Bandgap and doping effects in MoS2 measured by Scanning Tunneling Microscopy and Spectroscopy

TL;DR

This study uses scanning tunneling microscopy and spectroscopy to measure how doping and gating affect the bandgap and Fermi energy in MoS2, revealing defect-related doping sources and substrate interface effects.

Contribution

It provides detailed measurements of bandgap and Fermi level shifts in MoS2 under gating, identifying defect and substrate contributions to doping levels.

Findings

Bulk MoS2 has a bandgap of ~1.3 eV and n-doping from sulfur vacancies.

Thin films exhibit higher n-doping levels unrelated to sulfur vacancies.

Charge traps at the substrate interface cause increased doping in thin films.

Abstract

The discovery of graphene has put the spotlight on other layered materials including transition metal dichalcogenites (TMD) as building blocks for novel heterostructures assembled from stacked atomic layers. Molybdenum disulfide, MoS2, a semiconductor in the TMD family, with its remarkable thermal and chemical stability and high mobility, has emerged as a promising candidate for post-silicon applications such as switching, photonics, and flexible electronics. Since these rely on controlling the position of the Fermi energy (EF), it is crucial to understand its dependence on doping and gating. Here we employed scanning tunneling microscopy (STM) and spectroscopy (STS) with gating capabilities to measure the bandgap and the position of EF in MoS2, and to track its evolution with gate voltage. For bulk samples, the measured bandgap (~1.3eV) is comparable to the value obtained by photoluminescence, and the position of EF (~0.35eV) below the conduction band, is consistent with n-doping reported in this material. Using topography together with spectroscopy we traced the source of the n-doping in bulk MoS2 samples to point defects, which we attribute to S vacancies. In contrast, for thin films deposited on SiO2, we found significantly higher levels of n-doping that cannot be attributed to S vacancies. By combining gated STS with transport measurements in a field effect transistor (FET) configuration, we demonstrate that the higher levels of n-doping in thin film samples is due to charge traps at the sample-substrate interface.