Fe charge state kinetics in semi-insulating Fe-doped GaN

GaN is a wide bandgap semiconductor with applications in LEDs and high-power devices. One of the problems plaguing this material is a high concentration of residual donors. This issue can be resolved by doping GaN with deep acceptors such as Fe, which compensates donors and creates semi-insulating material. Recently, a photo-induced electron paramagnetic resonance (EPR) spectroscopy study of Fe-doped GaN showed significantly long relaxation times [1]. The study proposed a charge transfer mechanism between Fe$^{3+}$ and Fe$^{4+}$ as an explanation for the phenomenon. However, absorption data from the same samples showed the existence of both Fe$^{2+}$ and Fe$^{3+}$ which suggests that the proposed model involving Fe$^{4+}$ is incorrect and a theory involving an intermediate center is more likely. 3.5 K 10 GHz EPR was performed on HVPE grown free-standing Fe/Si co-doped GaN. Data show an unexpected situation where both donor and Fe$^{3+}$ acceptor signals exist simultaneously. Together with the photo-EPR results, these data reinforce the necessity of invoking a multi-step mechanism for compensation. A model for compensation based on charge transfer between Fe3$+$ and a donor will be described based on EPR and additional material characterization measurements.