Temperature dependence of electrical properties of gallium-nitride bulk single crystals doped with Mg and their evolution with annealing

Comprehensive studies of the electrical properties of Mg-doped bulk GaN crystals, grown by high-pressure synthesis, were performed as a function of temperature up to 750 degreesC. Annealing of the samples in nitrogen ambient modifies qualitatively their resistivity values rho and the rho (T) variation. It was found that our material is characterized by a high concentration of oxygen-related donors and that the charge transport in the studied samples is determined by two types of states, one of shallow character (Mg-related state, E-A approximate to 0.15 eV), and the second one much more deep, E-2 approximate to 0.95 eV (above the valence band). Depending on the effective concentration of either states, different resistivities rho can be observed: lower resistivity (rho < 10(4) Ohm cm at ambient temperature) in samples with dominant E-A states and very high resistivity (rho > 10(6) Ohm cm at ambient temperature) in samples with dominant E-2 states. For the first type of samples, annealing at T-ann < 500 degreesC leads to a decrease of their resistivity and is associated with an increase of the effective concentration of the shallow Mg acceptors. Annealing of both types of samples at temperatures between 600 and 750 degreesC leads to an increase of the deep state concentration. The presence of hydrogen ambient during annealing of the low-resistivity samples strongly influences their properties. The increase of the sample resistivity and an appearance of a local vibrational mode of hydrogen at 3125 cm(-1) were observed. These effects can be removed by annealing in hydrogen-free ambient. (C) 2001 American Institute of Physics.