Electron transport properties in degenerate magnesium tin oxynitride (Mg_1−xSn_1+xN_2−2yO_2y) with average wurtzite structure

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-Mg_1−xSn_1+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-Mg_1−xSn_1+xN₂ films as magnesium tin oxynitride films (wz-MTNO) because a certain amount of oxygen was unintentionally incorporated into the sputtered wz-Mg_1−xSn_1+xN₂ films. The wz-MTNO layers were n-type degenerate semiconductors with an electron density (n_e) of the order of 10²⁰ cm⁻³. In films with n_e > 8 × 10²⁰ cm⁻³, optically extracted resistivities (ρ_opt) obtained via a Drude-fit analysis of the infrared transmittance and reflectance spectra were almost identical to the direct-current resistivities (ρ_dc), 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 n_e. The Drude-fit analysis also allowed the determination of the conduction-band effective mass (m_c*) for the first time. A band edge mass of m_c*/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, m_c*/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.