Status and prospects of Ohmic contacts on two-dimensional semiconductors

DFT Simulation of a Gold Electrode Vapor-Deposition Growth Process and the Effect of Defects on the Electrode Work Function

While two-dimensional (2D) semiconductors are explored as field-effect transistor (FET) channel materials for decreasing the short channel effects, electrical contact with 2D semiconductors is a major issue. Many efforts have been made toward this issue. However, the discrepancy in the contact type and the Schottky barrier height from the same contact is present in experiments. This discrepancy supposedly should be associated with the vapor-deposition electrode structures, on which little attention had been focused. Here, the crystal growth of the gold vapor-deposition electrode is simulated by adding gold atoms to the gold substrate one by one in the framework of density functional theory, and for every step, the spontaneously searching adsorption site method is used to find thermodynamically stable adsorption sites and the climbing nudged elastic band method is used to find kinetically stable ones. Simulation shows that the Au(111) face grows according to the ABC sequence packing, and possible defects are interstitial, vacancy, and the partly filled nascent layer (PFNL). These defects have an unequal effect on the electrode work function. The PFNL may be a non-negligible factor responsible for the discrepancy.

The effects of point defect type, location, and density on the Schottky barrier height of Au/MoS2 heterojunction: a first-principles study

Using DFT calculations, we investigate the effects of the type, location, and density of point defects in monolayer MoS₂ on electronic structures and Schottky barrier heights (SBH) of Au/MoS₂ heterojunction. Three types of point defects in monolayer MoS₂, that is, S monovacancy, S divacancy and Mo_S (Mo substitution at S site) antisite defects, are considered. The following findings are revealed: (1) The SBH for the monolayer MoS₂ with these defects is universally higher than that for its defect-free counterpart. (2) S divacancy and Mo_S antisite defects increase the SBH to a larger extent than S monovacancy. (3) A defect located in the inner sublayer of MoS₂, which is adjacent to Au substrate, increases the SBH to a larger extent than that in the outer sublayer of MoS₂. (4) An increase in defect density increases the SBH. These findings indicate a large variation of SBH with the defect type, location, and concentration. We also compare our results with previously experimentally measured SBH for Au/MoS₂ contact and postulate possible reasons for the large differences among existing experimental measurements and between experimental measurements and theoretical predictions. The findings and insights revealed here may provide practical guidelines for modulation and optimization of SBH in Au/MoS₂ and similar heterojunctions via defect engineering.

Field-Effect Transistor Based on 2D Microcrystalline MoS2 Film Grown by Sulfurization of Atomically Layer Deposited MoO3

Atomically thin molybdenum disulfide (MoS₂) is a promising channel material for next-generation thin-body field-effect transistors (FETs), which makes the development of methods allowing for its controllable synthesis over a large area an essential task. Currently, one of the cost-effective ways of its synthesis is the sulfurization of preliminary grown oxide- or metallic film. However, despite apparent progress in this field, the electronic quality of the obtained MoS₂ is inferior to that of exfoliated samples, making the detailed investigation of the sulfurized films' properties of great interest. In this work, we synthesized continuous MoS₂ films with a thickness of ≈2.2 nm via the sulfurization of an atomic-layer-deposited MoO₃ layer. X-ray photoelectron spectroscopy, transmission electron microscopy, and Raman spectroscopy indicated the appropriate chemical composition and microcrystalline structure of the obtained MoS₂ films. The semiconductor quality of the synthesized films was confirmed by the fabrication of a field-effect transistor (FET) with an I_on/I_off ratio of ≈40, which was limited primarily by the high contact resistance. The Schottky barrier height at the Au/MoS₂ interface was found to be ≈1.2 eV indicating the necessity of careful contact engineering. Due to its simplicity and cost-effectiveness, such a technique of MoS₂ synthesis still appears to be highly attractive for its applications in next-generation microelectronics. Therefore, further research of the electronic properties of films obtained via this technique is required.