d-core transitions in ZnTe,CdTe and HgTe

Summary We have measured the reflectivity spectra of monocrystalline ZnTe, CdTe and HgTe between 5 and 35 eV. We have interpreted the sharp structures above approximately 10 eV as transitions originating in the metal uppermostd levels. The structures give a picture of the projected densities of states of the conduction bands and supply a better understanding of their states.

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Riassunto Abbiamo misurato gli spettri di riflettività di monocristalli di ZnTe, CdTe ed HgTe tra 5 e 35 eV. Oltre i 10 eV gli spettri presentano delle strutture fini, interpretate come transizioni che hanno origine nei livellid piú alti del catione. Questa interpretazione permette di ricavare una rappresentazione schematica della densità proiettata degli stati di conduzione e di ottenere una migliore comprensione sulla natura degli stati finali stessi.

     

Simple theoretical analysis of the thermoelectric power in quantum dot superlattices of non-parabolic heavily doped semiconductors with graded interfaces under strong magnetic field

We study theoretically the thermoelectric power in the presence of a large magnetic field (TPM) in heavily doped III–V, II–VI, PbTe/PbSnTe, strained layer and HgTe/CdTe quantum dot superlattices (QDSLs) with graded structures on the basis of newly formulated electron energy spectra and compare the same with that of the constituent materials. It has been found, taking heavily doped GaAs/Ga_1−xAl_xAs, CdS/CdTe, PbTe/PbSnTe, InAs/GaSb and HgTe/CdTe QDSLs as examples, that the TPM increases with increasing inverse electron concentration and film thickness, respectively, in different oscillatory manners and the nature of oscillations is totally band structure dependent. We have also suggested the experimental methods of determining the Einstein relation for the diffusivity–mobility ratio, the Debye screening length and the electronic contribution to the elastic constants for materials having arbitrary dispersion laws.     

Optoelectronic characteristics of CdTe/HgTe/CdTe quantum-dot quantum-well nanoparticles

Optoelectronic characteristics of CdTe/HgTe/CdTe quantum-dot quantum-well (QDQW) nanoparticles synthesized by the colloidal method are investigated in this study. Strong exciton bands were observed in absorption and photoluminescence (PL) spectra taken for the CdTe/HgTe/CdTe QDQW nanoparticles. The energy difference between the exciton absorption and PL bands is larger than those obtained with CdTe and HgTe nanoparticles. Photocurrent-voltage curves and time-dependent photocurrent curves were obtained for the CdTe/HgTe/CdTe QDQW nanoparticles. With regard to the photocurrent mechanism of these QDQW nanoparticles, those charge carriers participating in the formation of excitons may not contribute to the photocurrent, because of the large binding energy of the excitons. Moreover, it is suggested in this paper that free holes in the HgTe quantum-well in the valance band, rather than free electrons, are the main contributors to the photocurrent.     

Photoexcitation in two-dimensional topological insulators

One of the most fascinating challenges in Physics is the realization of an electron-based counterpart of quantum optics, which requires the capability to generate and control single electron wave packets. The edge states of quantum spin Hall (QSH) systems, i.e., two-dimensional (2D) topological insulators realized in HgTe/CdTe and InAs/GaSb quantum wells, may turn the tide in the field, as they do not require the magnetic field that limits the implementations based on quantum Hall effect. However, the band structure of these topological states, described by a massless Dirac fermion Hamiltonian, prevents electron photoexcitation via the customary vertical electric dipole transitions of conventional optoelectronics. So far, proposals to overcome this problem are based on magnetic dipole transitions induced via Zeeman coupling by circularly polarised radiation, and are limited by the g-factor. Alternatively, optical transitions can be induced from the edge states to the bulk states, which are not topologically protected though.Here we show that an electric pulse, localized in space and/or time and applied at a QSH edge, can photoexcite electron wavepackets by intra-branch electrical transitions, without invoking the bulk states or the Zeeman coupling. Such wavepackets are spin-polarised and propagate in opposite directions, with a density profile that is independent of the initial equilibrium temperature and that does not exhibit dispersion, as a result of the linearity of the spectrum and of the chiral anomaly characterising massless Dirac electrons. We also investigate the photoexcited energy distribution and show how, under appropriate circumstances, minimal excitations (Levitons) are generated. Furthermore, we show that the presence of a Rashba spin–orbit coupling can be exploited to tailor the shape of photoexcited wavepackets. Possible experimental realizations are also discussed.     

Thermoelectric properties of the trigonal and orthorhombic modifications of zinc telluride

Thermoelectric measurements are performed to study the phase transformations occurring in ZnTe under high pressure. It is shown that the thermoelectric power S of the cinnabar trigonal phase corresponds to a semiconductor with a hole-type conduction. In the Cmcm orthorhombic phase, the value of S≈+10 μV/K and the sign of the thermoelectric power testify to the metallic hole-type conduction, as in the high-pressure phases of other Group II chalcogenides (HgSe, HgTe, CdTe) with similar crystal lattices. In the transition region between the trigonal and orthorhombic phases, the pressure dependence of the thermoelectric power is found to exhibit an anomaly (a sharp dip), which leads to a change in the sign of S under decreasing pressure. This feature may presumably be related to the formation of the intermediate phase with the NaCl structure, which has an electron-type conduction in other zinc and cadmium chalcogenides.     

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?.     

Energy levels of vanadium ions in CdTe

In CdTe doped with vanadium the photoluminescence due to the ³ T ₂(F) ³ A ₂(F) transition of V³⁺(d ²) is detected. Its decay time is determined as (630±20) s, a result comparable to the analogous emissions in various host lattices. Further emissions around 5000 cm^–1 and 9000 cm^–1 are caused by charge-transfer transitions or bound-exciton decay. Excitation and sensitization spectra yield information on the positions of the energy levels within the gap, which are discussed using two different models. At T=4.2 K, the distance of the V²⁺/V³⁺ donor level is 7300 cm^–1 and 5700 cm^–1 referred to the valence and the conduction band edges, respectively. The absence of V²⁺(d ³) centres is tentatively ascribed to the existence of deeply bound excitons.     

Sputter cleaning and dry oxidation of CdTe,HgTe, and Hg0.8Cd0.2Te surfaces

The analyses of CdTe, HgTe, and Hg_0.8Cd_0.2Te surfaces by XPS and LEED after Ar⁺ sputtering and after the subsequent onset of a dry oxidation are described, and a quantitative evaluation of the XPS spectra is attempted. The results are: Ar⁺ sputtering yields a perfect unreconstructed CdTe surface of stoichiometric composition, whereas the composition of sputtered HgTe and Hg_0.8Cd_0.2Te surfaces generally deviates from the stoichiometry of the respective compound. This deviation is a function of the energy of the Ar ions (1 to 15 keV) and is characterized by an increasing deficit in Hg as the ion energy is raised. The Hg deficit of sputtered Hg_0.8Cd_0.2Te surfaces is substitutionally compensated by an equivalent increase in Cd, and due to this substitution the resulting surfaces are sufficiently ordered to display a distinct LEED pattern. The oxidation of sputtered CdTe, HgTe, and Hg_0.8Cd_0.2Te surfaces in an O₂ atmosphere is an extremely slow process. Therefore, the surfaces to be oxidized were additionally exposed to UV radiation (low pressure mercury lamp), and due to UV generated ozone as an oxidizing agent ultrathin native oxide layers of up to 15 Å thickness were readily obtained. The predominant constituents of these native oxide layers on Hg_0.8Cd_0.2Te are concluded to be CdTeO₃ and TeO₂.     

Spectral-Luminescence Characteristics of Carbazole, Dibenzofuran, and Dinaphthofuran in the Gas Phase in Excitation by Electrons and Photons

The spectra of electron-energy loss, the excitation functions, and the fluorescence spectra in excitation of carbazole, dibenzofuran, and dinaphthofuran by monoenergetic beams of electrons of different energies are determined. The singlet-triplet transitions S ₀–T ₁ and the singlet-singlet transitions up to S ₀–S ₇ are recorded, which covers the region 2–11 eV. In the spectra of electron-energy loss, bands that refer to the n^ast and ^* transitions are identified. The replacement of the heteroatom of nitrogen by the atom of oxygen in the five-membered ring has no substantial effect on the spectra of electron-energy loss.     

Silica-encapsulated CdTe/MPA quantum dots: microstructural,thermal, and chemical stability characterization

In this paper, we describe the synthesis of silica-encapsulated CdTe/MPA quantum dot (QD) nanocomposites (CdTe/MPA@SiO₂) at room temperature starting from water-soluble CdTe QDs capped with 3-mercaptopropionic acid (MPA) as surface ligand (CdTe/MPA), followed by their encapsulation with a thin layer of silica using the sol–gel process. The objective of this work was to evaluate the microstructural, thermal, and chemical properties of the encapsulated material as a way to evaluate these characteristics for possible applications in different technological areas. Transmission electron microscopy results proved that CdTe QDs (2.5 nm in size) were inside the SiO₂ nanoparticles, and the spherical CdTe/MPA@SiO₂ nanocomposite with a 69-nm thick shell. The photoluminescence intensity was evaluated as a function of pH in the range of 2–12. The CdTe/MPA@SiO₂ nanocomposite showed better chemical stability and greater intensity of photoluminescence in the pH 2–12 range than CdTe/MPA. In addition, the CdTe/MPA@SiO₂ nanocomposites showed good heat stability up to 125 °C. The changes in their optical properties were also evaluated by means of photoluminescence spectroscopy. The materials were also characterized by diffuse reflectance spectroscopy, X-ray diffraction, infrared spectroscopy, and thermogravimetric analysis.

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Growing of CdTe single crystals by static sublimation on a cool wall under the pressure of one of the components

The results obtained hitherto for equilibrium of the solid and gaseous phases of the CdTe semiconductor compound are summarized. The growth conditions of the static sublimation of CdTe on a cool wall were estimated from the relations at equilibrium. The single crystals grown had maximum dimensions 5×5×15 mm³, the concentration of the carriers (at 25C) wasp=4·8×10¹³ cm^–3, the mobility of the carriers (at 25C) was _p=66 cm²/Vsec.In conclusion the authors thank laboratory assistant J.Vlek for help in the work.     

CO-laser side-band spectrometer: Sub-Doppler heterodyne frequency measurements around 5 µm

We report the realization of a tunable sub-Doppler heterodyne spectrometer with high absolute accuracy, employing side-band generation with a CO laser. The fixed-frequency CO-gas laser, working from 4.7 to 8.4µm, is made partially tunable by the use of microwave side-band generation in a CdTe Electro-Optical Modulator (EOM). This leads to tunable radiation of high spectral purity. We describe the design of the microwave EOM, adapted to the CO laser, its performance and its first application to highly accurate frequency measurements. The side-band radiation is used for sub-Doppler stabilization of the CO laser, while the carrier frequency is mixed with the frequencies of two CO₂ reference lasers. As a first result, we present measurements of OCS transitions in the 4.9µm (61 THz) region, reaching an absolute accuracy of 30 kHz (/ = 5×1O^–10). Further application of our spectrometer to calibration gases will establish a variety of InfraRed (IR) calibration standards with a new quality of accuracy. On visit from: Fachbereich Physik, Humboldt-Universität zu Berlin     

窄带隙超晶格中载流子俄歇寿命和碰撞电离率的第一性原理研究

通过第一性原理的完整形式,基于全势能线性化增广平面波方法确定的精确能带结构和波函数,推算了技术上极为重要的窄带隙半导体超晶格中载流子俄歇复合时间.少数载流子的俄歇寿命由两种相关的方法来确定:1)由Fermi-金规则直接估算,2)联系俄歇复合和其相反过程碰撞电离,建立细致平衡公式,在一个统一的结构中进行间接估算.在n掺杂HgTe/CdTe和InAs/In_xGa_1－xSb超晶格中,由直接和间接的方法确定的寿命与一些实验结果相当一致.这说明该计算模式可以作为一种精确的手段用于窄带隙超晶格材料的性能优化. 关键词： 第一性原理 俄歇复合 碰撞电离 半导体超晶格     

Optical and Photoelectric Inhomogeneity of CdTe:V Crystals

The optical and photoelectric properties of CdTe:V crystals with the doping impurity concentration N _V = 5·10¹⁸–5·10¹⁹ cm^–3 are investigated and the possibility of their use as a photorefractive material is considered. As is seen from the spectra of optical transmission, the crystals of both types possess high transparency (50–65%), which for CdTe:V specimens with N _V = 5·10¹⁹ cm^–3 decreases sharply and in the range 12–14 m does not exceed 5%, whereas for CdTe:V crystals with vanadium concentration of 5·10¹⁸ cm^–3 such a value of transmission remains unchanged up to 25 m, implying a good optical quality of the latter crystals and their possible application in the spectral range 1.06–1.25 m in modern fiber-optic communication lines.     

Cyclotron resonance in HgTe/CdTe(013) narrowband heterostructures in quantized magnetic fields

Electron cyclotron resonance in HgTe/CdTe(013)-based narrowband heterostructures with a noninverted band structure in quantized magnetic fields has been investigated at different electron concentrations. Cyclotron transitions from the lower Landau levels of the conduction band have been discovered. The Landau levels have been calculated in the Kane model with the inclusion of strain. Comparison with the experimental data indicates that the energies of the observed cyclotron resonances are systematically higher than the calculated ones.     

CdTe和HgTe电子结构的紧束缚模型计算

基于局域密度近似(LDA或GGA)的密度泛函理论计算往往低估体系的禁带宽度,而这一低估对窄带隙半导体尤为严重.尽管基于混合泛函的密度泛函理论能有效地修正这一误差,但是由于计算量较大仍无法用于计算较大体系.本文发展了一组能够比较准确描述CdTe和HgTe晶体电子结构的紧束缚参数.将基于混合泛函的密度泛函计算结果作为输入,我们构建了正交的sp~3s~*基组下的紧束缚模型.此模型能够比较准确地描述能带结构在费米面附近4 eV范围内的色散关系.利用当前模型计算了CdTe和HgTe非晶的电子态密度,计算结果与他人的理论计算和实验值符合较好.     

Phase transitions in 2-dimensional systems of migrating orientable lattice defects

Phase transitions are studied in a system consisting of reorientating and migrating point defects in a two dimensional lattice. Due to the long range (r ^–2) nature of the dominant elastic interaction, surface effects are of central importance and have to be included. After diagonalizing the elastic interaction energy for defects characterized by arbitrary elastic dipole tensors the free energy of the system is minimized with respect to the tensor defect density (which describes the defect distribution in space and over a discrete number of orientations). Different types of phase transitions are obtained depending on the magnitude of the defect anisotropy. The phase below the paraelastic one is characterized for large by an anisotropic but homogeneous distribution, for small by an anisotropic and inhomogeneous distribution with a non linear space dependence. Similarities and differences with 3d results for=0 (or small) are discussed.     

K-forbidden allowed $ \beta$ transitions in heavy nuclei

The role of the band quantum number K in influencing the character of allowed transitions in heavy deformed nuclei is examined. The conditions for the occurrence of K -forbidden decays in this region are explored. Specific cases of “allowed” decays proceeding via K = 2 to K = 6 channels are presented to illustrate the phenomenon. The listed K = 2 transitions, which by themselves contribute over 10% of all the presently known allowed transitions for A 228 nuclei, are seen to have an average , which is clearly outside the normal range for allowed transitions. It is concluded that, wherever the -connected states can be confidently labelled using the quantum numbers, the K -forbiddenness is in general as significant as that involving the other two (spin and parity) quantum numbers.     

Hard-to-reach nuclei studied with deep-inelastic heavy-ion reactions

Spectroscopic studies performed with deep-inelastic heavy-ion reactions are reviewed for two regions of neutron-rich nuclei. The identification of isomers in nearly complete series of Sn isotopes and the resulting systematic of B(E2) values for isomeric transitions is presented and followed by the discussion of shell model states studied in neutron-rich Te isotopes including the new four neutron-hole isomers identified in ¹³⁰Te. Yrast spectroscopy studies of nuclei from the doubly magic ²⁰⁸Pb region are described by outlining the highest spin states observed in the ²⁰⁸Pb core nucleus. The E3 transitions observed abundantly in yrast decays are discussed within the framework of particle-octupole vibration coupling and the validity of a simple rule connecting energy shifts of octupole states built on one-particle states with the ones observed for two-particle states is demonstrated.Received: 15 January 2003, Published online: 23 March 2004PACS: 21.60.Cs Shell model - 23.20.Lv transitions and level energies - 27.60. + j - 27.80. + w      

Hot hole effects in uniaxially stressedp-type germanium under E∥P∥ [1 1 1]

High field behaviour of holes in uniaxially stressedp-type germanium for stressP and the electric fieldE along the [1 1 1] axis is studied. The field dependence of the drift velocity, the frequency dependence of the longitudinal differential mobility and the spectral dependence of the sub-band population for direct hole transitions between the upper and lower splitted sub-bands are calculated by the Monte-Carlo method. A Gunn-type negative differential mobility is demonstrated to appear atP 7 to 8 kbar,E 0.15 to 4 kV cm^–1 in a frequency rangev 0 to 100 GHz. It is shown, that the stimulated emission and current increase observed by I. V. Altuhovet al. [1] cannot be ascribed to the direct hole transitions between the sub-bands because the population inversion of the transitions is absent.