Crystals were grown without B2O3 encapsulation via 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. With increasing Ga content of the melt the omnipresent As precipitate concentration decreased in as-grown GaAs crystals. Semi-insulating GaAs crystals could been obtained even from melts with a composition of 46at%As. Comparative cathodoluminescence measurements on those crystals and on boron-contaminated GaAs showed that the luminescence bands at 1.316eV and at 1.441eV were related to BAs. Reduced concentrations of boron and EL2 result in significant lower near-band edge absorption, mainly caused by the photo-ionization cross section of As antisites in semi-insulating GaAs. These VCz GaAs crystals possess improved optical properties. Semi-conducting GaAs:Si crystals were grown from near-stoichiometric melts. Positron annihilation lifetime spectroscopy measurements showed that the low boron concentration in the semi-conducting VCz GaAs always resulted in 3-times higher Ga vacancy concentrations compared to standard semi-conducting GaAs. Cathodoluminescence bands due to optical transitions at 0.95eV and 1.15eV were related to auto-compensating (VGaSiGa)2- and (SiGaVGaSiGa)- complexes, respectively, and depended upon the doping level.

Boron- and Stoichiometry-Related Defect Engineering during B2O3-Free GaAs Crystal Growth. F.M.Kiessling, M.Albrecht, K.Irmscher, M.Rossberg, P.Rudolph, W.Ulrici, R.Fornari: Physica Status Solidi C, 2009, 6[12], 2778-84