Oxygen-related gettering of silicon during growth of bulk GaAs Bridgman crystals

The role of oxygen during the growth of Bridgman GaAs is studied by comparing chemical analysis, deep-level spectroscopy and electrical measurements for the same materials. The authors establish that the concentration of Si introduced in ingots is fixed by the thermal equilibrium conditions that prevail at the growth temperature in the presence of oxygen for all the values of initial Si doping in the melt used in this present study. Actual gettering of Si is observed in ingots that were intentionally doped with Si and simultaneously treated with Ga2O3. On the other hand, it is proved that addition of Ga2O3 in the melt does not increase the concentration of the deep-donor EL2 which is believed to be unrelated to oxygen, and the presence of oxygen is not observed to modify the concentration of other deep levels. Taking into consideration all the observations reported, a compensation scheme is proposed to account for the values of resistivity measured in undoped or slightly Cr-doped Bridgman crystals.