Microscale Electronic Properties of Epitaxial Gallium Nitride Surface Defects

The wide bandgap material GaN is desired for next-generation high power electronics beyond Si and SiC. However, the vertical structures needed for scalable devices suffer from issues related to dislocations and defects, which propagate from the substrate into the epitaxially grown device area. Substrate developments such as strain patterning to coalesce dislocation bundles and alternate scalable growth methods such as the ammonothermal process have produced higher structural quality substrates. However, in both cases, the remaining dislocations and defects are still a major concern for high power device performance and reliability. For example, threading dislocations with screw components are known to exhibit leakage current and are predicted to have mid-bandgap states, while newer ammonothermal GaN substrates may exhibit different defect densities and distributions compared to GaN grown on foreign substrates. Therefore, determining the electronic properties of dislocations and defects at the microscale will provide valuable information for identifying “killer” defects.<br/><br/>Here, we use a combination of laser-based photoemission electron microscopy (PEEM) and conductive atomic force microscopy (cAFM) to investigate the local surface electronic properties of defects and dislocations in GaN epitaxially grown via MOCVD on different substrates. In epitaxy on patterned substrates, we identify surface defects located near strain centers that have a star-shaped appearance with central pits and extending cracks. These star defects exhibit a larger work function and a shifted valence band maximum, and further, show increased reverse bias leakage current compared to the surrounding GaN. For epitaxy on ammonothermal GaN substrates, we instead observe elongated triangular patches that sporadically occur on a face of certain growth ridges. These regions also show altered electronic structure compared to the surrounding GaN, but are instead more resistive under both forward and reverse bias conditions. Our results provide evidence for defective sites likely related to extended dislocations that will lead to degraded device performance.