The origin of EL2 family evidenced by STM direct observations of individual photoquenching behaviors

Abstract

Abundant point defects in low-temperature grown (LT-) GaAs, presumably As antisite defects, exhibit a photo-induced transformation at low temperatures with an excitation spectrum very close to that obtained for the macroscopic photoquenching effect of EL2 centers. Some of the defects do not exhibit the photo-induced transformation, disfavoring the hypothesis of variants of EL2-like centers differing in the atomic structures. The unquenchable EL2 centers are commonly located near the interface between the LT-GaAs epi-layer and the n-GaAs substrate. Separate macroscopic photoluminescence experiments showed that the photoquenching efficiency is strongly decreased by external compressive stress, which suggests that the absence of the phototransformation behavior in some centers was due to a local stress field induced by the lattice mismatch between the epi-layer and the substrate. This is shown by STM-electric field modulation spectroscopy that the epi-layer was locally strained to a different degree depending on the position from the interface. Therefore, we conclude that the EL2 variants are the result of the spatial variation of the internal stress environment that is felt by the EL2 centers in an identical atomic structure.