Résumé
We characterize by transmission electron microscopy (TEM), GaN layers deposited by metal organic chemical vapor deposition (MOCVD) on (0001) sapphire. Different GaN films with different surface morphologies have been observed and their crystallographic quality determined. Polarity and surface diffusion are the important factors that determine the surface morphology. The lack of an adapted buffer layer leads to a layer with a dominant N-polarity that contains many inversion domains (IDs) (of Ga-polarity) that grow faster than the surrounding material and form pyramids. All the flat unipolar GaN films we have observed have a Ga-polarity. Unipolarity (Ga-polarity) is realized with the recrystallization of the low temperature buffer layer or/and of the nitridation of the sapphire substrate. An intermediate cubic phase has been observed at the sapphire/buffer layer interface of optimized nitridated samples. In non optimized samples, IDs (of N-polarity) can remain near the buffer layer, but they tend to disappear during the growth of the Ga-polar GaN layer. A high growth temperature (about 1000 degrees C) was necessary to obtain flat GaN layers. (C) 1997 Elsevier Science S.A.