Mechanism for homogenizing electrical properties of semi-insulating GaAs during ingot annealing

Concentrations of total ionized point defects, neutral and ionized EL2, carbon acceptor, and other shallow donor and acceptor levels in as-grown and as-annealed semi-insulating GaAs crystals were characterized using nuclear magnetic resonance, multiwavelength near-infrared photoabsorption, and Fourier-transformed infrared photoabsorption. In as-grown crystals, shallow donors (X_D) and acceptors (X_A) of about 2×10 ¹⁵ cm^-3 concentration were found to exist. The net donor concentration (N_XD - N_XA) of these levels showed an excellent positive correlation with the carbon acceptor in its distribution within the wafer, implying the donor's carbon-acceptor-related origin. An anneal at 850 °C for 24 h diminished these levels. By regarding the intrinsic donor as V_As, the improvement in electrical uniformity during ingot annealing is explained in terms of an annihilation of both intrinsic acceptors and precursors for Si_As acceptors.