Band alignment of 2D WS2/HfO2 interfaces from x-ray photoelectron spectroscopy and first-principles calculations

Abstract

Two-dimensional (2D) WS2 films were deposited on SiO2 wafers, and the related interfacial properties were investigated by high-resolution x-ray photoelectron spectroscopy (XPS) and first-principles calculations. Using the direct (indirect) method, the valence band offset (VBO) at monolayer WS2/SiO2 interface was found to be 3.97 eV (3.86 eV), and the conduction band offset (CBO) was 2.70 eV (2.81 eV).Furthermore, the VBO (CBO) at bulk WS2/SiO2 interface is found to be about 0.48 eV (0.33 eV) larger due to the interlayer orbital coupling and splitting of valence and conduction band edges. Therefore, the WS2/SiO2 heterostructure has a Type I energy-band alignment. The band offsets obtained experimentally and theoretically are consistent except the narrower theoretical bandgap of SiO2. The theoretical calculations further reveal a binding energy of 75 meV per S atom and the totally separated partial density of states, indicating a weak interaction and negligible Fermi level pinning effect between WS2 monolayer and SiO2 surface. Our combined experimental and theoretical results provide proof of the sufficient VBOs and CBOs and weak interaction in 2D WS2/SiO2 heterostructures.

WS2 Nanosheet/Si p–n Heterojunction Diodes for UV–Visible Broadband Photodetection

“…The valence band offset across the WS 2 /Si interface is calculated using the difference of respective VBO measured from the XPS data, and the results are shown in Figure 4. In the heterostructure, two valence band onsets are observed; one at 0.30 ± 0.2 eV, corresponding to the Si substrate, 47 and the other at 0.78 ± 0.2 eV. The as-grown film is found to be multilayered, as revealed by atomic force microscopy (AFM) measurements, which is in good agreement with the VBO values for WS 2 , available in the literature.…”

“…However, for thin 2D TMD films, it has been observed that the valence band onset (VBO) varies depending on the deposition techniques and the number of layers present in the sample. It has been reported that for few-layer nanosheets of WS 2 , the VBO increases with the layer number reduction . Moreover, depending on nanotexturing, the band alignment may also be affected.…”

“…Moreover, depending on nanotexturing, the band alignment may also be affected. It has been reported that the VBO values of bulk, bilayer, and monolayer WS 2 are 0.68, 0.77, and 0.97 eV, respectively …”

“…It has been reported that for few-layer nanosheets of WS 2 , the VBO increases with the layer number reduction. 47 Moreover, depending on nanotexturing, the band alignment may also be affected. It has been reported that the VBO values of bulk, bilayer, and monolayer WS 2 are 0.68, 0.77, and 0.97 eV, respectively.…”

“…It has been reported that the VBO values of bulk, bilayer, and monolayer WS 2 are 0.68, 0.77, and 0.97 eV, respectively. 47 To evaluate the band diagram of our fabricated heterostructure, we, therefore, measured the valence band positions directly through photoelectron spectroscopy. The valence band offset across the WS 2 /Si interface is calculated using the difference of respective VBO measured from the XPS data, and the results are shown in Figure 4.…”