Characterization of defects in Mg doped GaN epitaxial layers using conductance measurements

Mg-doped GaN films have been grown on sapphire by metalorganic vapour phase epitaxy. Two different buffer layer schemes between the film and the sapphire substrate were used, giving rise to differing threading dislocation densities (TDDs) in the GaN. Frequency-dependent capacitance and conductance measurements have been used to study the electronic states associated with the Mg doping, and to determine how these are affected by the TDD. Current-voltage and admittance spectroscopy measurements were carried out using Al/Au Schottky contacts and Ni/Au Ohmic contacts fabricated in a dot-and-ring pattern. Frequency-dependent measurements on these devices were corrected to remove the series resistance effects arising from the well known high resistivity of Mg-doped GaN using formulae derived from the equivalent parallel circuit model. Thermal admittance spectroscopy, in which the conductance is monitored as a function of temperature, verified the existence of a single impurity-related acceptor level in samples with different TDDs at 165 +/- 10 meV. For all of the samples the extrapolated capture cross-section values were very small, in the range of similar to 10(-20) cm(-2), suggesting that the acceptor level could be very repulsive. Since admittance spectroscopy detects only majority carrier traps, we believe these defect signature values are most probably associated with the Mg acceptor state as they are very close to the results previously reported by other research groups using different techniques. (C) 2011 Elsevier B.V. All rights reserved.