The Structural Evolution of Dot-Core GaN Substrates with Annealing Under Growth-Like Conditions

Abstract

The structural evolution before and after annealing of c-plane dot-core GaN substrates under growth-like conditions was studied using monochromatic X-ray topography. The anneal was performed at 1050 °C for 10 hours in a MOCVD growth chamber under conditions optimized for the growth of GaN drift layers but without the group-III alkyl. This replicates the thermal environment the substrates experiences during the growth of > 2 kV PN diodes. In the current literature, fundamental understanding of the defect structure of dot-core GaN substrates – and its direct impact on device performance – is limited. X-ray topography provides a nondestructive method for studying the structural nature of defects which can then be used to understand the impact of defects on device performance. The measurements were performed at the 1-BM beamline of the Advanced Photon Source, Argonne National Laboratory using an X-ray energy of 8.05 keV to measure the (1124) GaN reflection to generate single exposure images (images exposed at a specific ω angular position along the X-ray rocking curve) as well as rocked images (a composite image exposed at many positions along the rocking curve). The step size for the rocked images was 0.008°. The rocked images showed butterfly-shaped diffraction lines centered at each core – indicative of localized distortions due to a high density of defects aggregated at each core. Furthermore, the single exposure images revealed that in the regions between the cores, the lattice is less misoriented than at the cores. The defects present at the cores induce severe lattice tilt emanating radially from the cores, which suggests that these relatively high tilt regions (cores) would be detrimental to device performance. Before annealing, it was observed that adjacent low tilt regions were tilted with respect to each other. The radius of (lattice) curvature of the substrate before annealing was 17 ± 1 m, which indicates that globally throughout the entire substrate there was a high amount of lattice tilt misorientation. After annealing, the substrate flattens – the radius of curvature increased to 27 ± 2 m and an increase in diffracted area between adjacent “low tilt” areas also increased, which is evidence of both local and global structural reconfiguration of the lattice towards a smaller degree of misorientation due to the anneal. Figure 1