Piezospectroscopic evidence for tetrahedral symmetry of theEL2 defect in GaAs

Splittings of the EL2 zero-phonon line (ZPL) at 8378 ${\mathrm{cm}}^{\mathrm{-}1}$ under uniaxial stress applied along the [100], [111], and [110] directions have been measured. The observed splittings together with polarization selection rules clearly indicate the tetrahedral $T_d$ symmetry of the EL2 defect ruling out any other point groups, in particular, $C_{2v}$ and $C_{3v}$. The ZPL is due to the $A_1$→$T_2$ (or $A_2$→$T_1$) electric dipole transition. A state of $A_1$ symmetry lying 65±10 ${\mathrm{cm}}^{\mathrm{-}1}$ above the $T_2$ state is needed in quantitative analysis of observed splittings to account for the nonlinear stress dependence of certain split components. Contradiction between the symmetry of EL2 deduced from piezospectroscopic and optically detected electron-nuclear double resonance experiments is discussed.