Research On The Modulating Effects Of Electronic Properties On The Silicon And Germanium Materials

Mechanical properties are crucial for designing and manufacturing devices in practice. Stress-strain engineering is also a common and effective approach to tailor the electronic properties of materials and the performance of the devices. Today, researchers have high expectations to change characteristics of the materials, build new devices and extend new applications by stress-strain engineering. Supported by the National Natural Science Foundation of China (Grant Nos.61275201,61372037,61102024), this thesis is devoted to:(1) Electronic properties of tensile-strained germanium films;(2) Mechanical properties and phonon instabilities of silicene, H-silicane and F-silicane;(3) The modulating effect of stess-strain engineering on electronic properties of silicene, H-silicane and F-silicane. The main contents and some innovations are listed below:1) The hybrid HSE06functional with spin-orbit coupling effects is used to calculate the habituation of the electronic properties of Ge on the (001),(111),(101) in-plane equiaxial tensile strains (IPBTSs). The motivation of study is to realize Ge photonic devices monolithically integrated with Si-based electronics. The calculated results demonstrate that one of the most effective and practical approaches for transforming Ge into a direct transition semiconductor is to introduce (001) IPBTS to Ge. At2.3%(001) IPBTS, Ge becomes a direct bandgap semiconductor with0.53eV band gap. By investigating the dependence of valence band spin-orbit splitting on strain, we prove that the dependency relationship and the coupled ways between the valence-band states of tensile-strained Ge are closely related to the symmetry of strain tensor, i.e., the symmetry of the substrate orientation.2) Nonlinear elastic properties of silicene, H-silicane and F-silicane are investigated using the density functional theory (DFT) within local density approximation (LDA). The two continuum models which accurately describes nonlinear elastic behavior of2D materials are given. We respectively calculate the second-and third-order nonlinear elastic constants and find that they are good agreement with each other. Based on the calculated third-order elastic constants Cij,we calculate the pressure dependent elastic constants Cij(P) and their pressure derivatives C’ij. The results show that Cij(P) are linearly increase with increasing pressure. The two continuum models, nonlinear elastic constants and pressure dependent elastic constant can be directly applid into a finite element analysis model or experiments about the large scale applications of these materials.3) We study the stress, atomic structure, phonon instability, and electronic property of silicene as a function of strain for equiaxial tension and uniaxial tension along armchair and zigzag directions based on the DFT with LDA functional. Phonon calculations demonstrate that failure mechanisms of silicene under uniaxial and equiaxial tension are all due to elastic instability (phonon instability occurs behind the elastic instability). We find that phonon instabilities under two types of uniaxial tension occur near the center of the Brillouin zone and phonon soft modes are longitudinal acoustical (LA) modes along the pulling direction like graphene. However, the eigenvector of unstable phonon mode in silicene under equiaxial tension is flexural mode (ZA), which is normal to the sublattice plane and different than that of graphene. The calculations of band strucrure without SOC show silicene under three types of tension is gapless and SOC can open a band gap of1.44meV. The uniaxial tension along armchair and zigzag directions makes SOC-induced gap decrease and then increase with increasing strain. The reason results from two aspects:One side, the decrease of buckling height with the increasing uniaxial tension results in the reduction of SOC. On the other side, the break of sublattice symmetry and the increase of the inner displacement of atoms in the cell lead to the rise of SOC.4) We firstly investigate the deperdence of the stress, atomic structure, phonon dispersions, and electronic properties of H-silicane and F-silicane on the strain under three tpyes of tension based on DFT within LDA functional. The stress-strain relationships show that the nonlinear responses of H-silicane and F-silicane initiate at strain about0.04and0.02, respectively. Phonon calculations demonstrate that failure mechanisms of both H-silicane and F-silicane under three types of tension are also due to elastic instability. Then, HSE06functional with SOC is employed to investigate the modulating effect of strain on the electronic of H-silicane and F-silicane. The result show H-silicene is an indirect semiconductor and F-silicane is a direct semiconductor. H-silicene can be transformed into a direct semiconductor at0.02equiaxially tensile strain or at0.07uniaxial tension along armchair direcition. The valence band SOCs of both H-silicane and F-silicane significantly enhance under uniaxial tension, which is helpful to observe the quantum spin Hall effect (QSHE) by them.99%of the energy of sunlight is visible and infrared. We find that H-silicane and F-silicane through modulation of strain can be applied to solar energy materials for absorbing visible and infrared light.