First-Principles Investigation of Low-Energy Recoils and Nonradiative Carrier Capture Rates for Defect Complexes in GaN

High-radiation environments such as outer space and nuclear reactors pose unique threats to electronic devices, which are susceptible to failure via radiation damage. One mode of degradation, displacement damage, occurs due to non-ionizing energy loss in the device material which forms defects in the crystal structure. These defects can have deep energy levels that trap charge carriers, potentially facilitating carrier recombination. Wide band-gap semiconductor materials such as gallium nitride (GaN) have been observed as a relatively radiation-hard alternative for devices in these environments, and further analysis regarding the impact of the defects that are formed in these materials on device performance is of interest.

Molecular dynamics displacement simulations yield Frenkel pair defects, divacancy complexes, and larger defect clusters dependent upon the initial knockout energy and direction. Climbing image nudged elastic band calculations suggest that the gallium Frenkel pairs are relatively stable, as the large separation distance between the interstitial and the vacancy requires complex atomic motion for recombination of the interstitial and vacancy to occur. Nitrogen Frenkel pairs can be created with less energy, but they have an easier path for recombination and may be more difficult to observe experimentally. The consistent observations of divacancies, specifically V_Ga–V_N, from high-energy recoil events in GaN samples suggest they could significantly impact charge carrier dynamics.

Formation energies were calculated from first-principles for these radiation-induced defect complexes. Supercell energies are determined with VASP, and analysis is performed using pydefect, doped, and sxdefectalign. Additional analysis functionalities developed are included in radDefects. Equilibrium charge states for each defect are determined as a function of Fermi level throughout the band gap. Charge localization is observed for the complexes, as electron transfer occurs between the constituent point defects. Charge transition levels are noted across the band gap for each defect, which also define the energy difference between the charge states’ potential energy curve minima in configuration coordinate diagrams for nonradiative carrier capture calculations. The rest of the potential energy curves are generated using single-point energy calculations of interpolated and extrapolated structures around the two equilibrium defect configurations. These configurations are created using the CarrierCapture Julia code, which also computes the carrier capture coefficients and cross-sections. Electron-phonon matrix elements are determined from a linear fitting to the wavefunction overlaps between states along the configuration coordinate and a reference state (often the neutral equilibrium state). Carrier capture coefficients and cross-sections were computed for the V_Ga–V_N divacancy (considering the -2/-1 and -1/0 transitions) and the Ga_i–V_Ga Frenkel pair (for the -1/0 transition). Further calculations are performed for native point defects, nitrogen Frenkel pairs, and other types of radiation-induced defect complexes in GaN. These results can be utilized to characterize the rate and likelihood for the carrier capture to occur, overall showing how these defects may impact device performance in high-radiation environments.