טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBen-Melech Stan Gabriela
SubjectLateral Interface in Two-Dimensional Transition Metal
Dichalcogenide Metal/Semiconductor
Heterostructures
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Maytal Caspary-Toroker
Full Thesis textFull thesis text - English Version


Abstract

Two-dimensional (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 layers composing the sheet. Recently, 2D-transistors were successfully fabricated using monolayer or bilayer TMD as the channel semiconductor (mostly MoS2 or WS2), while the source and drain electrodes were made of 'classical' bulk metals or graphene sheets.

In our present study, we used TMD monolayers to design a novel metal/semiconductor heterojunction for nanoscale applications, such as an all-2D-transistor. Using VASP, an ab-initio simulation program suitable for DFT calculations, we scanned a dozen of different TMD monolayers and computed their structural constants and energy band alignment. Based on these results, six optimal metal/semiconductor heterojunctions were composed: FeS2/MoS2, FeS2/WS2, FeS2/CrS2, VS2/MoS2, VS2/WS2, and VS2/CrS2. The quality and performance evaluation of each heterojunction was based on the following calculated characteristics: structural mismatch between metal and semiconductor monolayers, Schottky barrier height, adhesion energy of the interface, projected density of states, existence of metal-induced-gap-states (MIGS) along the semiconductor, potential energy, and charge density differences though the junction.

We noticed that all the monolayers composing the heterojunctions form a strong covalent bond at the interface expressed by a strong hybridization between the transition metals d-orbitals on both sides of the interface, combined with a high adhesion energy of the interface. In addition, we found that VS2/ CrS2 heterojunction is especially interesting since it has a neglectable Schottky barrier for hole transport, low potential energy barrier at the interface, and CrS2 has a high charge density around Fermi level. Moreover, monolayer CrS2 has a direct bandgap of 0.9eV, which is close to that of Silicon (1.1eV), which is widely used as the channel semiconductor in transistors.

Metal/semiconductor interfaces based on 2D-TMD both as the 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 monolayers can produce metal/semiconductor interface with few and highly localized MIGS.