Abstract
X-ray topography defect analysis of entire 1.8-inch GaN substrates, using the Borrmann effect, is presented in this paper. The GaN wafers were grown by the ammonothermal method. Borrmann effect topography of anomalous transmission could be applied due to the low defect density of the substrates. It was possible to trace the process and growth history of the GaN crystals in detail from their defect pattern imaged. Microscopic defects such as threading dislocations, but also macroscopic defects, for example dislocation clusters due to preparation insufficiency, traces of facet formation, growth bands, dislocation walls and dislocation bundles, were detected. Influences of seed crystal preparation and process parameters of crystal growth on the formation of the defects are discussed.
Highlights
X-ray topography defect analysis of entire 1.8-inch Gallium nitride (GaN) substrates, using the Borrmann effect, is presented in this paper
The μm scale defects, hereafter referred to as microscopic defects, are typically threading dislocations (TDs) of the screw, edge, or mixed type; on the other hand, mm scale defects, hereafter called macroscopic defects, are extended defects or defect clusters
In this study it has been demonstrated that Borrmann effect X-ray topography (XRT) is an excellent technique for the analysis of strongly absorbing GaN substrates or crystals
Summary
Gallium nitride (GaN) wafers of the highest structural quality are needed for building optoelectronic and electronic device structures. GaN substrates of the highest structural quality are fabricated from crystals grown by the basic ammonothermal method [7,10] These substrates are characterized by flatness of crystallographic planes (bowing radius higher than 15 m for 2 inch substrates), relatively low elastic deformation and threading dislocation density (TDD) at the level of 5 × 104 cm−2 or even lower [11,12]. Conventional high resolution X-ray diffraction (HRXRD) analyses of the full width at half maximum (FWHM) prove to be unsuitable at a certain point, since the reflection broadening is in the range of the intrinsic resolution limits of the diffractometers and the sensitivity of this method is not sufficient to distinguish low-dislocation GaN crystals, with TDD below 104 cm−2 and crystals with even lower defect density. The observation of the Borrmann effect in the investigated Am-GaN wafers proves the high structural quality of the material
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