Theoretical study of cation-related point defects in ZnGeP₂

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_Zn²⁻ to Ge_Zn²⁺ 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_Zn²⁺ antisite are favorable in energy. The V_Ge 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.