Infrared absorption of silicon isotopes in gallium phosphide

Infrared absorption measurements at ∼80°K on 28Si- and (28Si+30Si)-doped GaP grown by liquid-phase epitaxy and compensated by electron irradiation are presented. Several new bands are observed with samples containing the mixed isotopes. The observed frequencies of 28Si-related bands agree with previously reported values, and the isotopic shifts for 30Si-related bands agree with values calculated from local mode theory. The results indicate that one band is attributable to a defect containing two Si atoms on nonequivalent sites, presumably nearest-neighbor substitutional pairs. The present measurements confirm the previous observations of a much larger concentration of pairs than that predicted by random statistics.