Heterostructures such as heterojunctions, quantum wells, and superlattices are core components of advanced optoelectronic devices. Herein, we attempted the first investigations on the band alignment of nonpolar m-plane oriented ZnO1−xSx/Mg0.4Zn0.6O heterojunctions by X-ray photoelectron spectroscopy. All the heterojunctions were revealed to show a type-I band alignment, and the valence band offset (VBO; ΔEV) increased significantly, while the conduction band offset (ΔEC) decreased insignificantly with increasing S content in the ZnO1−xSx layer. Specifically, for the ZnO1−xSx/Mg0.4Zn0.6O heterojunctions with x = 0, 0.13, and 0.22, ΔEV (ΔEC) was determined to be 0.24 (0.22), 0.61 (0.17), and 0.79 (0.11) eV, respectively. The VBOs of ZnOS/MgZnO heterojunctions are significantly larger than those of heterojunctions involving only cation-substituted alloys (ZnO/MgZnO or ZnO/CdZnO) due to the opposite shift in the VB maximum of ZnOS and MgZnO with respect to ZnO. Knowing band alignment parameters of the ZnOS/MgZnO interface can provide a better understanding of the carrier transport mechanism and rational design of ZnO-based optoelectronic devices.
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