Line-shape anomaly in the local vibrational mode of a shallow acceptor in GaAs

Infrared transmission measurements have been performed on the local vibrational modes (LVM) of carbon acceptors in GaAs as a function of compensation ratio. This study indicates that there are two distinct line shapes that are characteristic of the neutral and negative charge states of the carbon acceptors. Because of the interaction between the LVM and the bound hole, the linewidth of the LVM arising from neutral carbon acceptors is significantly wider than that observed for the negatively charged state. In addition, the pressence of the bound hole reduces the bonding strength of the carbon atom to the lattice and lowers the LVM frequency by 0.15 cm-1. The magnitude of the change in the frequency of the LVM as a consequence of the change in charge state of the carbon acceptor is significantly smaller than was proposed recently.