Abstract
We report on the successful growth of undoped semi-insulating GaAs single crystals with low dislocation densities by the vertical gradient freeze (VGF) method using a proper quantity of B_2O_3 and precisely controlling the As-pressure. Impurity analysis shows that the use of B2C>3 is effective in preventing contamination with silicon from the quartz ampoule and also in reducing carbon concentration in the crystal, and that high purity GaAs crystals are obtained by using B_2O_3. Hall measurements give the activation energy as 0.8 eV for the annealed samples having the resistivity of 10^8Ω・cm. The average melt composition was estimated by monitoring the As-pressure during crystal growth. It was confirmed that the EL2 concentration depended on the melt composition in the same fashion as for LEG (Liquid Encapsulated Czochralski)-grown crystals. This observation, combined with the spatial distribution of EL2, indicates that the gradient of melt composition in the vertical direction is important in controlling the stoichiometry.
