Compositional and structural studies of amorphous GaN grown by ion-assisted deposition
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Compositional and structural studies of amorphous GaN grown by ion-assisted deposition U. D. Lanke1, A. Koo1, B. J. Ruck1, H. K. Lee1,3, A. Markwitz2, V. J. Kennedy2, M. J. Ariza4, D. J. Jones4, J. Rozière4, A. Bittar5 and H. J. Trodahl1 1 School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand 2 Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand 3 Department of Physics, Kangwon National University, Chunchon, Korea 4 Laboratoire des Agrégats Moléculaires et Matériaux Inorganiques UMR CNRS 5072, Université Montpellier II, Montpellier, France 5 Measurement Standards Laboratory, Industrial Research Limited, Lower Hutt, New Zealand ABSTRACT Amorphous GaN films have been deposited onto various substrates by ion-assisted deposition. The films were deposited at room temperature using nitrogen ion energies in the range 40-900 eV. Rutherford backscattering spectroscopy and nuclear reaction analysis show that the Ga:N atomic ratio is approximately one for films grown with ion energy near 500 eV; these films have the highest transparency. Films grown with ion energies below 300 eV are Ga rich, and show reduced transparency across the visible. Raman spectroscopy, x-ray diffraction, and transmission electron microscopy confirm the amorphous nature of the films. Annealing studies on a-GaN establish that the films begin to crystallise at a temperature of about 700°C. To investigate the local bonding environment of the Ga or N atoms, we have measured the extended x-ray absorption fine structure (EXAFS) of the transparent GaN films. The EXAFS results indicate that the films are dominated by heteropolar tetrahedral bonding, with a low density of homopolar bonds. INTRODUCTION Group III-nitrides (AlN, GaN and InN) have attracted tremendous scientific attention during the last few years because of their potential application in green-blue-UV optoelectronic devices [1-3]. Due to a lack of lattice-matched substrates it is extremely difficult to deposit crystalline GaN (c-GaN) films, placing restrictions on the range and affordability of available GaN based devices. Indeed, it is noteworthy that dislocations and defects do not act as efficient non-radiative recombination centres in this nitride system [4]. Surprisingly, a high defect density of 2 x 1010 dislocations/cm2, which normally hampers light emission in other semiconducting materials such as III-As and III-P, does not stop light emission from GaN [4]. Why GaN continues to perform well is not yet completely understood. Recently, ab initio molecular dynamics calculations have indicated that there may be an absence of wrong pairs and no midgap states in amorphous GaN (a-GaN) [5-6]. Therefore, there is speculation that efficient light emission can be obtained from amorphous or disordered III - nitrides and they also might play a significant role in the fabrication of light emitting diodes [7] or UV detectors [3]. Advantages of developing amorphous materials include the wide range of substrate choice, large area deposition,
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