Pressure-Induced Transition from Wurtzite and Epitaxial Stabilization for Thin Films of Rocksalt MgSnN₂

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.