Gabriela Ben-Melech, MSc. Candidate
Department of Material Science and Engineering, Technion
2D materials are the subject of numerous studies over the past two decades. Graphene sheets are the pioneers in this field of research and their outstanding mechanical and electronic properties were the incentive for searching other 2D graphene-like structure materials; leading to the development of an extensive 2D materials library, in which Transition-Metal-Dichalcogenides (TMD) are prominent in their qualities. TMD can be
fabricated by conventional exfoliation methods, offering structural stability and a large variety of electronic properties (band gap and mobility) that can be relatively easy manipulated; for example, by changing the number of atomic layer composing the sheet. Recently, 2D-transistor were successfully fabricated using mono or bi-layer TMD as the channel semiconductor (mostly MoS2 or WS2), while the source/drain electrodes were made of ‘classical’ bulk metals or graphene sheets.
In our present study, we used TMD monolayers (MLs) to design a novel metal-semiconductor (M-SC) interface for nanoscale applications, such as an all-2D-transistor. Using VASP, an ab-initio simulation program suitable for DFT calculations, we scanned a dozen different TMD MLs and computed their structural constants and energy band alignment. Based on these results, six optimal M-SC interfaces were composed. The quality and performance evaluation of each interface was based on the following calculated characteristics: structural mismatch between metal and SC MLs, Schottky barrier height, adhesion energy at interface, projected density of states, existence of metal-induced-gap-states (MIGS) along the semiconductor, potential energy and charge density differences though the interface.
We noticed that high adhesion energy is closely related to small initial structure mismatch, as expected. In addition, High adhesion energy correlates with large charge dipole and high potential energy barrier at the interface. Moreover, this energy barrier created at the interface after relaxation is very little affected by the initial Schottky barrier height. We also found a correlation between coupling strength of transition metal d-orbitals around the fermi level, and the density of MIGS along the semiconductor layer.
M-SC interface based on 2d-TMD both as metal and semiconductor possess great potential in terms of performance control. High quality fabrication and defect free interface can be expected due to the low mismatch and similar chemical characteristics of the materials. Also, as showed in this study, TMD MLs can produce M-SC interface with negligible MIGS and energy barrier.
Supervised by Asst. Prof. Maytal Caspary Toroker