Infrared absorption of mixed silicon isotope pairs in gallium arsenide

The infrared absorption of GaAs doped with 28Si, 30Si, or 28Si + 30Si and compensated by 6Li diffusion or electron irradiation is reported. Isotopic shifts close to those predicted from local-mode theory are observed for all the silicon defect bands in 30Si-doped GaAs from frequencies previously reported for 28Si-doped material. a new band is observed at 456 cm−1 for the sample containing both 28Si and 30Si, and is attributed to (28Si–30Si) pairs. The frequency of the new band is in accord with simple theory, and its presence confirms the observation of the nearest-neighbor silicon pairs. The results indicate, however, that the indentification of one of the three absorption bands previously attributed to the 28Si pairs is questionable. The experimental result also indicates that an absorption band at 369 cm−1 previously observed in electron-irradiated samples in indeed due to a silicon-related defect.