Comment on: ``The effect of charge state on the local vibrational mode absorption of the carbon acceptor in semi-insulating GaAs''

A recent study of the effect of photoquenching of EL2 on the compensation of carbon acceptors in GaAs and the resulting effect on the carbon local vibrational mode, LVM, concluded that there was no broadening of the LVM associated with bound holes on neutral carbon acceptors. The failure to observe broadening of the LVM in this study was a result of the failure to neutralize a significant fraction of the carbon atoms by photoquenching EL2. We show that, in the experiment reported, at most only about 3%-4% of the carbon acceptors were neutralized. Such very small modifications of the ionized carbon concentration are unobservable in the LVM spectra unless the spectral stability and signal-to-noise ratio are exceptional.