Identification of the double acceptor state of isolated nickel in gallium arsenide

The zero-phonon line (ZPL) at 4615 ${\mathrm{cm}}^{-1\mathrm{}}$ in the absorption spectrum of $n$-type GaAs:Ni has been measured under uniaxial stress and magnetic field. The observed ZPL splittings conclusively demonstrate that the ZPL is due to the ${\Gamma }_7\left({}_{}{}^2{}_{}{}^{}T_2^{}{}_{}{}^{}\right)\to {\Gamma }_8\left({}_{}{}^2{}_{}{}^{}E\right)$ intracenter transition of isolated ${\mathrm{Ni}}^{+}\left(3\mathrm{}{d}^9\right)$, thus confirming that the doubly ionized state of the Ni acceptor is a stable charge state in $n$-type GaAs. The $g$ factors of the ground state ($g_7=-1.16\mathrm{}$) and of the excited state ($g_1=1.31,g_2=-0.29\mathrm{}$) are analyzed in terms of the Jahn-Teller $\epsilon$-mode coupling within the ${}_{}{}^2{}_{}{}^{}T_2^{}{}_{}{}^{}$ ground state.