Guided CdTe Nanowires Integrated into Fast Near-Infrared Photodetectors

Infrared photodetectors are essential devices for telecommunication and night vision technologies. Two frequently used materials groups for this technology are III–V and II–VI semiconductors, notably, mercury-cadmium-telluride alloys (MCT). However, growing them usually requires expensive substrates that can only be provided on small scales, and their large-scale production as crystalline nanostructures is challenging. In this paper, we present a two-stage process for creating aligned MCT nanowires (NWs). First, we report the growth of planar CdTe nanowires with controlled orientations on flat and faceted sapphire substrates via the vapor–liquid–solid (VLS) mechanism. We utilize this guided growth approach to parallelly integrate the NWs into fast near-infrared photodetectors with characteristic rise and fall times of ∼100 μs at room temperature. An epitaxial effect of the planar growth and the unique structure of the NWs, including size and composition, are suggested to explain the high performance of the devices. In the second stage, we show that cation exchange with mercury can be applied, resulting in a band gap narrowing of up to 55 meV, corresponding to an exchange of 2% Cd with Hg. This work opens new opportunities for creating small, fast, and sensitive infrared detectors with an engineered band gap operating at room temperature.

and (c), (d), (e) on R-plane sapphire.This analysis statistically supports the reported orientations for each (1102) plane, respectively.All the wires show the same growth direction.In the case of the epitaxial guided growth (on Rplane sapphire), all of the examined NWs exhibit uniform epitaxial relations to sapphire, with the same longitudinal and transversal planes.

Figure S2 .
Figure S2.A TEM image of a WZ-CdTe grown on top of R-plane sapphire.the extracted FFT pattern, from the area defined by the red square shows periodicity along a single direction, which is not enough for the determination of the crystal structure and orientation.

Figure S3 .
Figure S3.(a) SEM image of a non-planar NW grown on R-plane sapphire (yellow rectangle).(b) Cross-sectional TEM image of a non-planar CdTe NWs.The growth direction and crystal structure are similar to the planar NWs grown on R-plane sapphire [0001] in the WZ structure.(c) Cross-sectional TEM image of another non-planar CdTeNWs showing the parallel direction for ZB symmetry[111].It strengthens our understanding that the growth direction is dictated not exclusively by the substrate but also by the grown material, which plays a significant role.

Figure S4 .
Figure S4.Right, are additional EDS mapping for four wires in a particular lamella cut of NWs on annealed Mplane sapphire.The biphasic structure is clear and is a characteristic of all the examined.NWs.In the large image to the left, a Bismuth (Bi) map of an image of a NW cross-section in the region of the catalytic droplet.The droplet exhibits a high accumulation of the Bi in it, further, assuring its role as a co-catalyst aside from the gold.

Figure S5 .
Figure S5.(a) PL spectra were taken from different NWs grown on annealed M-plane and (b) on R-plane (1100) sapphire, showing deviations from one NW to another.These results correlated with the discussed crystal (1102) purity.While the CdTe NWs grown on annealed M-plane sapphire exhibit wide distribution ranges 800-820 nm due to differences in chemical composition, the NWs grown on R-plane sapphire show very narrow distribution with minimal deviations from one NW to another due to higher crystal purity.

Figure S6 .
Figure S6.Fitting of the PL spectra collected from single CdTe NW grown on annealed M-plane (a) and on (1100) R-plane (b) sapphire to a Voigt line shape with the respective fitting parameters reported in the paper.The (1102) fitting parameters support the narrower FWHM of the spectra taken from R-plane grown NWs.

Figure S7 .
Figure S7.(a) A sketch of gold pads in the size of 3x30 μm that are deposited via lithographic methods in controlled positions on the sapphire substrates prior to the growth process.(b) A sketch of NWs arrays grown from the edges of the lithographic gold pattern.(c) A sketch of photodetectors arrays created based on the previously grown NWs arrays.(d) An SEM image of the photodetectors array, and higher magnification of the channel showing 10 CdTe NWs bridging two gold electrodes.

Figure S8 .
Figure S8.(a) A SEM image of CdTe NWs after cation exchange in Hg 2+ solution.(b) EDS elemental map of typical NWs after cation exchange of Cadmium (magenta) and Tellurium (yellow) and (c) Oxygen (green) and aluminum (light blue) and (d) mercury (cyan) are presented.All scales are 2 μm.(e) The resulting spectrum showing clear signals of mercury, cadmium, and tellurium from the NWs.(f) Quantification of the elements calculated using the spectrum for the mercury, cadmium, and tellurium content in a single NW.

Figure S9 .
Figure S9.Main results of the cation exchange experiment were done by using HgCl 2 dissolved in EtOH solution.(a) The change in near band edge emission by peak position of eight different nanowires went over a cation exchange reaction in HgCl 2 .Here, a redshift is also observed in all examined wires up to 19 nm, corresponding to 36 meV.(b) A sample spectrum of NW number #8, reported in (a), shows the maximal bandgap narrowing between the starting nanowire (blue) and the exchanged nanowire (red), corresponding 36 meV.(c)-(h) Raman spectra of six different nanowires went over a cation exchange reaction in HgCl 2 reported in (a).The apparent decrease in the LO phonon mode intensity is also observed and presented for six different NWs.