Optical and EPR study of the nickel two-electron-trap state in GaP

The infrared luminescence and the absorption spectra of the nickel two-electron-trap state, ${\mathrm{Ni}}^{+}$ $3d^9$, have been observed and analyzed in GaP. The spectra do not exhibit full mirror symmetry relative to the common zero-phonon line (ZPL) at 5 354 ${\mathrm{cm}}^{-1\mathrm{}}$. Chemical identification of the defect is based on the fine structure of the ZPL which is shown to arise from an isotope effect. The Zeeman splitting of the ZPL in fields up to 5.3 T unambiguously demonstrates that the zero-phonon transition occurs between a cubic ${\Gamma }_8$ excited and a ${\Gamma }_7$ ground state, implying that the center is in the ${\mathrm{Ni}}^{+}$ $3d^9$ charge state. The analysis of the Zeeman data yields $g=-0.94\mathrm{}$ for the ${\Gamma }_7$ state and $g_1=1.45\mathrm{}$, $q{g}_2=-0.23\mathrm{}$ for the ${\Gamma }_8$-state $g$ factors. The ${\mathrm{Ni}}^{+}$ center was also identified in the EPR spectrum of GaP: Ni and the EPR $g$ value is fully consistent with the Zeeman results. Certain details of the optical spectra and the strongly quenched $g$ factors indicate a moderate Jahn-Teller (JT) coupling within the ${}_{}{}^2{}_{}{}^{}T_2^{}{}_{}{}^{}$, and also a moderate JT coupling within the ${}_{}{}^2{}_{}{}^{}E$ state of ${\mathrm{Ni}}^{+}$. It is shown that static-crystal-field theory is insufficient to account for the data, and a consistent interpretation based on existing JT theories is presented.