TEM Study of High Quality GaN Grown by OMVPE Using an Intermediate Layer
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TEM STUDY OF HIGH QUALITY GaN GROWN BY OMVPE USING AN INTERMEDIATE LAYER. M. Benamara, Z.Liliental-Weber, S. Kellermann, W. Swider, J. Washburn, J.H. Mazur and E. D. Bourret-Courchesne. E.O. Lawrence Berkeley National Laboratory, 1 Cyclotron Road M.S. 62-203, Berkeley CA 94720.
ABSTRACT We report on high quality GaN layers grown with the use of one intermediate layer. The defect analysis shows that the density of dislocation is only 8x107/cm2 in these layers, compared to over 1010/cm2 for layers grown without the intermediate layer (IL). Electron microscopy on cross-section samples shows that deposition under certain specific conditions of a lowtemperature IL directly benefits the quality of the subsequently deposited GaN layer. The growth of the GaN top layer appears to be similar to growth observed for lateral epitaxial overgrowth layers. This first time observation opens the possibility for using standard growth methods of GaN compounds to achieve a dislocation density comparable to that achieved with lateral overgrowth epitaxy. INTRODUCTION GaN and related alloys are attractive as substrate materials for light emitting and laser diodes because of their ability to cover a wide spectral range [1]. Due to the lack of lattice matched substrates, special techniques have been developed in order to grow GaN films with low threading dislocation densities. The deposition of a buffer layer at low temperature (LT-BL) before the growth of GaN at higher temperature [2] results in a large dislocation density of about 109-1010 dislocations/cm2. The lateral epitaxial overgrowth (LEO) technique is the one of the most sophisticated special process to reduce the density of dislocations [3-4]. It gives the best results to date but it still suffers from the large number of processing steps required. Furthermore, grain boundary dislocations are formed at the meeting front of the two overgrown areas which compensate a misorientation [5]. These grain boundaries limit the spatial utilization of the GaN film and are believed to deteriorate its optical properties. The growth of intermediate low temperature layers is a promising alternative to
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reduce the defect contents. This method was introduced by Iwaya et al. [7] and proved to be efficient. However, several interlayers are required in order to reach dislocation density lower than 109/cm2 [8]. Thus, the process is long and expensive. In an other study, Lahrèche et al [9] used silane to nucleate an intermediate layer of self-organized islands of GaN:Si and succeded in reducing the dislocation density to the 7. 108/cm2 range. However the presence of the strongly n-type dopant silicon might not always be suitable. In this letter, we report on TEM characterization of high quality GaN layers grown with only one intermediate-temperature undoped intermediate layer (IT-IL). The growth process is outlined here but details will be described separately [10]. EXPERIMENTS GaN was deposited on (0001) oriented sapphire substrates using a lowpressure chemical vapor deposition reactor. Triet
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