High Resolution X-Ray Diffraction Analysis of Gallium Nitride Grown on Sapphire by Halide Vapor Phase Epitaxy
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ABSTRACT We report a structural analysis of GaN layers with thicknesses ranging from 10 ýIm to 250 pm which have been grown on sapphire substrates by halide vapor phase epitaxy (HVPE). The effect of growth rate during HVPE growth has also been examined. The growth was performed using GaCI and ammonia as reactants; growth rates in excess of 90 um/hr have been achieved. The structural characteristics of these layers have been performed wit'i high resolution x-ray diffractometry. Longitudinal scans parallel to the GaN [0002] direction, transverse scans perpendicular to the [0002], and reciprocal space maps of the total diffracted intensity have been obtained from a variety of GaN layers. The transverse scans typically show broad rocking curves with peak breadths of several hundreds of arcseconds. In contrast, the longitudinal scans (or "0/20 scans") which are sensitive only to strains in the GaN layers (and not their mosaic distributions) showed peak widths that were at least an order of magnitude smaller and in some cases were as narrow as 16 arcseconds. These results suggest that the defect structure of the GaN layers grown by HVPE is dominated by a dislocation-induced mosaic distribution, with the effects of strain in these materials being negligible in comparison. INTRODUCTION Halide vapor phase epitaxy (HVPE) offers the advantage of a high deposition rate during the epitaxial growth of GaN. This high growth rate offers the prospect of a reduction in the dislocation density in the top device region of the grown layer by producing a layer of sufficient thickness (tens to hundreds of microns) so that misfit dislocations and Ather defects generated dtring growth will be restricted to the vicinity of the heterointerface. The availability of a GaN layer with a defect-free top layer may also be important to the subsequent growth of either homoepitaxial GaN or heteroepitaxial structures containing AIN or InN. The need to reduce or at least control the grown-in defect density is, of course, dictated by the fact that heteroepitaxial GaN grown on common substrates such as SiC or sapphire typically exhibit a high density of structural defects. The relatively large difference in lattice parameter between GaN and these substrate materials typically results in a dislocation density of 1010 cm"2 or higher [1]. The difference in the coefficient of thermal expansion between GaN and its substrate can lead to the generation of strains and (in the worst case) cracking in the epitaxial layer as it is cooled from the growth temperature. The problems with thermal expansion mismatch are expected to be particularly severe in GaN due to the relatively high temperatures that are typically used in its epitaxial growth. Finally, when growth is performed on sapphire substrates, the difference in crystallographic symmetry between the substrate and the hexagonal GaN wurtzite phase can lead to the generation of antiphase domains. In the current work we have used a variety of high resolution x-ra, diffraction methods to monitor the evolution of th
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