Tight-binding investigation of electron tunneling through ultrathin SiO2gate oxides

We investigate electron tunneling through ultrathin gate oxides using scattering theory within a tight-binding framework. We employ Si[100]/SiO₂/Si[100] model junctions with oxide thicknesses between 7 and 18 Å. This approach accounts for the three-dimensional microscopic structure of the model junctions and for the three-dimensional nature of the corresponding complex energy bands. The equilibrium positions of the atoms in the heterostructure are derived from first-principles density-functional calculations. We show that the present method yields qualitative and quantitative differences from conventional effective-mass theory.     

Electron and hole effective masses in self-assembled quantum dots

Electron and hole effective masses in self-assembled InAs/GaAs quantum dots are determined by fitting the energy levels calculated by a single-band model to those obtained by a more sophisticated tight-binding method. For the dots of various shapes and dimensions, the electron effective-mass is found to be much larger than that in the bulk and become anisotropic in the dots of large aspect ratio while the hole effective-mass becomes almost isotropic in the dots of small aspect ratio. For flat InAs/GaAs quantum dots, the most appropriate value for the electron and hole effective-mass is believed to be the electron effective-mass in bulk GaAs and the vertical heavy-hole effective-mass in bulk InAs, respectively.     

n-Si/SiGe quantum cascade structures for THz emission

In this work we report on modeling the electron transport in n-Si/SiGe structures. The electronic structure is calculated within the effective-mass complex-energy framework, separately for perpendicular (X_z) and in-plane (X_xy) valleys, the degeneracy of which is lifted by strain, and additionally by size quantization. The transport is described via scattering between quantized states, using the rate equations approach and tight-binding expansion, taking the coupling with two nearest-neighbour periods. The acoustic phonon, optical phonon, alloy and interface roughness scattering are taken in the model. The calculated U/I dependence and gain profiles are presented for a couple of QC structures.     

Radiative transitions involving impurities

A many-electron development of effective-mass theory is given, based on a Hartree-Fock viewpoint. The procedure provides a basis for the calculation of polarization effects other than those produced by single point charges. An exact effective-mass differential equation is obtained, taking into account non-parabolicities of the band and gradients of the perturbation. Results are obtained for the matrix-element for dipole emission between two impurity-states in a semiconductor. When applied to the problem of donor-acceptor transfer, the results offer an explanation as to why the no-phonon process is considerably stronger in Ge than in Si, even though dielectric constant and effective-mass magnitude effects predict the opposite. The highly non-parabolic conduction band of Ge is responsible.     

Anisotropic strain effects on light emission characteristics of CdZnO/ZnO quantum well structures

Anisotropic strain effects by strain relaxation on TE-polarized light emission characteristics of c-plane CdZnO/ZnO quantum well (QW) structures were theoretically investigated by using the multiband effective-mass theory. The CdZnO/ZnO QW structure with anisotropic strain has much larger emission intensity than conventional CdZnO/ZnO QW structure without the strain relaxation. In the case of the strain relaxation along x(or y)-direction, the x(or y)-polarized light emission is observed to be larger than the x(or y)-polarized light emission. In particular, in the case of the strain relaxation along both x- and y-directions, the increase in the spontaneous emission peak is significant. This can be explained by the fact that the internal field is reduced owing to the decrease in the piezoelectric field by the strain relaxation.     

Pressure-induced interband optical transitions in an InAs0.8P0.2/InP quantum wire

Hydrostatic pressure-induced exciton binding energy in an InAs_0.8P_0.2/InP quantum well wire is investigated taking into account the geometrical confinement effect. Numerical calculations are carried out using variational approach within the single-band effective-mass approximation. The compressive strain contribution to the confinement potential is included throughout the calculations. The energy difference of the ground and the first excited state is found with the consideration of spatial confinement effect in the influence of pressure. The second-order susceptibility of harmonic generation is carried out using the compact density method. The optical gain as a function of incident photon energy is computed in the presence of the hydrostatic pressure. The result shows that the range of wavelength for the potential applications of telecommunications (1.3–1.55 μm) can be obtained by the application of the hydrostatic pressure. We believe that the obtained results can be applied for tuning the ranges of fibre optical wavelength in telecommunications.     

Frequency-dependent spontaneous emission rate from CdSe and CdTe nanocrystals: influence of dark states

We studied the rate of spontaneous emission from colloidal CdSe and CdTe nanocrystals at room temperature. The decay rate, obtained from luminescence decay curves, increases with the emission frequency in a supralinear way. This dependence is explained by the thermal occupation of dark exciton states at room temperature, giving rise to a strong attenuation of the rate of emission. The supralinear dependence is in agreement with the results of tight-binding calculations.     

A tight-binding approach to the electronic structure of the silver halides—I: Silver chloride

In this paper we present a modified version of the tight-binding method for determining the electronic structure of the silver halides. The main advantage of this approach is that it enables the bandstructure of these materials to be derived for all energies adjacent to the band gap. This is achieved by including an s function into the tight-binding basis set of atomic orbitals. In addition to the basic simplicity of the conventional tight-binding method this approach allows the ionicity of the potentials to be determined empirically by fitting the experimental band-gap and incorporates into the bandstructure calculation s-p and s-d interactions which may well be important.The validity of the approach is examined for the case of silver chloride and good agreement is obtained with previous calculations.     

Electronic properties of silicon with ultrasmall germanium clusters

The electronic states of silicon with a periodic array of spherical germanium clusters are studied within the pseudopotential approach. The effects of quantum confinement in the energies and wave functions of the localized cluster states are analyzed. It is demonstrated that clusters up to 2.4 nm in size produce one localized s state whose energy monotonically shifts deep into the silicon band gap as the cluster size increases. The wave function of the cluster level corresponds to the single-valley approximation of the effective-mass method. In the approximation of an abruptly discontinuous potential at the heterointerface, the quantities calculated using the effective-mass method for clusters containing more than 200 Ge atoms are close to those obtained by the pseudopotential method. For smaller clusters, it is necessary to take into account the smooth potential at the interface.     

Strain effects on optical polarisation properties in （11^-22） plane GaN films

We present the theoretical results of the electronic band structure of wurtzite GaN films under biaxial strains in the （11^-22）-plane. The calculations are performed by the k.p perturbation theory approach through using the effectivemass Hamiltonian for an arbitrary direction. The results show that the transition energies decrease with the biaxial strains changing from -0.5% to 0.5%. For films of （11^-22）-plane, the strains are expected to be anisotropic in the growth plane. Such anisotropic strains give rise to valence band mixing which results in dramatic change in optical polarisation property. The strain can also result in optical polarisation switching phenomena. Finally, we discuss the applications of these properties to the （1132） plane GaN-based light-emitting diode and lase diode.     

Spontaneous emission in coupled microcavity-waveguide structures at the band edge

We examine spontaneous emission and photon dynamics in a microcavity coupled to a coupled-resonator optical waveguide (CROW) in a photonic crystal. We present an efficient tight-binding approach to obtain the Green tensor in large, arbitrary systems of coupled microcavities. We use this approach to examine spontaneous emission when the microcavity is strongly coupled to the CROW at the band center and band edge. We confirm the validity of weak-coupling theories for microcavities resonant at band center and obtain strong peak splitting in the previously inaccessible case of band-edge coupled structures.     

Numerical simulation of a coupling effect on electronic states in quantum dots

Lasers operating at 1.3 μm have attracted considerable attention owing to their potential to provide efficient light sources for next-generation high-speed communication systems. InAs/GaAs quantum dots (QDs) were pointed out as a reliable low-cost way to attain this goal. However, due to the lattice mismatch, the accumulation of strain by stacking the QDs can cause dislocations that significantly degrade the performance of the lasers. In order to reduce this strain, a promising method is the use of InAs QDs embedded in InGaAs layers. The capping of the QD layer with InGaAs is able to tune the emission toward longer and controllable wave-lengths between 1.1 and 1.5 μm. In this work, using the effective-mass envelope-function theory, we investigated theoretically the optical properties of coupled InAs/GaAs strained QDs based structures emitting around 1.33 μm. The calculation was performed by the resolution of the 3D Schrödinger equation. The energy levels of confined carriers and the optical transition energy have been investigated. The oscillator strengths of this transition have been studied with and without taking into account the strain effect in the calculations. The information derived from the present study shows that the InGaAs capping layer may have profound consequences as regards the performance of an InAs/GaAs QD based laser. Based on the present results, we hope that the present work make a contribution to experimental studies of InAs/GaAs QD based structures, namely the optoelectronic applications concerning infrared and mid-infrared spectral regions as well as the solar cells.     

A new approach to the calculation of tight-binding surface density of states in thin films

We present a new approach to the calculation of planar electronic density of states of thin films within the tight-binding scheme. The diagonal elements of the electronic Green's function are obtained by an iteration procedure derived from the transfer-matrix approach for semi-infinite crystals. An application is made to the study of the (100) planes of a model transition metal film with a simple cubic structure.     

Band structure and absorption coefficient in GaN/AlGaN quantum wires

The band structures of rectangular GaN/AlGaN quantum wires are modeled by using a parabolic effective-mass theory.The absorption coefficients are calculated in a contact-density matrix approach based on the band structure.The results obtained indicate that the peak absorption coefficients augment with the increase of the injected carrier density,and the optical gain caused by interband transition is polarization anisotropic.For the photon energy near 1.55 eV,we can obtain relatively large peak gain.The calculations support the previous results published in the recent literature.     

Peierls相变与磁场中碳纳米管的场发射

应用紧束缚模型和WKB方法研究碳纳米管的out-of-plane型Peierls相变,及其对碳纳米管的场发射的影响.结果发现Peierls相变会在室温出现,并使碳纳米管费米面附近出现能隙,导致碳纳米管发生金属—半导体转变,从而抑制碳纳米管的场发射.磁场也会抑制Peierls形变,Peierls相变和磁场相互竞争影响碳纳米管的能带结构,从而影响碳纳米管的场发射. 关键词： 场发射 碳纳米管 Peierls相变     

基于紧密耦合理论的3端分子桥的传导特性

利用基于紧密耦合理论的格林函数方法，考虑到每一个碳原子的π轨道电子，对3苯环构成的纳米分子桥的量子传输特性进行了理论研究。得出了从分子桥的一个端点到另外两个端点的电子传输概率。 利用基于Fisher-Lee公式的流密度方法，在传输概率出现峰值的能量点E=±0.42, ±1.06 和±1.5处，计算了分子桥内部的电流分布。其数值结果用图形模拟表示出来。我们发现传输谱与入射电子能量相关，并且紧密地依赖与分子能级。其结果显示电子流分布完全符合Kirchhoff量子动量守恒定律。     

Electronic structure of quantum spheres and quantum wires

The electronic structures of quantum spheres and quantum wires are studied in the framework of the effective-mass theory. The spin-orbital coupling (SOC) effect is taken into account. On the basis of the zero SOC limit and strong SOC limit the hole quantum energy levels as functions of SOC parameter λ are obtained. There is a fan region in which the ground and low-lying excited states approach those in the strong SOC limit as λ increases. Besides, some theoretical results on the corrugated superlattices (CSL) are given.     

Manifestation of LO-LA phonons in Raman scattering in graphene

We analyze the spectral density of Raman scattering in graphene accompanied by the emission of a pair of LO or LA phonons from the corner of the Brillouin zone. Using a minimal tight-binding model approach, we find that the lineshape of the corresponding Raman signal consists of two peaks with a strongly non-Lorentzian (almost triangular) form with their width and the splitting between the peaks being strongly dependent on the energy of the incoming photon. The asymmetric lineshape is determined by the kinematics of the fully-resonant two-phonon process, and it reflects a strong anisotropy of LO-LA phonons’ dispersion around the Brillouin zone corners.     

Tight-binding parameterization of α-Sn quasiparticle band structure

The diamond structure of tin (α-Sn) can be stabilized in nanocrystals embedded in a suitable host. We developed highly accurate parameterizations for tight-binding simulation of such structures incorporating strain and spin-orbit interaction. Parameters are obtained by fitting to ab initio GW quasiparticle band structures of unstrained α-Sn as well as geometries under uniform compressive or tensile strain. The optical response calculated from this fit is in excellent agreement with experiments. As an application, confinement induced band gaps in strained and unstrained 3 nm nanocrystals are computed. It is found that compressive and tensile strain raises and lowers the gap, respectively.