A study of the lattice imperfections in Li-diffused GaAs by transmission electron microscopy

Abstract

The lattice imperfections in GaAs single crystals introduced as a result of the in-diffusion of lithium at high temperatures were investigated by using transmission electron microscopy. The effects of diffusion temperature, time, postdiffusion annealing treatments, and the presence of other impurities (primarily Zn at concentration levels of 1×1019 cm−3) in the host crystal, on the defect structure have been determined. The results indicate that when Li-saturated GaAs single crystals are cooled to room temperature following high-temperature diffusion, precipitation of Li occurs readily in the form of tetrahedral particles, the size and density of which decrease with decreases in diffusion temperature. Postdiffusion annealing at temperatures of 600 °C and above caused the formation of additional precipitates, vacancy-type prismatic dislocation loops, and a few extrinsic stacking faults. Diffusion of Li into originally Zn-doped GaAs ([Zn]∼1×1019 cm−3) at 700 °C, on the other hand, did not lead to the formation of Li precipitates; however, a moderately high density of small vacancy-type prismatic loops was observed in the microstructure. On increasing the diffusion temperature to 850 °C the substructure revealed the presence of stacking faults and a small density of precipitates in addition to a number of large vacancy-type prismatic loops. The size density and distribution of these defects were found to be dependent on the diffusion time at 850 °C. The origins of the observed lattice defects have been discussed in the light of the previous localized-vibrational-mode (LVM) infrared spectroscopy measurements on materials with identical chemical composition and thermal history as in the present study. A definite correlation between the microstructural features and defect structure as determined by infrared absorption measurements was found to exist.