Microdefects in Si-doped GaAs single crystals grown by the vertical gradient freeze method have been studied with X-ray diffuse scattering. In the case of doping to majority carrier concentrations n ～ 1 × 1018 cm?3, large microdefects with positive dilatation that accompany the initial stage of arsenic precipitation at high temperatures were observed. It is shown that GaAs samples heavily doped with silicon (n ～ 3 × 1018 cm?3) contain large (several micrometers) interstitial microdefects, which can play the role of nucleation regions for new SiAs and SiAs2 phases.
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