Understanding and Control of Bipolar Self-Doping in Copper Nitride

Semiconductor materials that can be doped both n-type and p-type are desirable for diode-based applications and transistor technology. Copper nitride (Cu₃N) is a metastable semiconductor with a solar-relevant bandgap that has been reported to exhibit bipolar doping behavior. However, deeper understanding and better control of the mechanism behind this behavior in Cu₃N is currently lacking in the literature. In this work, we use combinatorial growth with a temperature gradient to demonstrate both conduction types of phase-pure, sputter-deposited Cu₃N thin films. Room temperature Hall effect and Seebeck effect measurements show n-type Cu₃N with an electron density of 10¹⁷ cm^-3 for low growth temperature (≈ 35 °C) and p-type with a hole density between 10¹⁵ cm^-3 and 10¹⁶ cm^-3 for elevated growth temperatures (50 °C to 120 °C). Mobility for both types of Cu₃N was ≈ 0.1 cm²/Vs to 1 cm²/V. Additionally, temperature-dependent Hall effect measurements indicate that ionized defects are an important scattering mechanism in p-type films. By combining X-ray absorption spectroscopy and first-principles defect theory, we determined that V_Cu defects form preferentially in p-type Cu₃N while Cu_i defects form preferentially in n-type Cu₃N; suggesting that Cu₃N is a compensated semiconductor with conductivity type resulting from a balance between donor and acceptor defects. Based on these theoretical and experimental results, we propose a kinetic defect formation mechanism for bipolar doping in Cu₃N, that is also supported by positron annihilation experiments. Overall, the results of this work highlight the importance of kinetic processes in the defect physics of metastable materials, and provide a framework that can be applied when considering the properties of such materials in general.