Deep levels of intrinsic point defects and the nature of “anomalous” optical absorption in ZnGeP2

Abstract

Deep levels of single vacancies and antisite defects in the structure of the ZnGeP2 compound are investigated using the pseudopotential method and an extended unit cell. The data obtained are compared with those for the GaP isoelectronic analog. It is demonstrated that, in the case of the ZnGeP2 crystal, deep levels (degenerate in the GaP structure) are substantially split as a result of lowering the lattice symmetry and anisotropy of the chemical bonding. In particular, the splitting of the V P 0 (t 2) level is equal to 1.58 eV. The averaged levels of defects in the ZnGeP2 compound are in close agreement with the levels of defects in the GaP compound. The absorption coefficients for polarized light are calculated with allowance made for the neutral and charged states of the defects. The optical transitions responsible for the absorption peaks in the IR range of the spectrum of the ZnGeP2 compound are revealed. It is shown that the first peaks are associated with the transitions of electrons from the valence band states located deep in the Brillouin zone to the V Zn −1 and V P 0 deep levels. This leads to a considerable shift (by ∼0.3 eV) of these peaks toward the high-energy range as compared to the energy positions of the deep levels in the band gap with respect to the top of the valence band. The experimental data on the photoinduced EPR spectra of postgrowth and electron-irradiated ZnGeP2 crystals are consistently interpreted by analyzing the electron density.