Structural, surface, electrical and UVC sensing properties of high temperature RF sputtered gallium oxide thin films

Abstract

Gallium oxide thin films have been deposited on a-, c-, r- plane sapphire and amorphous Si₃N₄ at 800 °C by RF sputtering from a 99.99 % purity Ga₂O₃ target then characterised structurally, optically and electrically. A fixed process pressure of 3.0 mTorr was employed with O₂:Ar ratios of 0:1 (0 % O₂), 1:18 (5 % O₂), 1:9 (10 % O₂) and 3:17 (15 % O₂). X-ray diffractograms attributable to β-Ga₂O₃ were collected from the films grown on a- and c- plane sapphire. The highest crystallinity was observed in the films grown on c-plane sapphire. Ga₂O₃ films on r-plane sapphire and Si₃N₄ produced no diffracted peaks and were deemed to be amorphous or nanocrystalline. Ga 3d X-ray photoelectron spectra showed only Ga-O bonding with no evidence of Ga-Ga bonding, even in the films deposited with only Ar introduced to the chamber. Direct optical bandgaps exceeding 5.0 eV were observed in the films on a- and c- plane sapphire. Valence band spectra showed the valence band maxima (VBM) and Fermi level (FL) were separated by ∼3 eV in the Ga₂O₃ films on a- and c- plane sapphire whilst films on r-plane sapphire exhibited VBM - FL gaps of ∼2.5 eV, indicative of low shallow impurity/defect doping density, most likely due to oxygen vacancies. Selected films were incorporated into metal-semiconductor-metal UV-C detectors. Solar-blind detection was confirmed and the maximum measured UV-C /dark current ratios (I_UVC:I_dark) exceeded 10³:1.