The thin-film synthesis of high-pressure phases in inorganic compounds remains a challenge. The synthesis of high-pressure phases in thin-film form opens potential opportunities for creating unique optoelectronic devices because high-pressure phases often exhibit intriguing characteristics that cannot be accessed in ambient phases. We investigated a high-pressure phase of MgSnN2 with the rocksalt structure (rs-MTN) which has only been identified in recent years. rs-MTN is a direct-gap compound, and its (111) plane matches perfectly with GaN(001), which implies that rs-MTN is a promising candidate for optoelectronic materials for light-emitting diodes and tandem solar cells. However, single-phase rs-MTN has never been synthesized in either thin-film or single-crystalline forms. Herein, single-phase rs-MTN thin films were successfully synthesized via two routes. One was the high-pressure heat treatment of wurtzite-type MTN precursor layers, and the other was direct growth onto isostructural MgO(111) substrates using reactive co-sputtering. The former route exploited the pressure-induced wurtzite-to-rocksalt transition and was designed based on first-principles calculations that predicted a transition pressure of ∼8 GPa. The latter route utilized epitaxial stabilization on the (111) plane of MgO. The direct growth of the rs-MTN films with smooth surfaces enabled the investigation into their optoelectronic properties. Consequently, the rs-MTN films were found to be n-type semiconductors with electron densities of an order of 1017 cm–3 and a band gap of 2.3 eV. These findings provide a platform for developing rs-MTN as an optoelectronic semiconductor.

MgSnN2 with an average wurtzite structure (wz-MgSnN2) has recently emerged as a pseudo-III-nitride semiconductor, studied for applications in tandem solar cells, green light-emitting diodes, and other optoelectronic devices. This compound has only been researched recently, and, therefore, its charge-carrier transport properties are poorly understood. Understanding these properties is essential for optoelectronic applications. In this study, we grew wz-Mg1−xSn1+xN2 biaxially oriented polycrystalline films with x = −0.08 to 0.29 by reactive sputtering and investigated the charge-carrier transport properties using both direct current and optical techniques. We regarded the wz-Mg1−xSn1+xN2 films as magnesium tin oxynitride films (wz-MTNO) because a certain amount of oxygen was unintentionally incorporated into the sputtered wz-Mg1−xSn1+xN2 films. The wz-MTNO layers were n-type degenerate semiconductors with an electron density (ne) of the order of 1020 cm−3. In films with ne > 8 × 1020 cm−3, optically extracted resistivities (ρopt) obtained via a Drude-fit analysis of the infrared transmittance and reflectance spectra were almost identical to the direct-current resistivities (ρdc), indicating that the contribution of grain boundary scattering to the electron transport was negligible. However, the contribution of grain boundary scattering became unignorable with decreasing ne. The Drude-fit analysis also allowed the determination of the conduction-band effective mass (mc*) for the first time. A band edge mass of mc*/m0 ≈ 0.2 (m0 denotes the free-electron mass) was obtained in the wz-MTNO layers with |x| < 0.1. As x was increased from −0.18 to 0.29, mc*/m0 substantially increased from 0.18 to 0.56, indicating that the conduction-band dispersion decreased. That is, the conduction-band dispersion may be affected by the cation composition x. The findings of this study will provide important information to establish this material as a practical nitride semiconductor.