A Transmission Electron Microscopy Investigation of GaN grown on Patterned, Step-Free 4H-SiC Mesas

Through the use of patterned SiC substrates that have regions that are free of surface steps, we have previously reported the growth of high-quality GaN heteroepitaxial films with threading dislocation densities on the order of 10⁷/cm² (Bassim et al., 2005). In addition to the low-defect densities, we have observed a defect substructure in which lateral a-type dislocations annihilate run parallel to the heterointerface both in the nucleation layer and early in the subsequent GaN growth (Bassim et al., 2005; Bassim et al., 2004). Conversely, regions that have surface steps also have accompanying threading dislocations with screw-components in the GaN layer. This study focuses further on the role of substrate surface steps in the generation of misfit, a-type, and threading dislocations at the heteroepitaxial interface. By using weak-beam and dark-field imaging of plan-view TEM samples of stepped and unstepped mesas as well as stereo microscopy, we show dislocations generated on hillocks and flat surfaces and compare their geometries