Role of threading dislocations and point defects in the performance of GaN-based metal-semiconductor-metal ultraviolet photodetectors

Abstract

The role of threading dislocations and point defects is investigated by comparing the performance of metal-semiconductor-metal ultraviolet photodetectors (PDs) fabricated on GaN epilayers grown by hydride vapour phase epitaxy (HVPE) and metal organic vapour phase epitaxy (MOVPE) techniques. It is found that the density of threading dislocations is higher in HVPE GaN epilayers, however, the devices fabricated on them show a higher photo response, lower leakage current and faster transient response when compared to those fabricated on MOVPE GaN epilayers. It is noticed that a high density of threading dislocations of HVPE grown GaN epilayers doesn't always restrict their usefulness in the development of specific devices. On the other hand, an inferior performance of PDs fabricated on MOVPE GaN epilayers is observed despite their low dislocation density, which is explained by considering the presence of point defects. Further, the room temperature electronic transport is found to be dominated by thermionic emission (thermionic field emission) mechanism in devices fabricated on GaN epilayers grown by HVPE (MOVPE) technique. In case of HVPE based devices, a switching of dominant transport mechanism is seen at ~200 K during cooling down whereas no such behaviour is observed for MOVPE based devices. Key factors affecting the performance of ultraviolet PDs fabricated on GaN epilayers grown by the two techniques and associated charge transport mechanisms are discussed. • Crystalline quality of MOVPE GaN templates is found to be better than HVPE GaN. • HVPE GaN based MSM devices turn out to be better than those fabricated on MOVPE GaN. • A high dislocation density of HVPE GaN doesn't always restrict their usefulness. • Fundamental physical reasons for a better performance of MSM devices are discussed. • Transport mechanism switches at ~200 K for HVPE based GaN MSM devices.