Microstructure changes after annealing of undoped and Cr-doped liquid-encapsulated Czochralski-grown GaAs

Abstract

Microinhomogeneities in semi-insulating, liquid-encapsulated Czochralski (LEC) GaAs crystals were investigated by selective photoetching and spatially resolved photoluminescence at 1.8 K. The interaction of point defects with dislocations was studied by annealing the samples in a metalorganic chemical vapor deposition reactor under AsH3 pressure. The changes in microstructure, induced by the anneal process, were investigated for both undoped and Cr-doped crystals. A weakening of the photoetching contrast after annealing was observed for both types of crystals, with no essential difference between undoped and Cr-doped ones. The overall near-band-gap photoluminescence (PL) intensities were found to have increased after annealing. This increase amounted to about one order of magnitude for the Cr-doped crystals, and two orders of magnitude for the undoped ones. In addition, for the undoped samples the contrast in PL intensity between dislocated areas and the undisturbed crystal matrix was increased by the anneal process. Analysis of the local PL spectra taken at different regions on these samples showed a strong enhancement of acceptor-bound exciton (A0,X) transitions relative to donor-bound exciton (D0,X) transitions, especially at dislocated areas. Furthermore, a new emission for LEC GaAs at 1.5116±0.0003 eV was also found to be strongly enhanced at these regions. The PL results are explained by the disappearance of deep, nonradiative, excess As-related PL killer centers during annealing. The disappearance is proposed to proceed via diffusion of interstitial As atoms and the formation of As precipitates, for which dislocations act as nucleation centers. Additional deep level photoluminescence spectra of the 0.65 and 0.80-eV emission bands are shown to be in accordance with the proposed model.