Electronic And Magnetic Properties Of Low Dimensional Smeiconductor Materials

With the development of the information technology, low dimentional materials have been the focus of scientific research. The low dimensional materials have specific physical properties, like mechanical,optical and magnetic properites, which differ from their bulk counterpart,as well have potential applications in wide range of fields. Based on first-principles and fully self-consistent GW approximation, this thesis mainly focuses on several low dimensional semiconductors and investigates their electronic and magnetic properties. We hope to predict and present the novel properties and potentials of these low dimensional materials from the basisc physical properties, the main results and innovations are as follows:1. The geometric structures and electronic properties of wurzite and zinc-blend GaAs nanowires are studied. A revised cohesive energy is provided based on the influence of different kinds of surface dangling bonds. The calculations indicate that GaAs nanowires display bistability namely both zinc-blende and wurtzite nanowires will form around 90 A diameters. Wurtzite nanowires are found to be more stable over zinc-blende nanowires when the diameter is small than 90 A, and vise verse. The GaAs nanowire is semiconductor and the band gap decreases with the increase of the diameter. The crystal structure, band gap and elastic coefficient of GaAs1-xBix alloy are studied using hybrid function.Three different structures in prototype wurtzite and orthorhombic symmetries are considered. The lattice constants of GaAs1-xBix alloy follow the Vegard's Law regardless of the three different structures.Although the band gap decreases with the increase of Bi concentration,the trends of the band gap energy in different structures are nearly identical. The calculated elastic coefficients and bulk modulus display a discernible downward bowing and there exists a direct correlation between the elastic stiffness coefficients and strains. The mechanical and electronic properties of monolayer and bilayer arsenenes under in-plain biaxial strains are studied. Under large enough tensile strains, the monolayer arsenene can transfer from buckled honeycomb structure to planar honeycomb phase. The variations of the band gap energy are diverse with respect to the compressive and tensile biaxial strains. In addition, the monolayer arsenene exhibits an indirect-to-direct band gap transition when the compressive strains reach to -10%.2. The electronic structure and magnetic properties of (Mn, Fe)codoped bulk ZnO and nanowrie are investigated, respectively. The total energies of ferromagnetic and antiferromagnetic states are calculated for several doping configurations. The ground state of codoped ZnO shows room temperature ferromagnetism and the ferromagnetic coupling is mediated by double exchange mechanism. In addition, defects corresponding to Zn-vacancy and O-vacancy cann't enchance the ferromagnetism of (Mn, Fe) codoped bulk ZnO. The electronic and magnetic properties of V-doped AIN nanosheet under in-plane biaxial strains are calculated. The magnetic coupling of three different configurations are studied and configuration I is demonstrated to possess room temperature ferromagnetism. The stable ferromagnetic coupling is mediated by double exchange mechanism. In addition, the in-plane biaxial strains corresponding to tensile and compressive strains can affect and manipulate the magnetic interaction of V-doped AIN nanosheet in different ways.3. The GW approximation is a well-known method to obtain the quasiparticle and spectral properties of systems ranging from molecules to solids. We describe the implementation of a fully self-consistent GW approach based on the solution of the Dyson equation using a plane wave basis set. The pseudopotential including semicore is used and the Green's function is expressed as a full matrix without truncation. Algorithmic,numerical, and technical details of the self-consistent GW approach are presented. No further approximations and truncations apart from the truncation on the plane wave basis set are made in our implementation of the GW calculation.