Determination of band alignments at 2D tungsten disulfide/high-k dielectric oxides interfaces by x-ray photoelectron spectroscopy

Energy-band alignments at two-dimensional tungsten disulfide (WS2)/high-k dielectric oxides interfaces have been studied by high-resolution x-ray photoelectron spectroscopy (XPS). Our XPS results reveal a Type I heterojunction for 2D WS2/Al2O3, WS2/Y2O3, and WS2/ZrO2 heterojunctions. A valence band offset (VBO) of 2.79 eV and a conduction band offset (CBO) of 3.56 eV were obtained at monolayer WS2/Al2O3 interface, while the VBO and CBO at monolayer WS2/Y2O3 (ZrO2) interface are measured to be 1.98 eV (2.21 eV) and 1.37 eV (1.28 eV), respectively. When increasing the WS2 thickness to its bulk limit, the small split-up energy of VBO indicates the existence of interfacial states and the pinning of the Fermi level of WS2 films at the hetero-interfaces. Our XPS results demonstrate the nature of WS2/dielectric interfaces and suggest that high-k dielectric oxides (Al2O3, Y2O3, and ZrO2) could serve as the gate dielectrics for WS2-based field-effect transistors in term of suppressing the gate leakage current.