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We have found that the surfaces of ZnO(0001) annealed in a box made of ceramic ZnO exhibit atomically flat stepped and terraced structures. Structural and optical properties of ZnO are also improved by this annealing technique. Full width at half maximum of X-ray rocking curves of the ZnO substrates were decreased and green photoluminescence caused by defects were eliminated by the annealing. We have also found that epitaxial growth of GaN on atomically flat ZnO(0001) substrates at room temperature proceeds in a layer-by-layer mode, while growth on the as-received substrates results in formation of amorphous materials. These results indicate that the use of the annealing technique improves, not only crystal quality of ZnO substrates, but also morphology of the GaN films grown on them.
Kobayashi et al. (Sat,) studied this question.