STRUCTURE AND METASTABILITY OF THE EL2 DEFECT IN GAAS

EL2, the dominant native deep donor in GaAs, is one of few observed in trinsic defects in III-V semiconducting compounds. It is particularly interesting because it can be transformed to an excited metastable sta te through illumination of the crystal at low temperature. This articl e reviews experimental data on EL2 with emphasis on the reliable resul ts that allowed the determination pf the microscopic structure of this puzzling defect. Theoretical results, which help to understand and sy stematize the experimental. data, are recalled. This article provides a survey of the present understanding of the microscopic structure and the mechanism of metastability of EL2. Particular attention is given to a recent experiment, performed by the present authors, that determi ned the spatial symmetry of EL2 in the metastable state. In our opinio n, the properties of EL2 are best understood in the framework of the m odel identifying the normal (ground) state of the defect with the isol ated arsenic-antisite As-Ga and the metastable state with the tightly bound gallium-vacancy-arsenic-interstitial VGaAsi defect pair of trigo nal symmetry.