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Published online by Cambridge University Press: 10 February 2011
The evolution of the microstructure of GaN grown by molecular beam epitaxy on {001} and {111} oriented GaAs substrates has been followed using transmission electron microscopy and reflection high energy electron diffraction. A thin layer of GaN has been shown to form during the nitridation of the GaAs surface prior to growth. Growth of GaN then proceeds by an island mechanism. Faulting on the four {111} planes of the cubic zinc-blende phase which grows on the {001 } surface occurs at an early stage as a consequence of misfit strain. The distribution of the {111} microtwins is initially isotropic, but growth of one pair of {111} twins proceeds much faster than that of the other pair, leading to a final microstructure which has an anisotropic distribution of microtwins. Doping of GaN with Si hinders the growth of the zinc-blende phase, leading to a textured, columnar (0001) wurtzite microstructure. Evidence is presented to show that addition of Mg as a dopant may reduce the stacking fault energy of wurtzite GaN.