Oxygen Incorporation as a Route to Nondegenerate Zinc Nitride Semiconductor Thin Films

Zinc nitride (Zn₃N₂) comprises earth-abundant elements, possesses a small direct bandgap, and is characterized by high electron mobility. While these characteristics make the material a promising compound semiconductor for various optoelectronic applications, including photovoltaics and thin-film transistors, it commonly exhibits unintentional degenerate n-type conductivity. This degenerate character has significantly impeded the development of Zn₃N₂ for technological applications and is commonly assumed to arise from incorporation of oxygen impurities. However, consistent understanding and control of the role of native and impurity defects on the optoelectronic properties of this otherwise promising semiconductor have not yet emerged. Here, we systematically synthesize epitaxial Zn₃N₂ thin films with controlled oxygen impurity concentrations of up to 20 at % by plasma-assisted molecular beam epitaxy (PA-MBE). Contrary to expectations, we find that oxygen does not lead to degenerate conductivity but instead serves as a compensating defect, the control of which can be used to achieve nondegenerate semiconducting thin films with free electron concentrations in the range of 10¹⁷ cm^-3, while retaining high mobilities in excess of 200 cm² V^-1 s^-1. This understanding of the beneficial role of oxygen thus provides a route to controllably synthesize nondegenerate O-doped Zn₃N₂ for optoelectronic applications.