Conductivity, photoconductivity and optical properties of amorphous GaN films
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Conductivity, photoconductivity and optical properties of amorphous GaN films A. Koo1, U. D. Lanke1, B. J. Ruck1, S. A. Brown2, R. Reeves2, I. Liem2, A. Bittar3 and H. J. Trodahl1 1 School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand 2 Nanostructure Engineering Science and Technology Group and Department of Physics and Astronomy, University of Canterbury, Christchurch, New Zealand 3 Measurement Standards Laboratory, Industrial Research Limited, Lower Hutt, New Zealand ABSTRACT It has been predicted that amorphous GaN has a low density of states in the gap, and therefore has potential as a useful opto-electronic material in the blue-green spectral region. We have synthesised amorphous GaN films on various substrates by ion assisted deposition and investigated the effects of sample preparation conditions on the conducting and optical properties. The room temperature resistivity ρ 0 of stoichiometric (Ga:N of 1:1) films is above 105 Ω cm, and these films exhibit a complex form for the temperature dependence of the resistivity. Films having an excess of Ga show a much lower ρ 0 . The optical absorption shows a band-gap of 3 eV, with the gap falling below that value when the amorphous network incorporates homopolar (Ga-Ga) bonds. The best films are thus transparent across the visible region with a low density of gap states, undetectable in optical absorption. The photoluminescence spectra obtained from these a-GaN films consist of a broad green light emission peaking at 528 nm. Preliminary photoconductivity measurements show sensitivity in the UV. INTRODUCTION While group-III nitrides have shown great promise as opto-electronically significant materials [1], particularly in the UV, GaN in its crystalline form (c-GaN) has proven expensive and difficult to produce. Due primarily to problems of lattice mismatch between GaN and the substrates used, its incorporation into useful opto-electronic devices is severely hampered. In contrast, amorphous or nanocrystalline GaN can be formed on almost any substrate [3], and may still have many of the critical properties which make the crystalline form so attractive. We have synthesized highly disordered GaN thin films [2-4] in which the film structure varies depending on the growth conditions. Here we report on the measured optical and electronic properties of the films. We discuss the results of optical transmittance and reflectance as well as subsequent calculations of the complex refractive index. The temperature dependence of the electrical conductivity will also be considered, as well as the temporal characteristics of the photoconductivity in the films. Photoluminescence has also been observed and its spectrum presented. Our results will be compared with literature on polycrystalline and amorphous GaN. EXPERIMENTAL Amorphous GaN films have been prepared by Ion Assisted Deposition (IAD) [2-3] on several substrates, including quartz, glass, silicon, and glassy carbon. Compositional and I10.10.1
structural analysis of the films has
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