First-Principles Studies of Radiation Damage Mechanisms

A radiation event can generate a significant number of vacancies and self-interstitials. A large fraction of these will recombine (“annihilate”) in-situ due to the high mobility of some of these point defects, often resulting in lower net damage than would be expected. Defect annihilation can be either assisted or impeded by Coulomb interactions. If the time scale is such that charge equilibration can occur (i.e., point defects can adjust their charge state according to the local Fermi level), certain vacancy−interstitial pairs may no longer be overall charge neutral, resulting in a barrier to recombination. Based on first-principles calculations using density functional theory we indeed find that the recombination process depends sensitively on the charge state. We will illustrate these effects with the example for interstitial/vacancy pairs, comparing results for GaN and ZnO.

Radiation events also generate large concentrations of electron-hole pairs, and these highly energetic carriers can potentially break bonds. Bonds between hydrogen and host atoms are frequently present at passivated defects, and such bonds have been recognized as particularly prone to breaking in the presence of hot carriers. The mechanism of this bond breaking differs depending on the energetic position of the hydrogen-related states: if the states are in the band gap, carrier capture can be modeled using first-principles techniques that have been developed over the past decade. For the case where states appear as resonances in the host bands, we have developed a new first-principles methodology to model the bond dissociation process.

We acknowledge collaborations with W. Lee, A. Grandhi, and M. Turiansky, fruitful discussions with S. T. Pantelides, M. Fischetti, D. Fleetwood, and R. Schrimpf, and support from AFOSR.