Diffusion mediated by native point defects does not generally occur in semiconductors at room temperature (RT) because of high activation energies. However, recent observations of plastic deformation in AlGaN/GaN High Electron Mobilty Transistors (HEMTs) in AlGaN epilayers on GaN in the presence of strain and electric fields have been attributed to diffusive processes. Here we report the results of first-principles density-functional calculations of formation and migration energies of vacancies under strain and electric fields that allowed us to identify the enablers of self-diffusion at RT in AlGaN/GaN structures: triply-negative cation vacancies with near-zero formation energy, driven by an electrostatic potential gradient. We show that the formation energies of Ga and Al vacancies in unstrained n-type AlGaN are near zero but their migration energies are too large, >1.5 eV, for appreciable diffusion at RT (typically the diffusion activation energy, i.e., sum of formation and migration energies, must be ~1 eV or lower for diffusive processes to be appreciable at RT). We find that application of strain, even at substantial levels, has little effect on either formation or migration energies. However, the Ga and Al vacancies are triply negative, and application of an electric field lowers the barrier for migration by more than 0.5 eV. At the observed critical values of the field, the net activation energy is lowered down to approximately 1 eV which makes thermally-activated atomic migration possible at RT. Simulation of the AlGaN/GaN HEMT shows that the electric field is highest in the region where the plastic deformation is observed. These results provide a mechanism for plastic deformation mediated by vacancies, much like Nabarro-Herring creep and dislocation climb, but with an electric field being the main driver (vacancy drift). In addition, unusually high local strain can also lead to dislocation glide and further dislocation formation via strain relaxation, compounding the role of vacancy migration processes.

 

The work was supported in part by ONR MURI grant N-00014-08-1-0655 and by the McMinn Endowment at Vanderbilt University.