| Description |
Gallium nitride has excellent potential in radiation-hard high-power electronic devices due to its wide bandgap and strong atomic bonding. However, the nature of radiation damage created as a function of GaN crystallographic orientation is unclear. We aim to better understand these questions by growing, irradiating, and analyzing GaN in three orientations: nitrogen face c-plane, gallium face c-plane, and m-plane nonpolar. Epitaxial pin diode device stacks were grown using organometallic vapor phase epitaxy (OMVPE) and then fabricated into diodes at the University of New Mexico. Samples were exposed to reactor neutron fluxes at Penn State University using the TRIGA reactor to fluences of 1014, 1015, and 1016 n/cm2. Other samples were irradiated with gamma rays at Pacific Northwest National Lab's (PNNL) High Exposure Facility with a 60CO source to total doses ranging between 100 kGy and 2160 kGy. We tested for radiation-induced changes in the devices by measuring current-voltage (IV) curves of diodes and characterizing their turn-on voltage and the electroluminescence (EL) emitted. Circular transfer length method (CTLM) patterns were measured to yield the specific contact resistance and sheet resistivity. We observed considerable variation between devices in the preirradiation state, as well as metastable behavior within individual devices consistent with charging and discharging of contact interface trap states. Variations from these factors in IV and CTLM results were larger than any systematic changes induced by irradiation. We thus used current-driven EL, which allows characterization of the GaN iv junction without complications from interface states. These represent some of the first measurements of the effects of radiation on GaN minority carrier properties. The overall EL yield systematically decreased with neutron dose, and the band-to-band emission decreased faster than other emission bands ascribed to defect-related transitions. We did not observe any variation in the linear decrease of radiative recombination based on crystal orientation. No systematic trend was observed from EL for the series of gammairradiated samples. Previous studies have been able to show that neutron irradiation decreases the conductivity of GaN. However, in this study, because EL is particularly sensitive to minority carrier recombination, and we did not observe changes in IV or CTLM, we conclude that minority carrier lifetime decreases linearly with neutron radiation dose. In future work, we hope to understand how gamma radiation effects GaN devices while under operation by testing samples in-situ. As well, we are looking to use deep level transient spectroscopy (DLTS) to characterize defects before and after irradiation. DLTS can identify nonradiative defects and a very small concentrations of defects. |
