Showing papers by "Tsutomu Uesugi published in 2023"

Reliability issues of gate oxides and $p-n$ junctions for vertical GaN metal–oxide–semiconductor field-effect transistors (Invited)

Abstract: We focus on reliability issues of gate oxides and $p-n$ junctions to realize vertical GaN metal-oxide-semiconductor field-effect transistors (MOSFETs). An annealed AlSiO gate oxide on GaN displayed a lifetime of over 20 years at 150 °C and suppressed positive bias instability in MOSFETs. The key to high channel mobility and stability under positive gate bias is the interface structure designed to minimize oxide border traps. We also evaluated the reliability of GaN p-n diodes (PNDs) on freestanding GaN substrates with different threading dislocation densities. The reverse leakage for PNDs involving threading dislocations was explained by variable-range hopping, while the reverse leakage for dislocation-free PNDs was dominated by band-to-band tunneling. The fabricated PNDs demonstrated excellent robustness under high-temperature reverse bias. However, after continuous forward current stress, reverse leakage pathways were formed at threading screw dislocations, which should be minimized in future GaN substrates.

Effects of proton irradiation-induced point defects on Shockley–Read–Hall recombination lifetimes in homoepitaxial GaN p–n junctions

Abstract: This work examined the intentional generation of recombination centers in GaN p– n junctions on freestanding GaN substrates. Irradiation with a 4.2 MeV proton beam was used to create a uniform distribution of vacancies and interstitials across GaN p+/ n− and p−/ n+ junctions through anode electrodes. With increasing proton dose, the effective doping concentrations were found to be reduced. Because the reduction in the doping concentration was much higher than the hydrogen atom concentration, this decrease could not be attributed solely to carrier compensation resulting from interstitial hydrogen atoms. In fact, more than half of the electron and hole compensation was caused by the presence of point defects. These defects evidently served as Shockley–Read–Hall (SRH) recombination centers such that the SRH lifetimes were reduced to several picoseconds from several hundred picoseconds prior to irradiation. The compensation for holes in the p−/ n+ junctions was almost double that for electrons in the p+/ n− junctions. Furthermore, the SRH lifetimes associated with p−/ n+ junctions were shorter than those for p+/ n− junctions for a given proton dose. These differences can be explained by variations in the charge state and/or the formation energy of intrinsic point defects in the p-type and n-type GaN layers. The results of the present work indicate the asymmetry of defect formation in GaN based on the fact that intrinsic point defects in p-type GaN readily compensate for holes.