First principles study of electronic structures of defects in zirconium germanium phosphate and defect chalcopyrites.

摘要

This thesis mainly focuses on a study of the point defects in ZnGeP 2. Density functional theory (DFT) is used in the local density approximation (LDA) in conjunction with the full-potential linearized muffin-tin orbital (FP-LMTO) method, modeling defects with the supercell approach. Under prevalent Zn-poor conditions, the GeZn double donor and VZn shallow acceptor are found to have the lowest formation energies, which explains the compensated p-type nature of the material. Good agreement is obtained with the energy levels deduced from optical quenching and activation of the EPR signals, if a direct transfer of electrons from V2-Zn to Ge2+Zn is assumed to occur rather than a process via the conduction band. The VGe is found to have high energy of formation under any chemical potential conditions and is found to be unstable towards formation of VZn and GeZn. Structural relaxation of all defects is performed but no symmetry breaking distortions are found. The defect wavefunctions 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 (ENDOR) data which indicate primary localization on a pair of P atoms. Several possible origins for this discrepancy are examined. Alternative assignments of the AL1 EPR signal to ZnGe, or complexes such as Zni-VZn, V-Zn -G2+Zn-V- Zn are discarded although the latter complex is found to be favorable in energy. The possibility of a failure of the LDA due to its incompletely cancelled self-interaction is examined using Hartree-Fock cluster calculations. A distortion is found to occur in Hartree-Fock but not in LDA. However, it is different from the experimental one. Finally, a dangling bond and group theory model is proposed for a Jahn-Teller distortion which can explain the localization observed by ENDOR.; In the final chapter, the electronic band structures of the ordered vacancy defect chalcopyrites with formula II-III2-VI4 for II=Zn, Cd, Hg, III=Al, Ga, In and VI=S, Se, Te are studied, including an empirical gap correction.