Abstract
We have performed positron-lifetime and Doppler-broadening experiments before and after illumination of undoped semi-insulating GaAs with 0.7--1.5-eV photons. When the EL2 defect is transformed to the metastable state by 1.1--1.3-eV photons, we observe a metastable vacancy defect. Its concentration is proportional to the EL2 concentration. The metastable vacancy has smaller open volume than isolated Ga and As vacancies and it is connected to a negative charge. The vacancy is generated with an optical cross section of 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ ${\mathrm{cm}}^{2}$ at the photon energy of 1.15 eV and it recovers at 120 K. As these properties are identical to those of the metastable state of EL2, we conclude that the observed vacancy is involved in its atomic configuration. The results are in good agreement with the isolated As antisite model for the structure of the EL2 defect.
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