High electron mobility single-crystalline ZnSnN2 on ZnO (0001) substrates

Abstract

Making a systematic effort, we have developed single-crystalline ZnSnN₂ on ZnO (0001) by reactive magnetron co-sputtering. Epitaxial growth was achieved at 350 °C by co-sputtering from metal targets in a nitrogen atmosphere, and confirmed by transmission electron microscopy (TEM) measurements. TEM verified that the layers are single-crystalline of hexagonal phase, exhibiting an epitaxial relationship with the substrate described by: [11−20]_ZnSnN2//[11−20]_ZnO and [0001]_ZnSnN2//[0001]_ZnO. The screw-type threading dislocations originating from the ZnSnN₂/ZnO interface were identified as the dominant extended defects. More specifically, we report a pioneering measurement of the dislocation density in this material of 1.5 × 10¹¹ cm⁻². Even though there are no literature data for direct comparison, such values are typical of heteroepitaxial growth of III-nitride layers without applying defect density reduction strategies. The films demonstrated a high electron mobility of 39 cm² V⁻¹ s⁻¹ and 63 cm² V⁻¹ s⁻¹ for stoichiometric and Zn-rich layers, respectively, while the electron carrier density remained in the low 10¹⁹ cm⁻³ range as determined by the Hall effect measurements at room temperature. Optical bandgaps of 1.86 eV and 1.72 eV were determined for the stoichiometric and Zn-rich samples, respectively. As such, we conclude that ZnSnN₂ is an earth-abundant, environmentally-friendly semiconductor and is a promising candidate for cost efficient components in optoelectronics and photovoltaics.