Chlorine-Infused Wide-Band Gap p-CuSCN/n-GaN Heterojunction Ultraviolet-Light Photodetectors

Abstract

Copper thiocyanate (CuSCN) is a p-type semiconductor that exhibits hole-transport and wide-band gap (∼3.9 eV) characteristics. However, the conductivity of CuSCN is not sufficiently high, which limits its potential application in optoelectronic devices. Herein, CuSCN thin films were exposed to chlorine using a dry etching system to enhance their electrical properties, yielding a maximum hole concentration of 3 × 10¹⁸ cm^-3. The p-type CuSCN layer was then deposited onto an n-type gallium nitride (GaN) layer to form a prototypical ultraviolet-based photodetector. X-ray photoelectron spectroscopy further demonstrated the interface electronic structures of the heterojunction, confirming a favorable alignment for holes and electrons transport. The ensuing p-CuSCN/n-GaN heterojunction photodetector exhibited a turn-on voltage of 2.3 V, a responsivity of 1.35 A/W at -1 V, and an external quantum efficiency of 5.14 × 10²% under illumination with ultraviolet light (peak wavelength of 330 nm). The work opens a new pathway for making a plethora of hybrid optoelectronic devices of inorganic and organic nature by using p-type CuSCN as the hole injection layer.

Keywords: X-ray photoelectron spectroscopy; copper thiocyanate; gallium nitride; p-CuSCN/n-GaN heterojunction photodetector; ultraviolet-based photodetector.

Figure 1

Optical and electrical characterization of the intrinsic CuSCN and the Cl₂-infused CuSCN thin films, and DFT simulation of Cl₂-infused CuSCN: (a) I–V curves of CuSCN thin films exposed to chlorine for different periods of time. (b) Hole concentration and hole mobility of CuSCN thin films exposed to Cl₂ for different periods of time. (c) Electronic DOS for pristine CuSCN (black line) and for CuSCN with a Cl interstitial impurity (red line) in the configuration shown in the inset. Zero energy is set at the highest occupied level, VB and CB refer to the valence band and the conduction band, respectively (colors used in Inset: Cu: brown, S: yellow, C: gray, N: blue, Cl: green spheres). (d) Transmittance and absorbance spectra of CuSCN/UV-graded quartz exposed to Cl₂ for different periods of time.

Figure 2

Secondary ion mass spectra of intrinsic CuSCN thin films and Cl₂-infused CuSCN thin films. Positive SIMS profiles of (a) intrinsic CuSCN and (b) Cl₂-infused CuSCN thin films. Negative SIMS profiles of (c) intrinsic CuSCN and (d) Cl₂-infused CuSCN thin films.

Figure 3

TEM and HR-STEM images of the interface of a p-CuSCN/n-GaN heterojunction. (a) Bright-field TEM image of the interface of a p-CuSCN/n-GaN heterojunction. (b) Bright-field TEM image of another cross-section of the interface of a p-CuSCN/n-GaN heterojunction. The areas indicated by white arrows are diffusion spots. (c) Atom-revealing HR-STEM image of the interface of a p-CuSCN/n-GaN heterojunction.

Figure 4

HAADF–STEM images and STEM–EELS measurements of the p-CuSCN/n-GaN structure. (a) HAADF–STEM image of a p-CuSCN/n-GaN structure. The area within the red line indicates the region of STEM–EELS scanning. (b) Copper signal from STEM–EELS measurements. (c) Sulfur signal from STEM–EELS measurements. (d) Nitrogen signal from STEM–EELS measurements. (e) Gallium signal from STEM–EELS measurements.

Figure 5

High-resolution XPS measurements. (a) Cu 2p core-level and valence band spectrum for CuSCN. (b) Ga 2p core-level and valence band spectrum for GaN. (c) Ga 2p and Cu 2p core-levels for the GaN/CuSCN heterojunction. (d) Schematic representation of band alignment at the GaN/CuSCN heterointerface.

Figure 6

Device characterization of the as-fabricated p-CuSCN/n-GaN UVPD. (a) I–V curve sweeps from −3 to 3 V. The inset plot shows the I–V curve in log-scale. (b) Photocurrent measurements of the p-CuSCN/n-GaN photodetector for incident wavelengths ranging from 230 to 380 nm. (c) Power-dependent photocurrent measurements of the p-CuSCN/n-GaN device. The incident wavelength is 310 nm. (d) Plots of responsivity versus wavelength at a reverse bias of 1 V. (e) Plots of EQE versus wavelength at an input power of 1.62 μW/cm² and reverse bias of 1 V.

Figure 7

Time response tests of the as-fabricated p-CuSCN/n-GaN photodetector under illumination with a 266 nm laser with an optical chopper at 0 bias. (a) Chopper frequency set to 100 Hz. The rise time and fall time of the PD are 625 μs and 1.38 ms, respectively (b) Chopper frequency set to 200 Hz. The rise time and fall time of the PD are 630 μs and 1.29 ms, respectively. (c) Chopper frequency set to 300 Hz. The rise time and fall time of the PD are 570 and 710 μs, respectively. (d) Chopper frequency set to 400 Hz. The rise time and fall time of the PD are 510 and 640 μs, respectively.