Theoretical study of cation-related point defects in ZnGeP2 -: art. no. 205212

First-principles calculations are presented for the V-Zn and V-Ge cation vacancies and the Zn-Ge and Ge-Zn antisites in ZnGeP2, using full-potential linearized muffin-tin orbital method supercell calculations in the local-density approximation to density-functional theory. Under Zn-poor conditions, the lowest Gibbs energy defects are found to be the Ge-Zn and V-Zn defects, leading to a compensated p-type material in agreement with experimental evidence. The occupation energy levels of the defects are determined and compared with available experimental information. As expected, the Ge-Zn is found to be a donor while the other three are acceptors. Good agreement is obtained with optical quenching and activation of electron paramagnetic resonance signal studies if a direct transfer of electrons from V-Zn(2-) to Ge-Zn(2+) is assumed rather than a process via the conduction band. This suggests a close association of the dominant acceptors and donors. This is further confirmed by showing that the formation of complexes consisting of two V-Zn with a single Ge-Zn(2+) antisite are favorable in energy. The VGe on the other hand is found to have high energy of formation under any chemical potential conditions and is found to be unstable toward formation of a V-Zn and Zn-Ge pair. Structural relaxation of all defects is performed but no symmetry breaking distortions are found. As a result, the defect wave functions of the unpaired electron in the V-Zn(-) is found to be spread equally over the four neighboring P atoms, in disagreement with electron nuclear double resonance data which indicate primary localization on a pair of P atoms. Several possible origins for this discrepancy are discussed.