Study on the mechanism of helium platelets formation at low temperatures in SiC from the perspective of atomic diffusion

Abstract

To explain the room-temperature formation of He bubbles in α-SiC, we present the influence of mono- and di-vacancies on the migration behavior of He atoms in the 6H-SiC. The solution energy of He atoms, inserted at different interstitial sites around vacancies, is calculated via density functional theory (DFT). The results show that single carbon (C) vacancy and silicon (Si) vacancy can reduce the solution energy of a He atom within the 4th nearest neighbor (NN) site and the 2nd NN site, which suggests that C vacancies are more likely to become He bubble nucleation sites than Si vacancies. Moreover, both single C and Si vacancies provide a circular migration channel with a radius of 0.2 nm for He atoms but render a little influence on the migration behavior of farther He atoms. Besides, it is demonstrated that the dissociation of He atoms from a vacancy to bulk is hindered by the high migration barrier between the first NN site and the second NN site. However, double C vacancies that are the 1st NN sites of each other exhibit a strong coupling effect and alter the site occupancy of He atoms between vacancies. Besides, the migration barrier of a He atom from a vacancy to another adjacent vacancy is smaller than that of a He atom dissociated from a single vacancy to the matrix, which indicates that He atoms can achieve a long-distance migration between vacancies at low temperatures. Hence, the room-temperature formation of He platelets is mainly achieved by vacancies-assisted migration of He atoms.