Composition-Dependent Properties of Wurtzite-Type Mg_1+xSn_1–xN₂ Epitaxially Grown on GaN(001) Templates

The variety of wurtzite-type semiconductors with band gaps of 1.8–2.5 eV is limited, which hinders filling the “green gap”. This remains an unresolved problem in green light-emitting diodes (LEDs). Wurtzite-type MgSnN2 is a ternary nitride with a band gap of ∼2 eV, and studying it as a potential candidate for green emitters in LEDs and an absorber material in tandem photovoltaics has been initiated recently; little is known about its basic properties. We examined the cation-composition dependence of its optoelectronic properties, in which the cation sites are randomly occupied by Mg and Sn. Mg1+xSn1–xN2 epitaxial layers were grown on the (001) planes of GaN by reactive co-sputtering from Mg and Sn targets. These epilayers retained the wurtzite structure in a broad x range of −0.34–0.28. All of the layers were unintentionally doped with oxygen, thereby exhibiting n-type conductivity. The Sn-rich layers had electron densities (ne) of the order of 1020 cm–3, whereas the ne value was suppressed to 7 × 1018 cm–3 in Mg-rich layers. These results suggest that the excess Sn (Mg) serves as a donor-type (acceptor-type) defect, and the electron transport properties can be tailored by varying the cation composition. The optical gap (Egopt) of the Mg1+xSn1–xN2 films exhibited a blue shift due to the Burstein–Moss (B–M) effect for the Sn-rich composition; the intrinsic band gap of MgSnN2 was approximately 2.3 eV. The tunability of the transport properties and the Eg value corresponding to green light makes wurtzite-type MgSnN2 an attractive semiconductor for green emitters and photovoltaic absorbers.