Surface-state conduction through dangling-bond states

The surface-state conduction through dangling-bond states on the (1 1 1) ideal surfaces of group-IV semiconductors is studied theoretically. Localization strength of wave functions is an important factor determining the surface-state conduction. Approximate expressions for the decay constants of the dangling-bond states are presented using a tight-binding model. The origin of the difference in the decay constants of diamond, Si, and Ge is clarified in terms of linear combination of the states at the top of valence bands and the bottom of conduction bands. The ballistic conductance from a probe to the surfaces is calculated using the Landauer formalism. The surface-state conductance is much larger than the bulk-state one, which is due to small band dispersions of the dangling-bond states.     

sp3sTight-binding parameters for transport simulations in compound semiconductors

A genetic algorithm approach is used to fit orbital interaction energies of sp3s* tight-binding models for the nine binary compound semiconductors consistent of Ga, Al, In and As, P, Sb at room temperature. The new parameters are optimized to reproduce the bandstructure relevant to carrier transport in the lowest conduction band and the highest three valence bands. The accuracy of the other bands is sacrificed for the better reproduction of the effective masses in the bands of interest. Relevant band edges are reproduced to within a few meV and the effective masses deviate from the experimental values typically by less than 10%.     

Electronic structure of two-dimensional semiconductor systems

The motion of two-dimensional carriers in semiconductor heterojunctions, quantum wells and superlattices is discussed, with emphasis on subband dispersion parallel to the interfaces and on quantization in a perpendicular magnetic field. The envelope function approximation is shown to provide an efficient and accurate method for the calculation of electronic states. It is shown that for states derived from degenerate or coupled bulk bands (valence bands, k . p-coupled conduction and valence bands in narrow-gap semiconductors) striking non-parabolicities of the subband dispersion and non-linearities of the Landau levels versus magnetic field occur. Results for GaAs-AlGaAs and InAs-GaSb systems are presented and compared with experiment.     

X-ray absorption and photoemission spectroscopy of 3C- and 4H-SiC

We have studied the electronic properties of 3C- and 4H-SiC with X-ray absorption (XAS). Particular emphasis is placed on the conduction bands because they exhibit larger differences between the various SiC polytypes than valence bands. XAS spectra at the Si2p and C1s edges provide projections onto Si3d, 4s and C2p conduction band states. We explain the observed differences in the Si L_2,3 XAS data to arise from transition into dispersive bands which occur at the M and K point of the hexagonal Brillouin zone. The XAS data are sensitive to a difference in the dispersion of the two lowest conduction bands. For 3C-SiC the dispersion is larger than for 4H-SiC in agreement with theory. We compare the XAS data at the Si L edge with CFS and CIS spectra and find that the SiLVV Auger is dominant.     

Application of semi-ab initio method of band structure calculation to semiconductor superlattices

We used an efficient direct-space orthogonalized linear combination of atomic orbitals method to calculate the band structures of most common elemental, III–V and II–VI semiconductors, obtaining correct locations and magnitudes of the band gaps. The atomic-like minimal basis and the atomic-like potentials obtained from these calculations can be used to calculate the band structures of semiconductor superlattices. As an example of such an application, we show the results of calculations on 100 Si bilayers (313 Å in width) randomly stacked in the [111] direction. This structure represents a realistic one-dimensional model system with off-diagonal disorder in the z-direction and hexagonal translational symmetry in the x-y plane. Highly localized states at both the conduction band and the valence band edges are identified and analyzed. These electron states are compared with those obtained from similar calculations on perfect Si-bilayers of same width as in diamond lattice and those containing intrinsic and extrinsic types of stacking faults.     

硅酸锌的电子结构

采用局域密度泛函理论和第一性原理的方法,计算四方结构和六角结构硅酸锌的平衡晶格常数、电子态密度和能带结构。计算结果表明,四方结构硅酸锌的平衡晶格常数为0.71048nm,六角结构为1.40877nm,两者与实验值的误差均在1%左右。态密度图显示,主要电子态分布在-7.18～0.00eV和2.79～10.50eV两个能量区域;同时,不同元素电子对导带和价带有不同贡献,其中氧的p态电子对价带顶贡献最大,锌的s态电子对导带底贡献最大。能带计算表明,四方与六角结构硅酸锌均为直接带隙半导体,禁带宽度分别为2.66,2.89eV。     

Coherently photoinduced ferromagnetism in diluted magnetic semiconductors

Ferromagnetism is predicted in undoped diluted magnetic semiconductors illuminated by intense sub-band-gap laser radiation. The mechanism for photoinduced ferromagnetism is coherence between conduction and valence bands induced by the light which leads to an optical exchange interaction. The ferromagnetic critical temperature T(C) depends both on the properties of the material and on the frequency and intensity of the laser and could be above 1K.     

Urbach energy and the tails of the density of states in amorphous semiconductors

Formulas are derived for the Urbach energy as a function of the parameters controlling the slope of the exponential tails in the valence and conduction bands of amorphous semiconductors. It is shown that the Urbach energy remains constant, even if one of these parameters varies over a wide range.     

Dependence of vibrational density of states and Raman spectra on size in non-polar and polar nano-crystalline semiconductors

The vibrational density of states (VDOS) of Si, diamond, SiC, and InSb clusters has been calculated using the cluster model for various cluster sizes. The results show that the peak frequency of optical-like bands of VDOSs of non-polar nano-crystalline (NC) semiconductors varies with size, but that of polar NC semiconductors does not vary with size. We attribute the origin of this different behavior of non-polar and polar NC semiconductors to different interactions in the optical-like modes of them. That is, the deformation potentials for the non-polar NC semiconductors and electrostatic potentials for the polar NC semiconductors. According to the amorphous crystal (aC) model, Raman spectra are directly related to VDOS. In this Letter, it is verified that the aC model can be applied to NC semiconductors. Calculated VDOS are compared with observed Raman spectra of corresponding samples, which show agreement.     

Ab initio calculations of band structure and thermophysical properties for SnS2 and SnSe2

The electronic band structure and elastic constants of SnS₂ and SnSe₂ have been calculated by using density-functional theory (DFT). The calculated band structures show that SnS₂ and SnSe₂ are both indirect band gap semiconductors. The upper valence bands originate mainly from S_p and Sn_d electrons, while the lowest conduction bands are mainly from (S, Se) _p and Sn_s states. The calculated elastic constants indicate that the bonding strength along the [100] and [010] direction is stronger than that along the [001] direction and the shear elastic properties of the (010) plane are anisotropic for SnS₂ and SnSe₂. Both compounds exhibit brittle behavior due to their low B/G ratio. Relationships among volumes, the heat capacity, thermal expansion coefficients, entropy, vibrational energy, internal energy, Gibbs energy and temperature at various pressures are also calculated by using the Debye mode in this work.     

Theory of free-electron optical absorption in n-GaAs

Free-electron optical absorption of Se-doped GaAs at room temperature is calculated and compared with existing experimental data. In addition to the standard features the theory takes into account a non-parabolic character of the conduction band, a short-range component of the Se donor potential and a plasmon generation in the presence of donors. Deformation potential constants for the conduction and the valence bands are calculated using the empirical pseudopotential method. A value of C = -15.7 eV is obtained for the conduction band and shown to agree with existing hydrostatic and uniaxial stress experiments. Assuming no impurity compensation and using no adjustable parameters we successfully describe the experimental free-electron absorption in GaAs as a function of photon wavelength for samples with various donor densities.     

Negative thermal expansion in crystals with the zincblende structure: an EXAFS study of CdTe

The extended x-ray absorption fine structure (EXAFS) has been measured at both the K edges of cadmium and tellurium in CdTe, from liquid helium to room temperature, in order to investigate the local thermodynamic behaviour. The temperature dependences of the structural parameters obtained from the separate analysis of the two edges are perfectly consistent. The positive contribution to the thermal expansion due to the bond stretching and the negative contribution due to the tension effects are disentangled and quantified in terms of the bond thermal expansion and the perpendicular mean square relative displacement. The comparison with previous EXAFS results for Ge and CuCl shows that relevant correlations can be established between a number of local parameters measured by means of EXAFS and the properties of the lattice negative thermal expansion of tetrahedrally bonded semiconductors. The effective force constants derived from the EXAFS are compared with the force constants of a valence force field model.     

First-principles study of electronic structure and magnetism of cubic Al1−xErxN using the LSDA+U approach

First-principles calculations, by means of the full-potential augmented plane wave method using the LSDA+U approach (local spin density approximation with Hubbard-U corrections), have been carried out for the electronic structure of the Al_0.75Er_0.25N. The LSDA+U method is applied to the rare-earth 4? states. We have investigated the electronic and magnetic properties.The Al_0.75Er_0.25N is shown to be a semiconductor, where the filled ? states are located in the valence bands and the empty ones above the conduction band edge. The magnetic interaction of the rare-earth ion with the host states at the valence and conduction band edges has been investigated and discussed.     

Convergence of conduction bands as a means of enhancing thermoelectric performance of n-type Mg2Si(1-x)Sn(x) solid solutions

Mg(2)Si and Mg(2)Sn are indirect band gap semiconductors with two low-lying conduction bands (the lower mass and higher mass bands) that have their respective band edges reversed in the two compounds. Consequently, for some composition x, Mg(2)Si(1-x)Sn(x) solid solutions must display a convergence in energy of the two conduction bands. Since Mg(2)Si(1-x)Sn(x) solid solutions are among the most prospective of the novel thermoelectric materials, we aim on exploring the influence of such a band convergence (valley degeneracy) on the Seebeck coefficient and thermoelectric properties in a series of Mg(2)Si(1-x)Sn(x) solid solutions uniformly doped with Sb. Transport measurements carried out from 4 to 800 K reveal a progressively increasing Seebeck coefficient that peaks at x=0.7. At this concentration the thermoelectric figure of merit ZT reaches exceptionally large values of 1.3 near 700 K. Our first principles calculations confirm that at the Sn content x≈0.7 the two conduction bands coincide in energy. We explain the high Seebeck coefficient and ZT values as originating from an enhanced density-of-states effective mass brought about by the increased valley degeneracy as the two conduction bands cross over. We corroborate the increase in the density-of-states effective mass by measurements of the low temperature specific heat. The research suggests that striving to achieve band degeneracy by means of compositional variations is an effective strategy for enhancing the thermoelectric properties of these materials.     

First-principles studies on the electronic and optical properties of CuAlSe2 and CuAl5Se8

The electronic structure, dielectric function, absorption coefficient, and reflectivity of two polycrystalline semiconductors CuAlSe₂ and CuAl₅Se₈ are studied using the density functional theory within the generalized gradient approximation. There are different peaks in the spectra of the imaginary part of the dielectric function. The transitions between the valence bands and conduction bands are discussed at length. In addition, we also notice that the reduced absorption coefficient and reflectivity near band-edge in Cu-poor CuAlSe₂ are closely related to a reduction of the density of states near the valence band maximum.     

Effect of overlap of localized orbitals on the band structure of insulators under pressure

A new method for calculating the energy spectra of compressed insulators based on the cluster expansion is proposed. The dependence on the compression of the spectra of both the conduction band and the valence bands is uniquely determined by the values of the overlap integrals of the orbitals of pairs of isolated atoms. The overlap integrals and the matrices defined by them are calculated numerically for inert-gas crystals and their properties and values are analyzed for different lattice constants. Numerical calculations of the bands of the compressed crystal in the various proposed models are carried out and discussed for neon. Fiz. Tverd. Tela (St. Petersburg) 40, 1464–1472 (August 1998)     

Positron Energy Levels in Cd-Based Semiconductors

Using the full potential linearized augmented plane wave FP-LAPW method within local density ap-proximation LDA, we have studied positron diffusion and surface emission in Cd-based semiconductors. This requires the calculation of electron and positron band structures. In the absence of experimental and theoretical data for CdX (X=S,Se,Te) we have treated the Si, which has been studied by several authors, as a test case. Predictive results on positron effective masses, deformation potentials, positron work functions, diffusion constants and positron mobilities are presented for CdX (X=S, Se, Te). Our calculated data for Si are compared with experimental and recent theoretical results.     

Effect of interband scattering on the thermoelectric properties of semiconductors and semimetals

This paper reports on the study of the effect of interband scattering on the thermoelectric figure-of-merit of semiconductors and semimetals. It has been shown that the inclusion of interband scattering in the case of two types of carriers of the same sign leads to a decrease in the thermoelectric figure-of-merit. By contrast, if the material contains carriers of two types of opposite signs and the conduction and valence bands overlap, as is the case with semimetals, the interband scattering favorably affects on the magnitude of the thermoelectric figure-of-merit.