Strain-dependent Band Offset And Optical Absorption Modulation Of Si/Ge Core/Shell Nanowires

As significant structural elements of the micro-nano optoelectronic devices, Si/Ge and Ge/Si core/shell nanowires(CSNWs) become the focus of attention by a great deal of scientists due to their potential physical and chemical properties. Si-Ge CSNWs are different from the corresponding bulk and single component nanowires. They have tremendous advantages, for example, the epitaxial layer can separate the carriers, tolerate more elastic deformations without dislocations, and provide an efficient passivation of the surface trap states, etc. In addition, with the discrepancies of lattice constant and thermal expansion coefficient between core and its shell, the whole system will be in self-equilibrium state with a minimum of total energy. Thus, the system will be subject to the modulation of surface and interface. Therefore, it is important to clearly understand the interaction mechanism at interface and the size and shape effect in preparation and application of CSNWs.The energy band structure, band gap and optical absorption coefficient are the important physical parameters of electronic and optical properties of semiconductive CSNWs. Up to date, such issues have been studied generally by taking various experiments and calculations. Nonetheless, the exploration of theoretical model from atomic levels and relevant theoretical mechanism is still lacking. Especially the analytical study of the influence of size and geometry effects on band offsets and optical absorption properties. In this paper, based on the atomic bond relaxation consideration and the continuum mechanics, we establish a core-shell configuration model to investigate the size and shape dependence of band gap shift and optical absorption coefficient. And on this basis, we systematically study the size and shape effect on band gap shift and light absorption coefficient of radial Si/Ge and Ge/Si CSNWs. The achievements are shown as follows:(1) Four types of free-standing(triangle, tetragon, hexagon, and circle) Si/Ge and Ge/Si CSNWs with polygonal cross-sections are investigated. We establish an analytical core-shell configuration model to investigate the size and shape effects on band gap shift of CSNWs. It is found that the CSNWs system is in the self-equilibrium state and the band gap of CSNWs experiences a relative blue shift compared to the bulk due to the combined role of the spontaneous contraction of atomic bond length and the lattice mismatch at interface. The CSNWs system with triangular cross-section shape has the maximum deformation potential compared to the other systems.(2) We study the size and shape dependence of optical absorption coefficient of Si/Ge and Ge/Si CSNWs. Results show that, with a increasing of cross-sectional area and thickness of epitaxial layer, the optical absorption coefficient increases. In the same situation, the optical absorption coefficient of four types CSNWs complies with the following relationship: circle>hexagon>tetragon>triangle.