. Broader ContextThin film photovoltaics (PV), based on materials that absorb light 10-100 times more efficiently than crystalline silicon, has the potential to drive down PV module cost by increasing efficiency and requiring less raw material. Currently, the market for thin film PV is dominated by CdTe and CuIn x Ga 1-x Se 2 technologies, which suffer from concerns of toxicity (Cd) or rarity (Te, In) when considering terawatt-scale deployment. As an alternative to these technologies, researchers have turned to studying new absorber materials that exhibit equivalent light absorbing properties but are comprised of Earth-abundant elements. In this work, we explore the properties of a thin film III-N analog, ZnSnN 2 , that until recently has received little attention in the literature. Classification as a III-N analog is advantageous for PV applications, considering that III-N materials are well-known for their stability. ZnSnN 2 possesses a direct bandgap and large absorption coefficient in a PV-relevant energy range, in addition to being composed of abundant and non-toxic elements. However, a few key optoelectronic properties (e.g. doping control and exact bandgap) of this material are not well understood. If this challenge is addressed, ZnSnN 2 has excellent potential as an absorber material for Earth-abundant thin film PV. AbstractZnSnN 2 is an Earth-abundant semiconductor analogous to the III-Nitrides with potential as a solar absorber due to its direct bandgap, steep absorption onset, and disorder-driven bandgap tunability. Despite these desirable properties, discrepancies in the fundamental bandgap and degenerate n-type carrier density have been prevalent issues in the limited amount of literature available on this material. Using a combinatorial RF co-sputtering approach, we have been able to explore a growth-temperature-composition space for Zn 1+x Sn 1-x N 2 over the ranges 35-340• C and 0.30-0.75 Zn/(Zn+Sn). In this way, we were able to identify an optimal set of deposition parameters for obtaining as-deposited films with wurtzite crystal structure and carrier density as low as 1.8 x 10 18 cm -3 . Films grown at 230• C with Zn/(Zn+Sn) = 0.60 were found to have the largest grain size overall (70 nm diameter on average) while also exhibiting low carrier density (3 x 10 18 cm -3 ) and high mobility (8.3 cm 2

Inorganic nitrides with wurtzite crystal structures are well-known semiconductors used in optoelectronic devices. In contrast, rocksalt-based nitrides are known for their metallic and refractory properties. Breaking this dichotomy, here we report on ternary nitride semiconductors with rocksalt crystal structures, remarkable optoelectronic properties, and the general chemical formula MgxTM1-xN (TM=Ti, Zr, Hf, Nb). These compounds form over a broad metal composition range and our experiments show that Mg-rich compositions are nondegenerate semiconductors with visible-range optical absorption onsets (1.8-2.1 eV). Lattice parameters are compatible with growth on a variety of substrates, and epitaxially grown MgZrN2 exhibits remarkable electron mobilities approaching 100 cm 2 V -1 s -1 . Ab initio calculations reveal that these compounds have disorder-tunable optical properties, large dielectric constants and low carrier effective masses that are insensitive to disorder. Overall, these experimental and theoretical results highlight MgG-3TMNG-2 rocksalts as a new class of semiconductor materials with promising properties for optoelectronic applications.