Isotopic fine structure of the local mode absorption from [Si-Ga-Ge-As] pairs in gallium arsenide

High-resolution infrared spectra from GaAs doubly doped with silicon and germanium show local vibrational mode lines at 373 and 403 cm(-1) due to [Si-Ga-Ge-As] nearest-neighbour pairs. The lower-frequency line, ascribed to the transverse mode, is sharp showing that the paired silicon impurity has three arsenic neighbours ( all As-75). The line from the longitudinal mode at 403 cm(-1) shows fine structure due to the naturally occurring germanium isotopes. A simple theoretical model involving a Coulombic interaction between the silicon and germanium impurities predicts a splitting of the modes of isolated Si-Ga impurities of the correct magnitude but in the opposite sense to that observed.