Quantum Transport Of Novel Two-Dimensional Semiconductor Materials And Spin-related Devices From First Principles

Since the mesoscopic of spintronics devices have attracted great attention for decades, lots of research work has been devoted to investigating their physical mechanism and improving the electrical properties. Meanwhile, the successfully fabricated of two-dimensional (2D) layered materials has opened a new fundamental areas as emerging candidate device materials for nanoelectronics due to their novel mechanical, electrical, and optical properties. With impinging circularly polarized light or applying external electric field on 2D layered semiconductor materials, lots of new quantum transport phenomenon has been predicted by theoretical study. In this thesis, quantum transport properties of corresponding spin-related devices and 2D layered semiconductor materials are investigated using first principles calculation based on Density Functional Theory and Non-Equilibrium Green Function. The mainly results are as follow.1). we propose and theoretically investigate a very attractive MTJ Fe(001)/O/NaCl(001)/O/Fe(001) as a two-terminal transport junction. By density functional theory totalenergy methods, we establish two viable device models: with or without mirrorsymmetry across the center plane of the structure. Large tunnel magnetoresistance ratio (TMR) is predicted, at over 3600% and 1800% depending on the symmetry. Microscopically, a spin filtering effect is responsible for the large TMR. This effect essentially filters out all the spin-down channels from contributing to the tunneling current. On the other hand, transport of the spin-up channel having △1 and A5 symmetry is enhanced by the FeO buffer layer in the MTJ.2). we investigate quantum transport properties for anomalous Hall effect (AHE) of Fe and Ni nanostructures in the form of four-probe system from first principle. Due to the presence of spin-orbit interaction (SOI), the Fe and Ni nanostructure give out the opposite nonzero AHE coefficient like their bulk form. In the framework of Landauer-Buttiker formalism, the intrinsic contribution origin of nonzero AHE is identified from the quantum transport point of view. Furthermore, by analyzing the local density of scattering states (LDOSS), we find that the scattering effect due to the presence of voltage probes play an important role in the negative AHE coefficient of Ni nanostructure. Finally, a similar Onsager relation in AHE is established by using scattering matrix approach.3). we study on quantum transport in a monolayer WSe2 field effect transistor (FET). Due to strong spin-orbit interaction (SOI) and the atomic structure of the two-dimensional lattice, monolayer WSe2 has an electronic structure that exhibits Zeeman-like up-down spin texture in the Brillouin zone. In a FET, the gate electric field induces an extra, externally tunable SOI that re-orients the spins into a Rashba-like texture thereby realizing electric control of the spin. The conductance of FET is modulated by the spin texture, namely by if the spin orientation of the carrier after the gated channel region, matches or miss-matches that of the FET drain electrode. The carrier current 1τ,s in the FET is labelled by both the valley index r and spin index s, realizing valleytronics and spintronics in the same device.4). Due to having strong spin-orbit interaction, quantum states can be labeled by a valley index defined in the reciprocal space and the spin index in monolayer WSe2. We developed a first-principles theoretical formalism to both qualitatively and quantitatively predict non-equilibrium quantum transport of valley-polarized currents. We propose a WSe2 transistor to selectively deliver net valley-and spin-polarized current lτ,s to the source or drain by circularly polarized light under external bias. Due to the lack of translational symmetry of the real-space device, we predict a depolarization effect that increases with the decrease of the channel length of the transistor.5). we investigate possible metal contacts to monolayer black phosphorus (BP). By analyzing lattice geometry, five metal surfaces are found to have minimal lattice mismatch with BP:Cu(111), Zn(0001), In(110), Ta(110), and Nb(110). Further studies indicate Ta and Nb bond strongly with monolayer BP causing substantial bond distortions, but the combined Ta-BP and Nb-BP form good metal surfaces to contact a second layer BP. By analyzing the geometry, bonding, electronic structure, charge transfer, potential, and band bending, it is concluded that Cu(111) is the best candidate to form excellent Ohmic contact to monolayer BP. The other four metal surfaces or combined surfaces also provide viable structures to form metal/BP contacts, but they have Schottky character. Finally, the band bending property in the current-in-plane (CIP) structure where metal/BP is connected to a freestanding monolayer BP, is investigated. By both work function estimates and direct calculations of the two-probe CIP structure, we find that the freestanding BP channel is n type.