Characterization of Epitaxial β-(Al,Ga,In)₂O₃-Based Films and Applications as UV Photodetectors

Abstract

Epitaxial films of β-(Al_xGa_1−x)₂O₃, β-Ga₂O₃, and β-(In_xGa_1−x)₂O₃ were grown on (001) sapphire substrates via metalorganic chemical vapor deposition (MOCVD). The compositions of the films as determined from energy dispersive x-ray analysis (EDX) and x-ray photoelectron spectroscopy (XPS) results were X_Al = 0.57 ± 0.05 and 0.76 ± 0.05 and X_In = 0.12 ± 0.05 and 0.21 ± 0.05. The optical bandgap was found to correspondingly vary between 6.0 ± 0.2 and 3.9 ± 0.1 eV, as a function of composition via XPS and UV–visible spectroscopy (UV–Vis). X-ray diffraction, scanning electron microscopy, and atomic force microscopy revealed the films to be highly-oriented $\left( {\bar{2}01} \right)$-epitaxial films with nanocrystalline domains. Schottky- and MSM-based solar-blind UV photodetectors were fabricated on the films and showed responsivities at 20 V varying from > 10⁴ A/W for the Ga₂O₃ devices, > 10³ A/W for the (Al_xGa_1−x)₂O₃ devices and > 10² A/W for the (In_xGa_1−x)₂O₃ devices. Modest shifts in wavelength selectivity corresponding with the changes in composition/bandgap were also measured. Time response measurements on Schottky and MSM detectors reveal rise and dwell times on the order of a minute, indicating the presence of photoconductive gain. Noise-equivalent powers were in the fW–pW regime with specific detectivities ($D^{*}$) between 10¹⁰ and 10¹² Jones. Scanning photocurrent maps display large photocurrent generation at the Schottky interface in the case of a β-Ga₂O₃ Schottky detector, whereas for an β-(In_xGa_1−x)₂O₃ MSM detector the photocurrent generation occurs in the device channel and at the Schottky interface.