Kick-out diffusion of Al in 4H-SiC: an ab initio study

摘要

As a semiconductor with a wide bandgap, 4H silicon carbide (4H-SiC) has considerable potential for high-temperature and high-power devices. It is widely established that p-type 4H-SiC is formed predominantly by doping Al. Although Al diffusion in 4H-SiC is often negligible at low temperatures due to the tight bonding of Al in 4H-SiC, the diffusion coefficient of Al dramatically rises when the temperature is rather high. While diffusion is the most fundamental physical processes, the diffusion mechanism of Al in 4H-SiC remains unknown. Due to the large atomic radius of Al relative to the host Si/C atoms and the fact that Al occupies the Si lattice in 4H-SiC, the diffusion of Al is typically mediated by point defects such as vacancies and self-interstitials. We now investigate the diffusion of Al in 4H-SiC using first-principles calculations and compare the activation energy of Al diffusion mediated by carbon vacancies (V_C) to that of Al diffusion mediated by Si interstitials (Si_i). It is found that Al diffusion is actually a Si_i-mediated process, in which a nearby Si_i first kicks a substitutional Al atom to an interstitial site. The kicked-out Al then spreads via interstitial sites. The diffusion coefficient is calculated, which is comparable to experimental results.