The White Beam Synchrotron X-Ray Topography (WBSXRT) technique was used to assess sapphire wafer quality. The dislocation distribution, dislocation density, Burgers vectors 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 study. The results reveal the high quality of the inspected Heat-Exchanger Method produced sapphire wafers and their subsequent improved performance as Bragg backscattering mirrors. Wing tilts in the epitaxial lateral overgrowth (ELO) of GaN on sapphire substrates using a Si02 mask were observed and measured with the WBSXT method. The wing tilt increases as the fill factor (ratio of stripe opening width to stripe period) increases and asymmetric wing tilts are observed in all samples with different fill factors. The crystal misorientation in the window regions and beneath the seed layer is approximately an order of magnitude less than the wing tilt. A regular wave-like two dimensional stress distribution was observed in the ELO GaN epilayer by using high resolution micro-Raman spectroscopy (p.RS). The average compressive stress in the ELO GaN was around 450-460 MPa, and the coalesced region between two ELO wings usually exhibits a lower compressive stress by about 60 MPa compared to the average stress in the epilayer. The ELO process increases the residual compressive stress in the GaN epilayer. The stress in both device silicon cap layers and the underlying Sii_xGex virtual substrates was characterized with fiRS using both the 488 nm Ar+ visible laser and 325 nm HeCd UV laser sources. The Si0.70Ge0.30 capping layer at the virtual substrate is fully unstrained as the result of formation two perpendicular <110> misfit crosshatches, while the top silicon cap layer is in extremely high tension with a measured tensile stress of 2.4 GPa.