Bandgap, Mid-Gap States, and Gating Effects in MoS₂

TLDR

The discovery of graphene has highlighted transition metal dichalcogenides, especially MoS₂, as promising materials for post‑silicon devices, where device performance hinges on controlling the Fermi energy through doping and gating. We performed gated scanning tunneling microscopy and spectroscopy and compared the results with field‑effect transistor transport measurements to probe the band structure and Fermi level evolution under gate bias. The measurements revealed a 1.3 eV bandgap and an EF 0.35 eV below the conduction band, indicating N‑doping from sulfur vacancies in bulk MoS₂, while thin films on SiO₂ exhibit much higher N‑doping dominated by interface charge traps.

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 postsilicon applications such as switching, photonics, and flexible electronics. Because these rely on controlling the position of the Fermi energy (EF), it is crucial to understand its dependence on doping and gating. To elucidate these questions we carried out gated scanning tunneling microscopy (STM) and spectroscopy (STS) measurements and compared them with transport measurements in a field effect transistor (FET) device configuration. This made it possible 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 (∼1.3 eV) is comparable to the value obtained by photoluminescence, and the position of EF (∼0.35 eV) below the conduction band, is consistent with N-doping reported in this material. We show that the N-doping in bulk samples can be attributed to S vacancies. In contrast, the significantly higher N-doping observed in thin MoS2 films deposited on SiO2 is dominated by charge traps at the sample–substrate interface.