Unintentionally doped GaAs crystals grown from Ga-rich melts without B2O3 encapsulation by the modified vapour pressure controlled Czochralski (VCz) method are analysed. The influence of this growth technique on dislocation density and distribution is presented. The concentration of As precipitates in dependence on the melt composition is shown. Finally, the contents of residual impurities and their influence on some characteristic electrical parameters are discussed. The attention is focused on the feasibility of in situ controlled near-stoichiometric semi-insulating (SI) GaAs crystals with minimized content of As precipitates. (c) 2006 Elsevier B.V. All rights reserved.