AlN/InN和AlN/GaN超晶格能带结构研究

利用Krönig-Penney 模型和形变势理论,从理论上探讨了纤锌矿型AlN/InN和AlN/GaN超晶格系统的能带结构及不同应变模式对能带结构的影响,计算得到了能带结构随各亚层参量变化的一般性规律、超晶格的能量色散关系、应变造成的影响以及不同亚层厚度的系统禁带宽度和导带第一子禁带宽度.研究发现,通过改变亚层厚度可以从不同形式设计能带结构,应变会改变系统禁带宽度,使带阶和子能带明显窄化,价带结构趋于复杂甚至生成准能带结构.与实验结果对比后发现,该模型适于模拟窄势阱结构超晶格,而对于宽势阱则必须 关键词： AlN/InN和AlN/GaN超晶格 Krönig-Penney模型 应变 子能带     

第一原理研究界面弛豫对InAs/GaSb超晶格界面结构、能带结构和光学性质的影响

通过广义梯度近似的第一原理全电子相对论计算, 研究了不同界面类型InAs/GaSb超晶格的界面结构、电子和光吸收特性. 由于四原子界面的复杂性和低对称性, 通过对InAs/GaSb超晶格进行电子总能量和应力最小化来确定弛豫界面的结构参数. 计算了InSb, GaAs型界面和非特殊界面(二者交替)超晶格的能带结构和光吸收谱, 考察了超晶格界面层原子发生弛豫的影响.为了证实能带结构的计算结果, 用局域密度近似和Hartree-Fock泛函的平面波方法进行了计算. 对不同界面类型InAs/GaSb超晶格的能带结构计算结果进行了比较, 发现界面Sb原子的化学键和离子性对InAs/GaSb超晶格的界面结构、 能带结构和光学特性起着至关重要的作用.     

剪切形变对硼氮掺杂碳纳米管超晶格电子结构和光学性能的影响

碳纳米管作为最先进的纳米材料之一, 在电子和光学器件领域有潜在的应用前景, 因此引起了广泛关注. 掺杂、变形及形成超晶格为调制纳米管电子、光学性质提供了有效途径. 为了理解相关机理, 利用第一性原理方法研究了不同剪切形变下扶手椅型硼氮交替环状掺杂碳纳米管超晶格的空间结构、电子结构和光学性质. 研究发现, 剪切形变会改变碳纳米管的几何结构, 当剪切形变大于12%后, 其几何结构有较大畸变. 结合能计算表明, 剪切形变改变了掺杂碳纳米管超晶格的稳定性, 剪切形变越大, 稳定性越低. 电荷布居分析表明, 硼氮掺杂碳纳米管超晶格中离子键和共价键共存. 能带和态密度分析发现硼氮交替环状掺杂使碳纳米管超晶格从金属转变为半导体. 随着剪切形变加剧, 纳米管超晶格能隙逐渐减小, 当剪切形变大于12%后, 碳纳米管又从半导体变为金属. 在光学性能中, 剪切形变的硼氮掺杂碳纳米管超晶格的光吸收系数及反射率峰值较未受剪切形变的均减小, 且均出现了红移.     

Diffraction effects in guided photonic band gap structure

The concept of the photonic band gap (PBG) structures stems from ideas of Yablonovitch. The idea is to design components so that they affect the properties of photons, in much the same way that ordinary semiconductor crystals affect the properties of electrons. In fact, the PBG structures forbid propagation of photons for a particular range of energy. They can be used to realise optical filters with large stop band and sharp transmission resonance. In the guided PBG structures, the existence of diffractive effects in the vertical dimension could limit the quality factor of such filters. In this paper, we have investigated the origin of diffraction losses in one-dimensional guided PGB structures using 2D and 3D numerical tools. We propose an analytical approach based on Bragg diffraction relation to explain these losses phenomenon. From this approach, the influence of some design parameters on the electromagnetic behaviour and the spectral response of PBG resonators will be explained.     

Optical properties and band structure of short-period GaAs/AlAs superlattices

We have studied six GaAs/AlAs superlattices with periods ranging from 18 to 60 Å and different average aluminum composition. Three of these samples are shown to be direct bandgap materials whose band structure differs strongly from that of the corresponding alloy, but is correctly described by an envelope function calculation. The three remaining samples are shown to be indirect both in real and reciprocal space. The lowest energy transitions are found to arise from an exciton involving a heavy hole state mostly confined in the GaAs layer and at the Brillouin zone center (Λ), and an electronic state of X character confined in the AlAs layers. Analysis of the time decay of the luminescence shows that this is a momentum-forbidden exciton made allowed by disorder scattering, which leads to a luminescence efficiency comparable to that of the direct bandgap samples. Partial lifting of the degeneracy of the three X orbitals by the superlattice potential is also observed. Finally, we take advantage of the strong dependence of these indirect transition energies on the band discontinuities to estimate the valence band offset to be about 550 meV in this system.     

GAAS-ALGAS superlattice band structure under hydrostatic pressure: An analysis based on the envelope function approximation

We propose new interface connection rules to determine the electronic properties of superlattices. Although similar to the so-called two-band model, these new rules are more general. Our approach is used to study the electronic structure of superlattices under high hydrostatic pressure. In our model, the influence of the pressure is taken into account by considering the modification of the energy gap and layer width of each material separately. It is demonstrated that the low value observed in the experimental determination of the pressure coefficient θ^SL of the GaAs layers is a consequence of the increasing rigidity of the energy band for energies away from the edges. Moreover the apparent homogeneity of θ^SL (GaAs) is the result of two opposing effects—the variation of the band rigidity and the narrowing of the layer thickness—which compensate each other.     

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.     

Photonic band structure of one-dimensional metal/dielectric structures calculated by the plane-wave expansion method

The plane-wave expansion(PWE) method is employed to calculate the photonic band structures of metal/dielectric(M/D) periodic systems. We consider a one-dimensional(1D) M/D superlattice with a metal layer characterized by a frequency-dependent dielectric function. To calculate the photonic band of such a system, we propose a new method and thus avoid solving the nonlinear eigenvalue equations. We obtained the frequency dispersions and the energy distributions of eigen-modes of 1D superlattices. This general method is applicable to calculate the photonic band of a broad class of physical systems, e.g. 2D and 3D M/D photonic crystals. For comparison, we present a simple introduction of the finite-difference(FD) method to calculate the same system, and the agreement turns out to be good. But the FD method cannot be applied to the TM modes of the M/D superlattice.     

Absolute band gaps of a two-dimensional triangular-lattice dielectric photonic crystal with different shapes

Absolute band gaps of a two-dimensional triangular-lattice photonic crystal are calculated with the finite-difference time-domain method in this paper.Through calculating the photonic band structures of the triangular-lattice photonic crystal consisting of Ge rods immersed in air with different shapes,it is found that a large absolute band gap of 0.098 (2c/a) can be obtained for the structures with hollow triangular Ge rods immersed in air,corresponding to 19.8% of the middle frequency.The influence of the different factors on the width of the absolute band gaps is also discussed.     

The electronic band structures of the wide band gap semiconductors GaN and A1N

A non-local model potential has been developed for nitrogen and used to calculate the energy band structures of GaN and A1N. The resulting band structures are found to be in good agreement with optical data for these materials.     

Electronic band structure of (001) GaAs-AlAs superlattices

We present the results of a theory of electronic energy bands of superlattices using an application of a recent novel empirical tight binding method. The theory is applied to GaAs-AlAs (001) superlattices with very good agreements with available experimental results for both the direct and indirect energy gaps.     

Valence band photoemission spectroscopy of a ternary layered semiconductor: ZnIn2S4

UV photoemission spectroscopy (UPS) experiments have been carried out on the layer compound ZnIn₂S₄ employing several different photon energies in the range $\text{h}\text{?}{}_{\mathrm{\omega }}\text{= 9.5?21.2}\text{eV}$. The energy distribution curves (EDC's) exhibit four valence band density of states structures besides the Zn 3d peak. These five peaks appear 0.90 eV, 1.6 eV, 4.3 eV, 5.8 eV and 8.7 eV respectively below the top of the valence band, E_v. The atomic orbital character of the shallowest peak A appears different from that of the three deeper valence band peaks B, C and D and this is discussed in terms of the more or less pronounced ionic character of the intralayer chemical bonds. These results demonstrate that an overall understanding of the electronic states in complex structures can be achieved by an approach based on photoemission experiments and chemical bonding considerations which has been widely used in the past to study simple binary layer compounds.     

氢化纳米硅薄膜中氢的键合特征及其能带结构分析

对氢化纳米硅薄膜中氢的键合特征和薄膜能带结构之间的关系进行了研究.所用样品采用螺 旋波等离子体化学气相沉积技术制备，利用Raman散射、红外吸收和光学吸收技术对薄膜的 微观结构、氢的键合特征以及能带结构特性进行了分析.Raman结果显示不同衬底温度下所生 长薄膜的微观结构存在显著差异，从非晶硅到纳米晶硅转化的衬底温度阈值为200℃.薄膜中 氢的键合特征与薄膜的能带结构密切相关.氢化非晶硅薄膜具有较高的氢含量，因键合氢引 起的价带化学位移和低衬底温度决定的结构无序性，使薄膜呈现较大的光学带隙和带尾宽度 .升 关键词： 氢化纳米硅 螺旋波等离子体 能带结构     

基于集中质量法的一维声子晶体弹性波带隙计算

通过将一维声子晶体中的原胞简化为有限多个自由度的弹簧振子结构，引入了一种基于集中 质量法的一维声子晶体弹性波带隙计算方法.与传统平面波展开法相比，该方法的计算结果 与之相符合，而且在收敛性方面较之有很大改善.通过使用集中质量法，可在得到同样计算 精度的条件下，显著降低计算量，提高计算速度. 关键词： 声子晶体 弹性波带隙 集中质量法 平面波展开法     

Electronic properties of HgTe-CdTe superlattices

HgTe-CdTe superlattices are new quasi-two-dimensional systems which are of great technical and fundamental interest. We report here calculations of the band structure of these superlattices, evidencing in particular the existence of interface states. Magneto-optical investigations of HgTe-CdTe superlattices are described and interpreted from their theoretical band structures, yielding the interface valence band offset Λ. Finally, we discuss the value obtained for this parameter and also some aspects of these superlattices which might be useful for manufacturing infrared photodetectors.     

Transmission resonances in magnetic-barrier structures

Quantum transport properties of electrons in simple magnetic-barrier (MB) structures and in finite MB superlattices are investigated in detail. It is shown that there exists a transition of transmission resonances, i.e., from incomplete transmission resonances in simple MB structures consisting of unidentical blocks, to complete transmission resonances in comparatively complex MB structures (, n is the number of barriers). In simple unidentical block arrangements in double- and triple-MB structures we can also obtain complete transmission by properly adjusting parameters of the building blocks according to k_y-value (k_y is the wave vector in y direction). Strong suppression of the transmission and of the conductance is found in MB superlattices which are periodic arrangements of two different blocks. The resonance splitting effect in finite MB superlattices is examined. It is confirmed that the rule (i.e., for n-barrier tunneling the splitting would be (n-1)-fold) obtained in periodic electric superlattices can be extended to periodically arranged MB superlattices of identical blocks through which electrons with tunnel, and it is no longer proper for electrons with k _y <0 to tunnel. Received: 18 August 1997 / Revised: 20 September 1997 / Accepted: 13 October 1997     

The band gap and transmission characteristics investigation of local resonant quaternary phononic crystals with periodic coating

In this article, the investigation of the Lamb wave propagation in two-dimensional phononic crystals (PCs) composed of an array of periodic coating on a thin plate is presented. Compared with the traditional PCs usually consist of cylindrical scatters with uniform coatings in their exterior structure, the newly exterior coating structures with periodic alternant arrangement of two different materials are proposed. The band structures are calculated using finite element method. We discover that a complete band gap can be exhibited at low frequency. Furthermore, for a finite PCs plate, the computed transmission and resonance spectra shown an evident resonance nature which can be directly related to formation of the low-frequency gaps. The effects of different material parameters and arrangement mode of coating on the acoustic energy transmission and attenuation are also studied. Finally, the experimental transmission spectrum of the periodic coating PCs are also presented and compared with the numerical results. This study will provide useful support to the design of tuning band gaps and isolators in the low-frequency range.     

大弹性常数差二维声子晶体带隙计算中的集中质量法

通过引入振动力学中的连续系统离散化的思想，将一维集中质量法延伸至二维，提出一种二维声子晶体带隙特性计算的集中质量法. 进而采用该算法对两种正方晶格的二维声子晶体的带结构进行了计算，计算结果与传统的平面波展开法相符合. 通过对计算结果以及两种算法收敛性的分析，发现集中质量法的收敛性对组成声子晶体的不同材料弹性参数差不敏感，这使得该算法在计算大弹性常数差二维声子晶体的带隙特性时较平面波展开法收敛速度更快. 此外，集中质量法对二维声子晶体单元形状没有特殊要求，这使得它更加适用于声子晶体带隙特性的计算. 关键词： 声子晶体 声子带隙 集中质量法     

First-principles study of interface relaxation effect on interface and electronic structures of InAs/GaSb superlattices with different interface types

We have implemented first-principles relativistic pseudopotential calculations within general gradient approximation to investigate the structural and electronic properties of quaternary InAs/GaSb superlattices with an InSb or GaAs type of interface. Because of the complexity and low symmetry of the quaternary interfaces, the interface energy and strain in the InAs/GaSb superlattice system have been calculated to determine the equilibrium interface structural parameters. The band structures of InAs/GaSb superlattices with InSb and GaAs interfaces have been calculated with respect to the lattice constant and atomic position relaxations of the superlattice interfaces. The calculation of the relativistic Hartree–Fock pseudopotential in local density approximation has also been performed to verify the calculated band structure results that have been predicted in other empirical theories. The calculated band structures of InAs/GaSb superlattices with different types of interface (InSb or GaAs) have been systematically compared. We find that the virtual–crystal approximation fails to properly describe the quaternary InAs/GaSb superlattice system, and the chemical bonding and ionicity of anion atoms are essential in determining the interface and electronic structures of InAs/GaSb superlattice system.     

Photonic band structures of quadrangular multiconnected networks

By means of the network equation and generalized dimensionless Floquet-Bloch theorem,this paper investigates the properties of the band number and width for quadrangular multiconnected networks (QMNs) with a different number of connected waveguide segments (NCWSs) and various matching ratio of waveguide length (MRWL).It is found that all photonic bands are wide bands when the MRWL is integer.If the integer attribute of MRWL is broken,narrow bands will be created from the wide band near the centre of band structure.For two-segment-connected networks and three-segment-connected networks,it obtains a series of formulae of the band number and width.On the other hand,it proposes a so-called concept of two-segment-connected quantum subsystem and uses it to discuss the complexity of the band structures of QMNs.Based on these formulae,one can dominate the number,width and position of photonic bands within designed frequencies by adjusting the NCWS and MRWL.There would be potential applications for designing optical switches,optical narrow-band filters,dense wavelength-division-multiplexing devices and other correlative waveguide network devices.