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Isotopic fine structure of the local mode absorption from [SiGa-GeAs] 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 [SiGa-GeAs] 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 75As). 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 SiGa impurities of the correct magnitude but in the opposite sense to that observed.