Correlation between surface morphologies and crystallographic structures of GaN layers grown by MOCVD on sapphire
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 33
Correlation between surface morphologies and crystallographic structures of GaN layers grown by MOCVD on sapphire J. L. Rouviere, M. Arlery CEA/Grenoble, Département de Recherche Fondamentale sur la Matière Condensée/SP2M R. Niebuhr, K. H. Bachem Fraunhofer Institut für Angewandte Festkörperphysik Olivier Briot Groupe d'Etude des Semiconducteurs, GES-CNRS This article was received on June 20, 1996 and accepted on November 5, 1996.
Abstract GaN layers deposited by MOCVD on sapphire have been characterized by Transmission Electron Microscopy (TEM). Two substrate orientations were used, (0 0 0 1) and(2 0).We determine the crystallographic structures (defect content and layer polarity) of three different types of GaN layers with different surface morphologies. Convergent Beam Electron Diffraction studies were particularly important to determine the polarity of the GaN layers. We find that polarity and surface diffusion are the factors that control the different growth modes. Unipolarity is obtained thanks to the annealing of the low temperature buffer layer or/and thanks to the nitridation of the sapphire substrate. Hexagonal pyramids and flat tops are formed when the material has a dominant N-polarity. The pyramids contain many tiny hexagonal columnar Inversion Domains (IDs). These pyramids are formed when the tiny Ga-polar IDs grow faster than the surrounding N-polar matrix. Flat GaN layers are unipolar, with a Ga polarity. Rough grainy layers which are unipolar (Ga-polarity) are obtained when surface diffusion is not high enough.
1. Introduction With the realization of gallium nitride diodes and more recently laser diodes [1], GaN has attracted much attention and there is now little doubt that GaN will be an important semiconductor in optoelectronic applications. Although all the major semiconductor devices have now been realized, the material is far from being mastered and many challenging problems still remain. The long-standing problem of GaN growth is the lack of a suitably adapted substrate. (0001) sapphire plane is the most popular and most successful substrate in spite of its huge lattice mismatch with GaN (-13%). Many alternative substrates have been tested [2]. The success of GaN growth on sapphire comes from the use of a low temperature buffer layer [3], but the exact role of this buffer layer has not yet been determined. It has recently appeared that the nitridation of the sapphire substrate could be as important as the buffer layer itself [4] [5] [6]. In this communication, we report Transmission Electron Microscopy on MOCVD grown GaN layers on (0 0 0 1) sapphire. We determine the crystallographic quality of different GaN layers with different surface morphology. We then determine the factor that controls the different growth modes. These results could help in improving further the GaN film structure.
2. Experimental details Downloaded from https://www.cambridge.org/core. RMIT University Library, on 06 Dec 2017 at 01:34:05, s
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