Differences in electrical responses and recovery of GaN p+n diodes on sapphire and freestanding GaN subjected to high dose 60Co gamma-ray irradiation

We investigate the effects of high-rate and high total doses of Co-60 gamma rays on the current-voltage (IV) characteristics of GaN p(+)n diodes grown by metal-organic chemical vapor phase epitaxy on Ga-face (0001) sapphire and hydride vapor phase epitaxy freestanding GaN substrates. We show that diodes grown on sapphire undergo more permanent changes upon irradiation at doses up to 3900 kGy than those grown on freestanding GaN. By combining diode and circular transfer length method measurements, we show that the p-type contact interface and adjacent p(++) Mg-doped layer are sensitive to irradiation. In initial experiments, diodes grown on sapphire exhibited p-type contacts with Schottky characteristics, while those on freestanding GaN were Ohmic. Serendipitously, we identified and subsequently irradiated a freestanding sample with a pre-irradiation spatial gradient of p-contact Schottky vs Ohmic behavior across the die. This sample allowed the root cause of induced change to be identified as differences in the p(++) contacting layer. We show that the p-type contact's pre-irradiation Schottky behavior is predictive of diodes' IV characteristics changing significantly upon gamma-ray irradiation. Further, we observe that the IV curves of diodes on freestanding GaN recover fully over several weeks at room temperature to be indistinguishable from pre-irradiation. IV curves from diodes on sapphire do not fully recover; we thus hypothesize that interactions between radiation-induced point defects and threading dislocations affect the evolution of radiation damage.

Automated summary

The study reveals that GaN p(+)n diodes grown on freestanding substrates exhibit higher radiation resistance compared to those grown on sapphire substrates. The p-type contact interface and adjacent p(++) Mg-doped layer are found to be sensitive to irradiation. Additionally, the pre-irradiation Schottky behavior of the p-type contact can predict significant changes in the diodes' IV characteristics upon gamma-ray irradiation.