishev, A. V.; Ferrera, M.; Boltasseva, A. Dynamic control of nanocavities with tunable metal oxides. Nano Lett. 2018, 18 (2), 740– 746,  DOI: 10.1021/acs.nanolett.7b03919
      42
      Dynamic Control of Nanocavities with Tunable Metal Oxides
      Kim, Jongbum; Carnemolla, Enrico G.; DeVault, Clayton; Shaltout, Amr M.; Faccio, Daniele; Shalaev, Vladimir M.; Kildishev, Alexander V.; Ferrera, Marcello; Boltasseva, Alexandra
      Nano Letters (2018), 18 (2), 740-746CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Fabry-Perot metal-insulator-metal (MIM) nanocavities are widely used in nanophotonic applications due to their extraordinary electromagnetic properties and deeply subwavelength dimensions. However, the spectral response of nanocavities is usually controlled by the spatial sepn. between the 2 reflecting mirrors and the spacer's refractive index. Here, the authors demonstrate static and dynamic control of Fabry-Perot nanocavities by inserting a plasmonic metasurface, as a passive element, and a Ga doped-Zn oxide (Ga:ZnO) layer as a dynamically tunable component within the nanocavities' spacer. Specifically, by changing the design of the Ag metasurface one can statically tailor the nanocavity response, tuning the resonance up to 200 nm. To achieve the dynamic tuning, the authors use the large nonlinear response of the Ga:ZnO layer near the epsilon near zero wavelength to enable effective subpicosecond (<400 fs) optical modulation (80%) at reasonably low pump fluence levels (9 mJ/cm2). The authors demonstrate a 15. nm red-shift of a near-IR Fabry-Perot resonance (λ ≃ 1.16 μm) by using a degenerate pump probe technique. The authors also study the carrier dynamics of Ga:ZnO under intraband photoexcitation via the electronic band structure calcd. from 1st-principles d. functional method. This work provides a versatile approach to design metal nanocavities by using both the phase variation with plasmonic metasurfaces and the strong nonlinear response of metal oxides. Tailorable and dynamically controlled nanocavities could pave the way to the development of the next generation of ultrafast nanophotonic devices.
    43. 43
      Campabadal, F.; Milian, V.; Aymerich-Humet, X. Trap-Assisted Tunneling in MIS and Schottky Structures. Physica Status Solidi (a) 1983, 79 (1), 223– 236,  DOI: 10.1002/pssa.2210790125
      There is no corresponding record for this reference.
    44. 44
      Racko, J.; Pecháček, J.; Mikolášek, M.; Benko, P.; Grmanova, A.; Harmatha, L.; Breza, J. Trap-assisted tunneling in the Schottky barrier. Radioengineering 2013, 22 (1), 240– 244
      There is no corresponding record for this reference.
    45. 45
      Masetti, G.; Severi, M.; Solmi, S. Modeling of carrier mobility against carrier concentration in arsenic-, phosphorus-, and boron-doped silicon. IEEE Trans. Electron Devices 1983, 30 (7), 764– 769,  DOI: 10.1109/T-ED.1983.21207
      There is no corresponding record for this reference.
    46. 46
      Kim, G.; Kim, H.; Jeon, Y.-U.; Kim, I. S.; Kim, S. J.; Kim, S.; Kim, J. Scalable hot carrier–assisted silicon photodetector array based on ultrathin gold film. Nanophotonics 2024, 13 (7), 1049– 1057,  DOI: 10.1515/nanoph-2023-0656
      There is no corresponding record for this reference.
    47. 47
      Huang, S.; Wu, Q.; Jia, Z.; Jin, X.; Fu, X.; Huang, H.; Zhang, X.; Yao, J.; Xu, J. Black Silicon Photodetector with Excellent Comprehensive Properties by Rapid Thermal Annealing and Hydrogenated Surface Passivation. Adv. Opt. Mater. 2020, 8 (7), 1901808  DOI: 10.1002/adom.201901808
      47
      Black Silicon Photodetector with Excellent Comprehensive Properties by Rapid Thermal Annealing and Hydrogenated Surface Passivation
      Huang, Song; Wu, Qiang; Jia, Zixi; Jin, Xiaorong; Fu, Xianhui; Huang, Hui; Zhang, Xiaodan; Yao, Jianghong; Xu, Jingjun
      Advanced Optical Materials (2020), 8 (7), 1901808CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Silicon-based photodetectors show attractive prospects due to their convenient prepn., high detectivity, and complementary metal-oxide-semiconductor compatibility. However, they are currently limited by low responsivity and sharp decay at sub-bandgap wavelength. Although the aforementioned limitation can be partly solved by femtosecond laser processing, the surface defects and carrier activation rates result in a large dark current and narrow spectral response, which are unsatisfactory. Herein, rapid thermal annealing and hydrogenated surface passivation are introduced to elevate the broad-bandgap responsivity and signal to noise ratio and to suppress the dark current. At optimal conditions, a sub-bandgap responsivity of 0.80 A W-1 for 1550 nm at 20 V at room temp. is obtained, comparable with com. germanium photodiodes and much higher than previously reported silicon photodiodes. Moreover, the prepd. photodetector responded to spectral range from 400 to 1600 nm, with responsivity reaching 1097.60 A W-1 for 1080 nm at 20 V, which is the highest in reported silicon photodetectors. Simultaneously, the device shows competitive detectivity (1.22 × 1014 Jones at -5 V) due to the post-processing procedures and suppressed dark current (7.8μA at -5 V). The results show great prospects for black silicon in IR light detection, night vision imaging, and fiber-optic communication.
    48. 48
      Park, M.; Kang, G.; Ko, H. Plasmonic-tape-attached multilayered MoS2 film for near-infrared photodetection. Sci. Rep. 2020, 10 (1), 11340,  DOI: 10.1038/s41598-020-68127-7
      48
      Plasmonic-tape-attached multilayered MoS2 film for near-infrared photodetection
      Park, Minji; Kang, Gumin; Ko, Hyungduk
      Scientific Reports (2020), 10 (1), 11340CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
      Molybdenum disulfide has been intensively studied as a promising material for photodetector applications because of its excellent elec. and optical properties. We report a multilayer MoS2 film attached with a plasmonic tape for near-IR (NIR) detection. MoS2 flakes are chem. exfoliated and transferred onto a polymer substrate, and silver nanoparticles (AgNPs) dewetted thermally on a substrate are transferred onto a Scotch tape. The Scotch tape with AgNPs is attached directly and simply onto the MoS2 flakes. Consequently, the NIR photoresponse of the MoS2 device is critically enhanced. The proposed tape transfer method enables the formation of plasmonic structures on arbitrary substrates, such as a polymer substrate, without requiring a high-tempareture process. The performance of AgNPs-MoS2 photodetectors is approx. four times higher than that of bare MoS2 devices.
    49. 49
      Qi, Z.; Zhai, Y.; Wen, L.; Wang, Q.; Chen, Q.; Iqbal, S.; Chen, G.; Xu, J.; Tu, Y. Au nanoparticle-decorated silicon pyramids for plasmon-enhanced hot electron near-infrared photodetection. Nanotechnology 2017, 28 (27), 275202,  DOI: 10.1088/1361-6528/aa74a3
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      Au nanoparticle-decorated silicon pyramids for plasmon-enhanced hot electron nearinfrared photodetection
      Qi, Zhiyang; Zhai, Yusheng; Wen, Long; Wang, Qilong; Chen, Qin; Iqbal, Sami; Chen, Guangdian; Xu, Ji; Tu, Yan
      Nanotechnology (2017), 28 (27), 275202/1-275202/9CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      The heterojunction between metal and silicon (Si) is an attractive route to extend the response of Si-based photodiodes into the near-IR (NIR) region, so-called Schottky barrier diodes. Photons absorbed into a metallic nanostructure excite the surface plasmon resonances (SPRs), which can be damped non-radiatively through the creation of hot electrons. Unfortunately, the quantum efficiency of hot electron detectors remains low due to low optical absorption and poor electron injection efficiency. In this study, we propose an efficient and low-cost plasmonic hot electron NIR photodetector based on a Au nanoparticle (Au NP)-decorated Si pyramid Schottky junction. The large-area and lithog.-free photodetector is realized by using an anisotropic chem. wet etching and rapid thermal annealing (RTA) of a thin Au film. We exptl. demonstrate that these hot electron detectors have broad photoresponsivity spectra in the NIR region of 1200-1475 nm, with a low dark current on the order of 10-5 A cm-2. The obsd. responsivities enable these devices to be competitive with other reported Si-based NIR hot electron photodetectors using perfectly periodic nanostructures. The improved performance is attributed to the pyramid surface which can enhance light trapping and the localized elec. field, and the nano-sized Au NPs which are beneficial for the tunneling of hot electrons. The simple and large-area prepn. processes make them suitable for large-scale thermophotovoltaic cell and low-cost NIR detection applications.
    50. 50
      Kim, C.; Yoo, T. J.; Chang, K. E.; Kwon, M. G.; Hwang, H. J.; Lee, B. H. Highly responsive near-infrared photodetector with low dark current using graphene/germanium Schottky junction with Al2O3 interfacial layer. Nanophotonics 2021, 10 (5), 1573– 1579,  DOI: 10.1515/nanoph-2021-0002
      There is no corresponding record for this reference.
    51. 51
      Wu, D.; Guo, J.; Wang, C.; Ren, X.; Chen, Y.; Lin, P.; Zeng, L.; Shi, Z.; Li, X. J.; Shan, C.-X. Ultrabroadband and High-Detectivity Photodetector Based on WS2/Ge Heterojunction through Defect Engineering and Interface Passivation. ACS Nano 2021, 15 (6), 10119– 10129,  DOI: 10.1021/acsnano.1c02007
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      Ultrabroadband and High-Detectivity Photodetector Based on WS2/Ge Heterojunction through Defect Engineering and Interface Passivation
      Wu, Di; Guo, Jiawen; Wang, Chaoqiang; Ren, Xiaoyan; Chen, Yongsheng; Lin, Pei; Zeng, Longhui; Shi, Zhifeng; Li, Xin Jian; Shan, Chong-Xin; Jie, Jiansheng
      ACS Nano (2021), 15 (6), 10119-10129CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Broadband photodetectors are of great importance for numerous optoelectronic applications. Two-dimensional (2D) tungsten disulfide (WS2), an important family member of transition-metal dichalcogenides (TMDs), has shown great potential for high-sensitivity photodetection due to its extraordinary properties. However, the inherent large bandgap of WS2 and the strong interface recombination impede the actualization of high-sensitivity broadband photodetectors. Here, we demonstrate the fabrication of an ultrabroadband WS2/Ge heterojunction photodetector through defect engineering and interface passivation. Thanks to the narrowed bandgap of WS2 induced by the vacancy defects, the effective surface modification with an ultrathin AlOx layer, and the well-designed vertical n-n heterojunction structure, the WS2/AlOx/Ge photodetector exhibits an excellent device performance in terms of a high responsivity of 634.5 mA/W, a large specific detectivity up to 4.3 x 1011 Jones, and an ultrafast response speed. Significantly, the device possesses an ultrawide spectral response spanning from deep UV (200 nm) to mid-wave IR (MWIR) of 4.6μm, along with a superior MWIR imaging capability at room temp. The detection range has surpassed the WS2-based photodetectors in previous reports and is among the broadest for TMD-based photodetectors. Our work provides a strategy for the fabrication of high-performance ultrabroadband photodetectors based on 2D TMD materials.
    52. 52
      Choi, W.; Cho, M. Y.; Konar, A.; Lee, J. H.; Cha, G.-B.; Hong, S. C.; Kim, S.; Kim, J.; Jena, D.; Joo, J. High-Detectivity Multilayer MoS2 Phototransistors with Spectral Response from Ultraviolet to Infrared. Adv. Mater. 2012, 24 (43), 5832– 5836,  DOI: 10.1002/adma.201201909
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      High-detectivity multilayer MoS2 phototransistors with spectral response from ultraviolet to infrared
      Choi, Woong; Cho, Mi Yeon; Konar, Aniruddha; Lee, Jong Hak; Cha, Gi-Beom; Hong, Soon Cheol; Kim, Sangsig; Kim, Jeongyong; Jena, Debdeep; Joo, Jinsoo; Kim, Sunkook
      Advanced Materials (Weinheim, Germany) (2012), 24 (43), 5832-5836CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      This study relatest to the optoelectronic properties of multilayer MoS2 TFTs and shows a compelling case of multilayer MoS2 phototransistors for applications in photodetectors. In particular, the interesting optoelectronic properties of our multilayer MoS2 phototransistors could potentially lead to their integration into touch screen panels for flat panel or flexible display devices.