Conduction-band effective mass and bandgap of ZnSnN ₂ earth-abundant solar absorber

Abstract

Pseudo III-V nitride ZnSnN ₂ is an earth-abundant semiconductor with a high optical absorption coefficient in the solar spectrum. Its bandgap can be tuned by controlling the cation sublattice disorder. Thus, it is a potential candidate for photovoltaic absorber materials. However, its important basic properties such as the intrinsic bandgap and effective mass have not yet been quantitatively determined. This paper presents a detailed optical absorption analysis of disordered ZnSnN ₂ degenerately doped with oxygen (ZnSnN _{2− x} O _x ) in the ultraviolet to infrared region to determine the conduction-band effective mass ( m _c ^*) and intrinsic bandgap ( E _g). ZnSnN _{2− x} O _x epilayers are n-type degenerate semiconductors, which exhibit clear free-electron absorption in the infrared region. By analysing the free-electron absorption using the Drude model, m _c ^* was determined to be (0.37 ± 0.05) m ₀ ( m ₀ denotes the free electron mass). The fundamental absorption edge in the visible to ultraviolet region shows a blue shift with increasing electron density. The analysis of the blue shift in the framework of the Burstein-Moss effect gives the E _g value of 0.94 ± 0.02 eV. We believe that the findings of this study will provide important information to establish this material as a photovoltaic absorber.