Electrical And Optical Properties Of Layered MoS₂ Field Effect Transistor From First Principle

Recently, much more attentions have been focused on the two dimensional（2D） MoS₂ with many novel properties. Monolayer MoS₂ is a direct semiconductor with a band gap（¹.90 e V） in the visible frequency range, ideal for exploring optoelectronic applications. In 2D MoS₂ there are some intriguing physics associated with valley, such as the valley-orbital coupling, the trion valley-Hall effect, valley Zeeman splitting and valley selective optical Stark effect, which implies potentials for valley electronics. In the thesis by using the nonequilibrium Green’s function（NEGF） formalism combined with density functional theory（DFT） the transport properties of MoS₂ field effect transistor（FET） are studied. Moreover by introducing the electron photon coupling Hamiltonian the valley polarized photocurrent is calculated. Our attentions are focused on two aspects:（1） The electronic spin and valley transport properties of monolayer MoS₂ FET are investigated. Due to the SOI the electronic valence bands of monolayer MoS₂ are split into spin up and spin down Zeeman-like texture near the K and K’ valleys of the first Brillouin zone. When the gate voltage is applied in the scattering region, an additional strong RSOI is induced which generates an effective magnetic field. As a result, electron spin precession occurs along the effective magnetic field. From our results the conductance can be modulated periodically by the magnitude of the gate voltage and the length of the gate region with oscillation periods approximately 2.2 V and 20.03 a B（a B is Bohr radius）, respectively.（2） The relationship between photocurrent and the gate voltage in layered MoS₂ FET are studied. Crystal symmetry governs the nature of electronic Bloch states. For bilayer MoS₂, the inversion symmetry is kept, however, the symmetry can be broken simply by applying a perpendicular electric field. At the same time, the optical interband transition from the valence band top to the conduction band bottom is totally decided by the coupling strength with optical fields of different circular polarization, this means the valley polarization of photon current can be controlled by the circular polarization of light and electronic field. The results show that the valley polarization of photon current in monolayer MoS₂ is robust with the change of gate voltage and that in bilayer MoS₂ is very sensitive to the gate voltage. The results can b e explained by k·p modelThis thesis is divided into six chapters. The first chapter serves as a summary of the properties and research background about MoS₂. In second chapter,the formulas and calculation method of NEGF are described. In third chapter, the general theoretical framework of DFT is developed in details. In fourth and fifth chapters the results from the first principle calculation and discussions are presented. Finally, a summary and outlook are given in last chapter.