Identification Of The Double Acceptor State Of Isolated Nickel In Gallium-Arsenide

The zero-phonon line (ZPL) at 4615 ${\mathrm{cm}}^{\ensuremath{-}1}$ 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 ${\ensuremath{\Gamma}}_{7}(^{2}T_{2})\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{8}(^{2}E)$ intracenter transition of isolated ${\mathrm{Ni}}^{+}(3{d}^{9})$, 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 (${g}_{7}=\ensuremath{-}1.16$) and of the excited state (${g}_{1}=1.31,{g}_{2}=\ensuremath{-}0.29$) are analyzed in terms of the Jahn-Teller $\ensuremath{\epsilon}$-mode coupling within the $^{2}T_{2}$ ground state.