Fourier-transform infrared-absorption studies of intracenter transitions in theEL2level in semi-insulating bulk GaAs grown with the liquid-encapsulated Czochralski technique

The intracenter transitions that are responsible for the characteristic absorption band of EL2 in the spectral region of 8060–10?0 ${\mathrm{cm}}^{\mathrm{-}1}$ were studied using a high-resolution Fourier-transform infrared spectrophotometer. The results show that the zero-phonon line observed at 8379 ${\mathrm{cm}}^{\mathrm{-}1}$ has a complex structure and therefore cannot be assigned to a unique center. The phonon replicas associated with the zero-phonon line indicate that couplings other than transverse acoustic phonon may exist. A group-theoretical analysis of the present results shows that the EL 2 center has a symmetry lower than that of $T_d$ suggesting that EL 2 can be identified as a pair defect which involves an arsenic antisite with a vacancy or interstitial.