A Study of the Decomposition of GaN during Annealing over a Wide Range of Temperatures
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A Study of the Decomposition of GaN during Annealing over a Wide Range of Temperatures M.A. Rana1, H.W. Choi2, M.B.H. Breese1, T. Osipowicz1, S.J. Chua2 and F. Watt1 1 Research Center for Nuclear Microscopy, Department of Physics, National University of Singapore, Singapore 117542 2 Department of Electrical Engineering, Center for Optoelectronics, National University of Singapore, Singapore 119260 ABSTRACT Annealing experiments were carried out on gallium nitride layers, which were grown on sapphire through Metal Organic Chemical Vapor Deposition (MOCVD). Rutherford Backscattering Spectrometry (RBS) was performed on as-grown and annealed GaN samples using a 2 MeV proton beam to study the stoichiometric changes in the near-surface region (750 nm) with depth resolution better than 50 nm. No decomposition was measured for temperatures up to 800 oC. Decomposition in the near-surface region increased rapidly with a further increase of temperature, resulting in a near-amorphous surface-region for annealing at 1100 oC. The depth profiles of nitrogen and incorporated oxygen in the decomposed GaN are extracted from the nanoscale RBS data for different annealing temperatures. The surface roughness of the GaN layers observed by atomic force microscopy (AFM) is consistent with RBS decomposition measurements. We describe the range of annealing conditions under which negligible decomposition of GaN is observed, which is important in assessing optimal thermal processing conditions of GaN for both conventional and nanoscale optoelectronic devices.
INTRODUCTION Ultraviolet (UV) and blue wavelengths have numerous applications in optical communication systems, full-color displays and data storage etc. [1]. Therefore, wide bandgap semiconductors, specifically gallium nitride and related materials have great potential for highly efficient blue and UV light emitting diodes (LEDs) and laser diodes (LDs). These materials are also suitable for high temperature devices. Gallium nitride and its alloys are now also of interest due to their use in electronic and optoelectronic nanoscale devices as nanowires and nanorods [23]. All these applications are possible due to the wide direct band gap of GaN, the possibility of band gap engineering and its high bond strength and high melting temperature (>1700 oC) [4-7]. High temperature annealing of GaN is an important step in producing blue and UV light emitters and high temperature devices. However, annealing above a certain temperature can cause decomposition of gallium nitride, especially near the surface, where evaporated nitrogen and gallium leave vacancies, which may result in the incorporation of oxygen. This incorporation of oxygen can cause n-type conductivity in initially p-type gallium nitride as discussed in theoretical studies from Mattila and Nieminen [8] and Park and Chadi [9]. A number of authors [10-14] have reported annealing results of gallium nitride but there have not been any studies of the elemental depth distributions in annealed GaN. We have determined the elemental
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