Factors Affecting Performance Of Monolayer MoS₂-based Field Effect Transistors

As the process of nano-crystallization of silicon-based devices is approaching to its technical limitation,the research of the two-dimensional materials has become the key part of all researches about substitutes of silicon material.Monolayer molybdenum disulfide?MoS₂?,as a typical transition-metal dichalcogenides,which possesses not only a two-dimensional structure of graphene,but also an appropriate direct bandgap in its band structure,has been considered as a promising replacement of channel material for field effect transistors?FETs?.Some fundamental questions related to devices,such as electrode contacts,await for solutions on the way of commercialization of monolayer MoS₂-based nano-devices.Because of its nano-scale characteristics,the implication of interfaces to the performance of devices,is different from that in conventional physics.In this article,the influences of two important interfaces to the performance of MoS₂-based FETs device were discussed: the first interface between metal electrode and monolayer MoS₂,the implications of its interface stress-matching to the performance of metal-electrode were studied systematically;the second interface between different gate insulator with different dielectric properties and monolayer MoS₂,the implications to transport properties were reviewed.The analysis of the two important interfaces corresponding to two key directions of optimization of MoS₂-based FETs,is helpful to enhance the recognition of MoS₂-based FETs devices in fundamental issues.In the first chapter,general two dimensional materials are demonstrated,and the crystal structure,electronic structure and applications of MoS₂,as well as the problems of metal contact and channel scatter of MoS₂-based FET devices are discussed.Finally,we point out the objective and significance of our work.In chapter two,the basic theory of Density Functional Theory?DFT?and Non-Equilibrium Green's Function?NEGF?,and VASP,ATK simulation packages are briefly introduced.In the third and fourth chapters,we discussed and analyzed our results detailedly.In the third chapter,the implications of interface mismatch strain to the performance of Au?111?/MoS₂ interface were clarified based on first principles calculations within DFT.The research has shown that:?1?the interfacial distance,tunneling barrier and Schottky barrier are bigger when the Au substrate or monolayer MoS₂ is in the state of compression than that in non-strained interface system,which will be gradually increased with increasing compression;while the charge density of interfaces is smaller than that in non-strained interface system,which will be gradually decreased with increasing compression.Thus,it is wised to avoid the Au substrates or monolayer MoS₂ in the compressive strain state when considering the Au/MoS₂ contacts;?2?in the stretched strain states,we found that applying a smaller tensile stress on Au substrate can obtain a nice contact;for monolayer MoS₂,although a larger tensile strain applied on it can optimizing the contacts which benefits the electron injunction from Au to MoS₂,but the bandgap of MoS₂ decreased with increasing tensile strain,the reason why we choose MoS₂ as the channel material for FETs is exactly that it has a considerable bandgap.Thus,the small stretch strain to monolayer MoS₂ can keep its both advantages of nice interface contacts and appropriate bandgap.In the fourth chapter,we have studied the electrical transport characteristics of monolayer MoS₂ FETs under various gate dielectric environments.We found that a high-k gate insulator does not increase the carrier mobility of MoS₂-based FET when neglecting its function of damping Coulombic scattering.However,a high-k gate insulator under various gate voltages can properly shift the valence?p-type?or conduction?n-type?bands around the bias energy window,leading to a larger on current but lower off current for the device.From the theoretical point of view,in the article,we have studied the effects of strain engineering and dielectric engineering on the performance of MoS₂-based FETs.We expect that our research provide with meaningful theoretical reference for design and applications of MoS₂-based FETs.