Localized vibrational modes of Si DX centers were estimated for various pressures by using the Green’s function technique; with account being taken of lattice relaxation. This yielded very good agreement with experimental results on the localized vibrational modes of DX centers. Since such very good agreement with experiment was obtained, especially in the case of DX centers, it was deduced that the assumption of a negative charge state for the DX centers gave a better result. This supported the prediction, of a quantum mechanical approach, that DX centers could be considered as being negatively charged centers. The calculation assumed that a lattice relaxation of more than 50% of the Kellermann A-parameter, which was associated with the formation of a DX center, was reflected by a change in the Kellermann D-parameter when Si was substituted at the Ga site. The degree of lattice relaxation could be determined from the percentage change in the Kellermann B-parameter upon doping. In general, when the change in the B-parameter exceeded 50% of the change in the A-parameter, this was defined to be a large lattice relaxation. The present problem could be considered to be a case of large lattice relaxation. In the present lattice dynamics investigation, this also implied that there was breathing around the substitutional impurity. Even if there was an opportunity for the impurity to go into the interstitial site, the Td site symmetry was expected to be retained and symmetry breaking was thought to be less probable when the impurity was shallow. When the impurity went into the deep level, additional Jahn-Teller type distortions could reduce the site symmetry to C3v. The very good agreement with experimental results for DX localized vibrational modes supported the predictions that DX centers could be negative centers which involved large lattice relaxations. Overall, the existence of DX centers in GaAs was well-demonstrated by the Green’s function technique. This basically took account only of the bulk properties such as lattice modes, and no detailed knowledge of the band structure was required.

R.Pothiraj, K.Ramachandran: Semiconductor Science and Technology, 1995, 10[11], 1470-4