Abstract
Pseudo III-V nitride ZnSnN2 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 ZnSnN2 degenerately doped with oxygen (ZnSnN2−xOx) in the ultraviolet to infrared region to determine the conduction-band effective mass (mc*) and intrinsic bandgap (Eg). ZnSnN2−xOx 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, mc* was determined to be (0.37 ± 0.05)m0 (m0 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 Eg 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.
Highlights
Photovoltaics represent a promising approach to achieving truly sustainable energy, and considerable efforts have been directed toward the development of high-efficiency photovoltaic cells to meet the increasing global energy demand
The conduction-band effective mass and intrinsic bandgap of disordered ZnSnN2 were optically determined to get an insight into the fundamental properties
Oxygen-doped ZnSnN2−xOx epitaxial layers were grown with various electron densities
Summary
Photovoltaics represent a promising approach to achieving truly sustainable energy, and considerable efforts have been directed toward the development of high-efficiency photovoltaic cells to meet the increasing global energy demand. As for semiconductor alloys, InxGa1−xN is a candidate material for a photovoltaic absorber because the Eg can be tuned to ~1.4 eV by adjusting the indium content (x) to ~0.653 These compounds are composed of rare or toxic elements, and it is difficult to produce cost-effective photovoltaic cells based on these semiconductors on large-area substrates. Veal et al conducted the first principle calculations and indicated that the ordered- and disordered-phases have Eg values of 2.3 and 0.98 eV, respectively[9] Based on these values, we thought that the disordered ZnSnN2 is more ideal for a photovoltaic absorber. The theoretical study predicted small mc* values of 0.12m0 (m0 denotes the free-electron mass)[9], whereas the experimental study yielded a value four times larger at 0.5m0 This large discrepancy motivated us to discover which value is correct. Mc* in many semiconductors has determined by this method
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