The insulating nature of about 0.5 of a monolayer of Si -doped material was attributed to extended defects which consisted of self-compensated SiGa-SiAs pairs. The longitudinal and transverse optical modes of chains and bi-layers which were made up of these pairs were calculated by using first-principles methods. It was found that the chain possessed a Raman active longitudinal mode at 470/cm; which dropped to 465/cm, when the chain was depleted of Si, and increased to about 500/cm for Si rafts: that is, (110) chains cross-linked by adjacent (11¯0) chains. These results were in good agreement with recent Raman scattering studies which revealed a band that increased, from 470 to 490/cm, when [Si] was increased from 0.5 of a monolayer to 2 monolayers. The theory also predicted the existence of 3 transverse branches, as well as a longitudinal resonant band. None of these had so far been observed.

R.Jones, S.Oberg: Materials Science Forum, 1995, 196-201, 415-20