Electronic properties of extended surface defects in homoepitaxial GaN diodes

Abstract

GaN on GaN vertical power devices are promising for high voltage applications beyond Si and SiC. Developing GaN substrates with low defect densities for high-quality epitaxial growth has been an ongoing challenge. Consequently, extended defects are a concern for high power device performance and reliability and must be mitigated. In particular, newer ammonothermally grown GaN substrates with lower extended defect densities have not yet been evaluated as extensively as hydride vapor phase epitaxy GaN. Here, we identify different electrically active defects on epitaxial p-GaN layers grown on strain-patterned (dot-core) and ammonothermal GaN substrates. On dot-core substrates, “star” defects were observed that consist of pits and cracks, have an increased hole carrier density, display occupied bandgap states, and show reduced and variable forward bias currents. In contrast, on ammonothermal substrates, triangular patches were found on certain rough epitaxy ridges and show increased gap states along with a lower carrier density and reduced conductivity. The star and triangular defects likely correspond to threading dislocations and stacking faults, respectively, and may act as killer defects. As a result, p–i–n diode devices on ammonothermal substrates frequently exhibited poor reverse and forward bias behavior due to the random extended defect distribution that cannot be intentionally avoided as in patterned dot-core substrates, as well as different impurity defects in the epitaxy. Therefore, we have found that ammonothermal GaN substrates require tighter control over both point and extended defect distributions in order to achieve reliable high power electronic devices.