Grain growth of precursor-derived nanocrystalline gallium nitride powder
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D.J. Kisailus and F.F. Lange Materials Department, University of California, Santa Barbara, California 93106
T. Wagnera) Max-Planck-Institut fu¨r Metallforschung, Seestrasse 92, 70174 Stuttgart, Germany (Received 6 August 2001; accepted 16 November 2001)
Nanocrystalline gallium nitride powder was synthesized from a gallium dimethylamidederived polymeric precursor by pyrolysis in ammonia atmosphere to study the grain growth mechanisms in the temperature range 800–1150 °C. In particular, growth exponents and activation energies were determined. Up to 900 °C, grain growth was inhibited, whereas, above 1000 °C, evaporation–condensation was identified as the dominant material transport path.
I. INTRODUCTION
Gallium nitride (GaN) and related III–V semiconductors have become important materials for optoelectronic devices such as blue light emitting diodes and laser diodes or high power field effect transistors.1,2 GaN is almost exclusively used in thin films typically deposited by chemical vapor deposition or molecular beam epitaxy.3 Very recently, GaN films derived from liquid precursors were produced by chemical solution deposition.4– 8 Relatively little attention has been spent on GaN powder material. Studies have dealt with the growth of GaN single crystals or powders from liquid Ga and ammonia (NH3), e.g., Refs. 9 and 10, by the reaction of Ga2O3 with ammonia, e.g., Refs. 11 and 12, or by the evaporation and condensation of GaN powder material.13 Gonsalves et al. demonstrated the formation of cubic GaN powder by the reaction of gallium dimethylamide with ammonia.14 Fischer and co-workers prepared azidebased precursors to GaN powders and thin films.15 However, grain growth in GaN powder has not been studied, more than likely because of the thermal decomposition of GaN at 900 °C in 1 bar N216 before significant grain growth occurs. In ammonia, on the other hand, GaN was reported to be stable up to about 1200 °C.17,18 Recently, a gallium dimethylamide-based polymeric precursor solution has been used to study the epitaxy of ␣–GaN thin films on c-plane sapphire (␣–Al2O3). The resulting films were successfully used as buffer layers for the deposition of GaN by metalorganic chemical vapor a)
Address all correspondence to this author. J. Mater. Res., Vol. 17, No. 2, Feb 2002
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deposition.6 In this study we demonstrate the fabrication of nanocrystalline hexagonal ␣–GaN powder by pyrolysis (decomposition during heating) of a gallium dimethylamide-based polymeric precursor in an ammonia atmosphere. Grain growth was investigated both as a function of temperature and as a function of time between 800 and 1150 °C. Linear shrinkage of agglomerates and mass loss were also investigated to determine the dominant material transport paths. In addition to understanding the densification behavior of GaN powders, these studies also shed light on the phenomena associated with chemical solution routes for the epitaxy of GaN on sapphire substrates. II. EXPERIMENTAL PROCEDURE
The prec
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