Compensation Mechanisms in Ge-Doped Al_xGa_1-xN

Al_xGa_1-xN, a semiconductor with a 3-6 eV bandgap, has applications in the next generation of UV optoelectronics and high-power electronics. N-type doping of AlGaN is crucial for the realization of these devices, but charge carrier density decreases as the amount of aluminum increases. Si is usually used as a dopant, but Ge is an n-type dopant that has received consideration lately. For either, compensation commonly causes an insufficient number of charge carriers. We investigated Ge-doped Al_xGa_1-xN for x=0.5 and 0.65 using electron paramagnetic resonance (EPR) at 25-150 K. Typically, for a donor, an EPR signal is seen in the dark. However, a signal was seen only under sufficient illumination at low temperatures, it persisted after illumination, and was quenched by heating. In situ annealing experiments, where the sample was annealed at increasing temperatures and rescanned after cooling, revealed that the x=0.5 sample quenched at 150 K, while the x=0.65 sample was quenched at a lower temperature, 100 K. This decrease in activation energy with increasing Al content is contrary to what is expected for a donor compensated by an acceptor. Rather, these experiments lead us to argue that the Ge dopant forms a DX center, a point defect which has a negative correlation energy and large structural relaxation.