GaAs crystals have been grown without B2O3 encapsulation by the vapour-pressure-controlled Czochralski (VCz) method to reduce undesired boron incorporation and to enable in situ control of the Ga/As ratio in the melt. Semi-insulating boron-reduced 2 and 3 in diameter twin-free GaAs crystals were grown from Ga-rich melts with compositions down to 46 at% arsenic. Cathodoluminescence (CL) measurements on crystals grown from Ga-rich melts show that the luminescence bands at 1.316 and at 1.441 eV are related to the B-As. Semiconducting GaAs crystals were grown from stoichiometric melts. The donor was incorporated by silicon doping or using fused silica crucibles. Performing positron annihilation lifetime spectroscopy shows that the low boron concentrations in our semi-conducting VCz GaAs result always in higher Ga vacancy concentrations compared to standard GaAs. CL bands due to optical transitions at 0.95 and 1.15 eV are related to auto-compensating (VGaSiGa)(2-) and (SiGaVGaSiGa)(-) complexes, respectively, and depend on the doping level. (C) 2007 Elsevier B.V. All rights reserved.