(Invited) Synchrotron White-Beam X-Ray Topography Analysis of the Defect Structure of HVPE-GaN Substrates
In this work we present a detailed analysis of defect structures of freestanding GaN (0001) crystals grown by HVPE and the ammonothermal method from different vendors. Synchrotron white-beam X-ray topography (SWXRT), high resolution X-ray diffraction (HRXRD) and microphotoluminescence (µPL) were used for this combined study. SWXRT in two principal X-ray topography geometries was used in the course of this analysis, namely the large-area back-reflection mode and section transmission mode. HRXRD reciprocal space mappings and in-plane measurements were used for mosaicity analyses. The combination of these methods allows a detailed statement of the nature, density and distribution of typical defects in GaN bulk crystals with a large field of view. As an example, Fig. 1 shows a large-area back-reflection topograph (0006 reflection) of a HVPE GaN substrate. The topograph shows dislocation free areas, surrounded by rather sharp lines, indicating good material quality with low defect density. These features are interpreted as dislocation cell walls, suggesting the formation of a cellular defect network within this GaN sample, analogous to what was commonly observed in LEC GaAs .
With the combination of the results from SWXRT, HRXRD and µPL a consistent model for different defect structures of the freestanding GaN samples grown by HVPE and the ammonothermal method is discussed.
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