Self consistent energy band structures for HgTe and CdTe

Self consistent energy band calculations have been performed for HgTe and CdTe with local density functional potentials, using the LMTO method in the atomic sphere approximation. Equilibrium volumes and bulk moduli are obtained in good agreement with experiment. In the case of HgTe the effect of spin-orbit interation is found to be important in obtaining the equilibrium volume. For both materials the energy bands (although in qualitative agreement with experiment) show a semiconducting gap (inverted in the case of HgTe) which is almost 1 eV lower than experiment. This seems to be a feature common to all calculations for semiconductors using existing local exchange and correlation potentials.     

Design principles and coupling mechanisms in the 2D quantum well topological insulator HgTe/CdTe

We present atomistic band structure calculations revealing a different mechanism than recently surmised via k · p calculations about the evolution of the topological state (TS) in HgTe/CdTe. We show that 2D interface (not 1D edge) TSs are possible. We find that the transitions from a topological insulator at critical HgTe thickness of n = 23 ML (6.453 [corrected] ?) to a normal insulator at smaller n is due to the crossing between two interface-localized states: one derived from the S-like Γ?(c) and one derived from the P-like Γ?(v) light hole, not because of the crossing of an interface state and an extended quantum well state. These atomistic calculations suggest that a 2D TS can exist in a 2D system, even without truncating its symmetry to 1D, thus explaining the otherwise surprising similarity between the 2D dispersion curves of the TS in HgTe/CdTe with those of the TS in 3D bulk materials such as Bi?Se?.     

Spectroscopic ellipsometry of layer by layer deposited colloidal HgTe nanocrystals exhibiting quantum confinement

We have explored the optical properties of bilayers of Mercury telluride (HgTe) nanocrystals (NCs) embedded in polymer which were prepared from a colloidal solution. These NCs show strong luminescence in the near infrared at room temperature, which makes them an interesting material for the telecommunication area. The emission wavelength can efficiently be tuned by controlling the size of the NCs. We report spectroscopic ellipsometry measurements, which clearly show an energy shift of the critical points (CPs) in the dielectric function to higher energies compared to the HgTe bulk properties. This is caused by quantum confinement in the crystals. The exact peak energies of the transitions are fitted with line-shape models for CPs. Surprisingly, concepts coming from semiconductor bulk optics, as CPs, can be applied to NCs with a diameter of less than 5 unit cells.     

Magneto-transport properties of thin flakes of Weyl semiconductor tellurium

As an elemental semiconductor, tellurium has recently attracted intense interest due to its non-trivial band topology, and the resulted intriguing topological transport phenomena. In this study we report systematic electronic transport studies on tellurium flakes grown via a simple vapor deposition process. The sample is self-hole-doped, and exhibits typical weak localization behavior at low temperatures. Substantial negative longitudinal magnetoresistance under parallel magnetic field is observed over a wide temperature region, which is considered to share the same origin with that in tellurium bulk crystals, i.e., the Weyl points near the top of valence band. However, with lowering temperature the longitudinal magnetoconductivity experiences a transition from parabolic to linear field dependency, differing distinctly from the bulk counterparts. Further analysis reveals that such a modulation of Weyl behaviors in this low-dimensional tellurium structure can be attributed to the enhanced inter-valley scattering at low temperatures. Our results further extend Weyl physics into a low-dimensional semiconductor system, which may find its potential application in designing topological semiconductor devices.     

Quantum spin Hall effect in inverted type-II semiconductors

The quantum spin Hall (QSH) state is a topologically nontrivial state of quantum matter which preserves time-reversal symmetry; it has an energy gap in the bulk, but topologically robust gapless states at the edge. Recently, this novel effect has been predicted and observed in HgTe quantum wells and in this Letter we predict a similar effect arising in Type-II semiconductor quantum wells made from InAs/GaSb/AlSb. The quantum well exhibits an "inverted" phase similar to HgTe/CdTe quantum wells, which is a QSH state when the Fermi level lies inside the gap. Due to the asymmetric structure of this quantum well, the effects of inversion symmetry breaking are essential. Remarkably, the topological quantum phase transition between the conventional insulating state and the quantum spin Hall state can be continuously tuned by the gate voltage, enabling quantitative investigation of this novel phase transition.     

Quantum confinement in layer-by-layer deposited colloidal HgTe nanocrystals determined by spectroscopic ellipsometry

We report spectroscopic ellipsometry studies in the energy range of 0.5-5 eV on samples of 1-10 bilayers of polymer and HgTe nanocrystals, which exhibit strong transitions at higher critical points in the dispersion relation. We show that the dispersion relation for nanocrystals can be modelled with the same concepts for critical points as used in semiconductor bulk optics. We find an energy shift of up to 0.4 eV of the critical points to higher energies compared to the HgTe bulk properties, caused by quantum confinement in the nanocrystals, which increases with decreasing nanocrystal size.     

Quantum Hall effect from the topological surface states of strained bulk HgTe

We report transport studies on a three-dimensional, 70-nm-thick HgTe layer, which is strained by epitaxial growth on a CdTe substrate. The strain induces a band gap in the otherwise semimetallic HgTe, which thus becomes a three-dimensional topological insulator. Contributions from residual bulk carriers to the transport properties of the gapped HgTe layer are negligible at mK temperatures. As a result, the sample exhibits a quantized Hall effect that results from the 2D single cone Dirac-like topological surface states.     

Helical quantum states in HgTe quantum dots with inverted band structures

We investigate theoretically the electron states in HgTe quantum dots (QDs) with inverted band structures. In sharp contrast to conventional semiconductor quantum dots, the quantum states in the gap of the HgTe QD are fully spin-polarized and show ringlike density distributions near the boundary of the QD and spin-angular momentum locking. The persistent charge currents and magnetic moments, i.e., the Aharonov-Bohm effect, can be observed in such a QD structure. This feature offers us a practical way to detect these exotic ringlike edge states by using the SQUID technique.     

Influence of Size Effects on the Electronic Structure of Hexagonal Gallium Telluride

Using methods of the density functional theory, the electronic band structure of a hexagonal modification of the layered GaTe semiconductor has been calculated. The structural parameters of a bulk crystal with the β-polytype symmetry have been determined taking into account van der Waals interactions and agree with experimental data for polycrystalline films within 2%. Estimates for the position of extrema of the upper valence band and the lower conduction band have been obtained with respect to the vacuum level for bulk β-GaTe and for ultrathin plates with the number of elementary layers ranging from 1 to 10, which corresponds to a thickness range of 0.5–8 nm. The calculations demonstrate that hexagonal GaTe is an indirect band gap semiconductor with a forbidden band width varying from 0.8 eV in the bulk material to 2.3 eV in the monolayer.     

Electronic structure of HgTe nanocrystals: An observation of p-d weakening

Photoemission studies to identify the electronic structure of the HgTe nanocrystals revealed a new phenomenon of p-d weakening, as a consequence of size quantization effect associated with the mean crystalline size, 5.35 ± 0.83 nm. The weakening of the p-d hybridization by a factor of 0.33, to that of the bulk HgTe suggests the valence band maxima and core level shifts toward higher binding energy. The widening of the band gap due to size quantization is confirmed from optical absorption and photoluminescence measurements. The upward and downward shift of the conduction band minima and the valence band maxima with respect to the bulk value of HgTe are found to be 1.6 eV and 0.54 eV respectively.     

Transmission Spectra of HgTe-Based Quantum Wells and Films in the Far-Infrared Range

Strained 80-nm-thick HgTe films belong to a new class of materials referred to as three-dimensional topological insulators (i.e., they have a bulk band gap and spin-nondegenerate surface states). Though there are a number of studies devoted to analysis of the properties of surface states using both transport and magnetooptical techniques in the THz range, the information about direct optical transitions between bulk and surface bands in these systems has not been reported. This study is devoted to the analysis of transmission and reflection spectra of HgTe films of different thicknesses in the far-infrared range recorded in a wide temperature range in order to detect the above interband transitions. A peculiarity at 15 meV, which is sensitive to a change in the temperature, is observed in spectra of both types. Detailed analysis of the data obtained revealed that this feature is related to absorption by HgTe optical phonons, while the interband optical transitions are suppressed.

     

A study on the electrical property of HgSe under high pressure

Using a microcircuit fabricated on a diamond anvil cell, we have measured in-situ conductivity of HgSe under high pressures, and investigated the temperature dependence of conductivity under several different pressures. The result shows that HgSe has a pressure-induced transition sequence from a semimetal to a semiconductor to a metal, similar to that in HgTe. Several discontinuous changes in conductivity are observed at around 1.5, 17, 29 and 49GPa, corresponding to the phase transitions from zinc-blende to cinnabar to rocksalt to orthorhombic to an unknown structure, respectively. In comparison with HgTe, it is speculated that the unknown structure may be a distorted CsCl structure. For the cinnabar-HgSe, the energy gap as a function of pressure is obtained according to the temperature dependence of conductivity. The plot of the temperature dependence of conductivity indicates that the unknown structure of HgSe has an electrical property of a conductor.     

Growth and studies of Hg1−xCdxTe based low dimensional structures

The band structure of HgTe quantum wells (QWs) has been determined from absorption experiments on superlattices in conjunction with calculations based on an 8×8 k·p model. The band structure combined with self-consistent Hartree calculations has enabled transport results to be quantitatively explained.Rashba spin–orbit, (SO) splitting has been investigated in n-type modulation doped HgTe QWs by means of Shubnikov–de Haas oscillations (SdH) in gated Hall bars. The heavy hole nature of the H1 conduction subband in QWs with an inverted band structure greatly enhances the Rashba SO splitting, with values up to 17 meV.By analyzing the SdH oscillations of a magnetic two-dimensional electron gas (2DEG) in modulation-doped n-type Hg_1−xMn_xTe QWs, we have been able to separate the gate voltage-dependent Rashba SO splitting from the temperature-dependent giant Zeeman splitting, which are of comparable magnitudes. In addition, hot electrons and Mn ions in a magnetic 2DEG have been investigated as a function of current.Nano-scale structures of lower dimensions are planned and experiments on sub-micrometer magneto-transport structures have resulted in the first evidence for ballistic transport in quasi-1D HgTe QW structures.     

半导体微结构物理效应及其应用讲座第三讲半导体的激子效应及其在光电子器件中的应用

人们对半导体中的电子空穴对在库仑互作用下形成的激子态及其有关的物理性质进行了深入研究.激子效应对半导体中的光吸收、发光、激射和光学非线性作用等物理过程具有重要影响,并在半导体光电子器件的研究和开发中得到了重要的应用.与半导体体材料相比,在量子化的低维电子结构中,激子的束缚能要大得多,激子效应增强,而且在较高温度或在电场作用下更稳定.这对制作利用激子效应的光电子器件非常有利.近年来量子阱、量子点等低维结构研究获得飞速的进展,已大大促进了激子效应在新型半导体光源和半导体非线性光电子器件领域的应用.     

Wave-vector filtering effect of electric–magnetic barrier in HgTe quantum wells

Because of helicity of electrons in HgTe quantum wells (QWs) with inverted band structure, the electrons cannot be confined by electric barriers since electrons can tunnel the barriers perfectly without backscattering in HgTe QWs. This behavior is similar to Dirac electrons in graphene. In this paper, we propose a scheme to confine carriers in HgTe QWs using an electric-magnetic barrier. We calculate the transmission of carriers in 2-dimensional HgTe QWs and find that the wave-vector filtering effect of local magnetic fields can confine the carriers. The confining effect will have potential application in nanodevices based on HgTe QWs.     

限域量子态调控研究

电子、激子和声子等量子态在固体中的行为早已被人们所熟知. 然而,当体系的尺寸只有纳米量级的时候,已有的固体理论常常不能适用,需要新的低维物理理论的建立. 我们系统研究了低维体系限域量子态（包括电子、激子和声子）的行为对环境、应力、压力及光的响应和性质的调控. 较早认识到低维体系之显著的表面-体积比对量子态性质调控之有效性,系统地揭示了低维体系的一系列由表面和应力决定的新颖性质,证明了低维体系的表面和应力效应同量子限域效应同等重要. 本文概况了如下五个方面的结果：（1）一种使用应力效应调控电子能带结构的方法和（2）一种使用表面效应调控电子能带结构的方法（这两个方法都可将低维体系能带从间接能隙调控至直接能隙能带结构）;（3）一种低维体系表面掺杂方法,该方法将在低维体系掺杂中取代传统方法;（4）量子点表面诱导的光致异构现象;（5）基于表面自催化半导体低维结构的形成机理. 希望我们的研究工作有助于促进低维体系在光电子、纳电子、环境、能源、生物和医学等领域的应用.     

Backscattering of Dirac fermions in HgTe quantum wells with a finite gap

The density-dependent mobility of n-type HgTe quantum wells with inverted band ordering has been studied both experimentally and theoretically. While semiconductor heterostructures with a parabolic dispersion exhibit an increase in mobility with carrier density, high-quality HgTe quantum wells exhibit a distinct mobility maximum. We show that this mobility anomaly is due to backscattering of Dirac fermions from random fluctuations of the band gap (Dirac mass). Our findings open new avenues for the study of Dirac fermion transport with finite and random mass, which so far has been hard to access.     

低维纳米材料热电性能测试方法研究

通过近几十年的研究,人们对于块体及薄膜材料的热电性能已经有了较全面的认识,热电优值ZT的提高取得了飞速的进展,比如碲化铋相关材料、硒化亚铜相关材料、硒化锡相关材料的最大ZT值都突破了2.但是,这些体材料的热电优值距离大规模实用仍然有较大的差距.通过理论计算得知,当块体热电材料被制作成低维纳米结构材料时,比如二维纳米薄膜、一维纳米线,热电性能会得到显著的改善,具有微纳米结构材料的热电性能研究引起了科研人员的极大兴趣.当块体硅被制作成硅纳米线时,热电优值改善了将近100倍.然而,微纳米材料的热电参数测量极具挑战,因为块体材料的热电参数测量方法和测试平台已经不再适用于低维材料,需要开发出新的测量方法和测试平台用来研究低维材料的热导率、电导率和塞贝克系数.本文综述了几种用于精确测量微纳米材料热电参数的微机电结构,包括双悬空岛、单悬空岛、悬空四探针结构,详细介绍了每一种微机电结构的制备方法、测量原理以及对微纳米材料热电性能测试表征的实例.     

岩盐结构SrC(111)表面和(111)界面的d~0半金属性

利用第一性原理方法,本文研究了岩盐结构的SrC块材、(111)表面和(111)界面的电子结构和磁性.块材的SrC被证实是一个良好的d~0半金属铁磁体.计算结果显示(111)方向的C表面和Sr表面都保持了块材的半金属性.对于(111)方向四个可能的界面,态密度的计算显示C-Pb界面呈现半金属特性.本文对岩盐结构SrC块材、(111)表面和(111)界面半金属性的研究结果,将为高性能自旋电子器件的实际应用提供一定的理论指导.     

Detection of directed electron-hole recombination energy transfer from an ionic crystal matrix to self-assembled nanocrystals

It is found that the energy released in the spin-dependent tunneling recombination of electron-hole pairs and self-trapped excitons in an ionic crystal matrix is directionally transferred to low-dimensional semiconductor structures embedded in the matrix as a result of self-assembled growth. The EPR spectra of electron and hole centers in the matrix crystal are detected by tunneling afterglow and photostimulated luminescence that are excited in the low-dimensional structure.