Long range, non-destructive characterization of GaN substrates for power devices

It has been well established that GaN grown on native substrates yields higher quality films compared to those grown on non-native substrates, such as SiC or Si. However, homoepitaxy is hindered by the lack of inexpensive high-quality GaN substrates. The importance of substrate quality, in particular defects and impurities, arises from its impact on device performance. Many of the imperfections in GaN substrates are inhomogeneous, thus are often not detected when short-range techniques are used. Therefore, methods for evaluating substrate quality over long ranges need to be developed. This research reports the study of these defects on four different GaN wafers from four different vendors using quick, long-range, non-destructive techniques that can be performed in a few hours. All samples arespected to be n+ and should theoredically be suited for use in GaN homoepitaxial vertical power devices, though had different properties that could impact device performance. Through evaluation by Raman spectroscopy, photoluminescence, white light optical profilometry, and Nomarski imaging concentrations of grain boundaries, impurity incorporations, point defects, v-shaped cone defects, polishing defects, crystal stress damage, and insulating and conducive regions were identified across the various GaN substrates studied.