Theory of the carbon vacancy in 4H-SiC: Crystal field and pseudo-Jahn-Teller effects

The carbon vacancy in 4H-SiC is a powerful minority carrier recombination center in as-grown material and a major cause of degradation of SiC-based devices. Despite the extensiveness and maturity of the literature regarding the characterization and modeling of the defect, many fundamental questions persist. Among them, we have the shaky connection of the EPR data to the electrical measurements lacking sublattice site resolution, the physical origin of the pseudo-Jahn-Teller effect, the reasoning for the observed sublattice dependence of the paramagnetic states, and the severe temperature dependence of some hyperfine signals, which cannot be accounted for by a thermally activated dynamic averaging between equivalent Jahn-Teller distorted structures. In this work, we address these problems by means of semilocal and hybrid density functional calculations. We start by inventorying a total of four different vacancy structures from the analysis of relative energies. Diamagnetic states have well defined low-energy structures, whereas paramagnetic states display metastability. The reasoning for the rich structural variety is traced back to the filling of electronic states which are shaped by a crystal-field-dependent (and therefore site-dependent) pseudo-Jahn-Teller effect. From calculated minimum energy paths for defect rotation and transformation mechanisms, combined with the calculated formation energies and electrical levels, we arrived at a configuration-coordinate diagram of the defect. The diagram provides us with a detailed first-principles picture of the defect when subject to thermal excitations. The calculated acceptor and donor transitions agree well with the binding energies of electrons emitted from the Z(1/2) and EH6/7 traps, respectively. From the comparison of calculated and measured U-values, and correlating the site-dependent formation energies with the relative intensity of the DLTS peaks in as-grown material, we assign Z(1) (EH6) and Z(2) (EH7) signals to acceptor (donor) transitions of carbon vacancies located on the h and k sublattice sites, respectively.