Theory of the temperature dependence of the direct gap of germanium (original) (raw)
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In order to explain the dc electrical conductivity in amorphous germanium, a new model for the one-electron states in the gap is proposed. It is based on the notion of strong interaction between a carrier and the lattice. The model explains a number of experimental observations apart from the dc electrical conductivity, notably the thermopower and induced optical absorption. It also leads to some predictions about the dielectric response at low frequencies (ac conductivity below 10 Hz). The proposed model leads naturally to the concept of diamagnetic, doubly occupied, localized states for strong enough coupling of the carrier to the lattice and predicts disappearance of mobility edges from one-particle energy spectrum in the study of electrical transport phenomena.
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We predict a new way to achieve a direct band gap in germanium, and hence optical emission in this technologically important group-IV element: tensile strain along the h111i direction in Ge nanowires. Although a symmetry-breaking band splitting lowers the conduction band at the corner of the Brillouin zone (at the L point), a direct gap of 0.34 eV in the center of the Brillouin zone (at À) can still be achieved at 4.2% longitudinal strain, through an unexpectedly strong nonlinear drop in the conduction band edge at À for strain along this axis. These strains are well within the experimentally demonstrated mechanical limits of single-crystal Ge (or Ge x Si 1Àx ) nanowires, thereby opening a new material system for fundamental optical studies and applications.
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Significant advances have been made recently toward understanding the properties of materials through theoretical approaches. These approaches are based either on first-principles quantum mechanical formulations or semiempirical formulations, and have benefitted from increases in computational power. The advent of parallel computing has propelled the theoretical approaches to a new level of realism in modelling physical systems of interest. The theoretical methods and simulation techniques that are currently under development are bound to become powerful tools in understanding, exploring and predicting the properties of existing and novel materials.
Physical Chemistry Chemical Physics, 2003
The electronic properties of two room temperature persistent phases of germanium dioxide have been studied by means of experimental and theoretical techniques. We collected the Ge-K edge XANES spectra of these materials at the GILDA beamline of ESRF. The density of states of the two crystal phases, obtained from fully periodic Hartree-Fock and density functional calculations, is taken as the reference term to rationalise and assign the manifolds of the XANES spectra. Although this scheme requires a number of severe approximations, we obtained a good overall agreement between experiment and theory. The topological analysis of the theoretical electron density distribution in the crystals gave further information regarding the electronic properties of germanium dioxide.
Ab initio calculation of the formation energy of charged vacancies in germanium
Density functional theory (DFT) with local density approximation (LDA) has been used to calculate the formation energy (E f ) of the neutral and charged vacancies in germanium single crystal. The standard (four valence electrons) and harder (which treat the semicore 3d states of Ge as valence) projector augmented wave (PAW) potentials were used. Additionally, the effect of including on-site Coulomb interaction, U, for Ge semicore d states within the LDA+U approach was investigated. The LDA+U method improves the LDA band gap which allows investigating the dependence of formation energy of charged vacancies on Fermi level position in the band gap. It was shown that the calculated formation energies of the neutral and charged vacancies are in good agreement with published experimental data. r