  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 10^17 electrons/cm^3 for low
growth temperature (~35 degrees C) and p-type with 10^15-10^16 holes/cm^3 for
elevated growth temperatures (50-120 degrees C). Mobility for both types of
Cu3N was ~0.1-1 cm^2/Vs. 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. 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.
