The white beam Synchrotron X-Ray Topography (SXRT) technique was used to assess the quality of sapphire wafers grown by the Heat-Exchanger Method (HEM) and the Modified Czochralski Method (MCM). Sapphire is a potential new material for X-ray crystal optics, especially for use as Bragg backscattering mirrors for X-rays and Mö ssbauer radiation. The dislocation distribution, dislocation density and Burgers vector of selected dislocations and stacking faults in the sapphire wafers were studied. A correlation between the sapphire quality and its performance as an X-ray backscattering mirror was established in this paper. The results reveal the high quality of the inspected HEM sapphire wafers and their subsequently improved performance as Bragg backscattering mirrors.
The Epitaxial Lateral Overgrowth (ELO) of GaN on Al2O3 using a SiO2 mask with different fill factors (ratio of stripe opening width to stripe period) is examined with White Beam Synchrotron X-ray Topography (WBSXT) and X-ray rocking curve analysis. The sapphire substrate was identified with a dislocation density of the order of ~106cm-2. WBSXT in both transmission and back reflection mode is used to image the ELO GaN and confirms that crystal planes in the lateral overgrown part (wing) are tilted, and that the wing tilt increases as the fill factor increases. X-ray rocking curve and WBSXT measurements confirm the same wing tilt tendency as the fill factor changes. The WBSXT method provides a measure of the maximum wing tilt, while the X-ray rocking curve method gives the average wing tilt. The average wing tilt reaches about 1602 arcsec at a fill factor of 0.625, but the maximum wing tilts can reach values as large as 2372 arcsec when the fill factor is only 0.571. This study shows that WBSXT is an effective method in dislocation and wing tilt determination for the GaN on Al2O3 ELO epilayer system or indeed for similar systems. The tilted wings induce a slightly lower compressive stress in the coalesced region of the GaN epilayer.
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