Electronic states and vibration spectra of CdTe/ZnTe quantum dot superlattices

Electronic and vibrational states in CdTe/ZnTe quantum dot superlattices are studied using optical spectroscopy techniques (photoluminescence in a wide temperature range, IR reflection, and Raman scattering). The effect of the ZnTe barrier layer thickness on the luminescence spectra of the structures is discussed. The luminescence from electronically coupled islands is assumed to be due to spatially indirect excitons because of the specific features of the CdTe/ZnTe heterostructure band structure. A combination of quantum-dot vibrational modes, which has not been observed earlier, is detected in the Raman spectra. Analysis of the lattice IR reflection spectra shows that, in the case of large barrier thicknesses between the quantum-dot planes, elastic stresses are concentrated in the Zn_1?xCd_xTe layers, whereas in structures with lower barrier thicknesses the elastic-strain distribution exhibits a more complicated pattern.     

Electro-Optic Coefficient Measurements for Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript> Te Single Crystals at 1550 nm Wavelength

Nonlinear optical materials of the type Zn _x Cd_1-x Te single crystals, where x = 0.0, 0.2, 0.4, 0.6, 0.7, 0.8, and 1.0, have been grown by the Bridgman method, using a vertical furnace. We have investigated the electro-optic (E-O) coefficient and refractive index of Zn _x Cd_1-x Te single crystals at optical communication wavelength (1550 nm). In the case of CdTe crystal, the E-O coefficient was 15.5 × 10^-12 m/V, which is the biggest among the E-O coefficients of Zn _x Cd_1-x Te crystals. The E-O modulation signal was very big in low frequency range (DC-100kHz), but the signal amplitude became smaller as we increased frequency above 100 kHz. We also found the acousto-optic modulation at CdTe single crystals.     

Optical properties of Cd<Subscript>0.6</Subscript>Mn<Subscript>0.4</Subscript>Te/Cd<Subscript>0.5</Subscript>Mg<Subscript>0.5</Subscript>Te quantum-well structures

Emission spectra of three Cd_0.6Mn_0.4Te/Cd_0.5Mg_0.5Te superlattices with Cd_0.6Mn_0.4Te quantum-well (QW) widths of 7, 13, and 26 monolayers, respectively, and the same thickness (46 monolayers) of the Cd_0.5Mg_0.5Te barriers have been studied. The QW width affects the shape and spectral position of the Mn²⁺ intracenter luminescence (IL) band as a result of the crystal field being dependent on the position of the manganese ion with respect to the interface. Measured in identical experimental conditions, the exciton luminescence as compared to the IL is substantially higher in intensity in a QW than in a bulk CdMnTe crystal. Some samples of superlattices and bulk crystals exhibit, in addition to the conventional IL band near 2.0 eV, a weaker band at about 1.45 eV. This band apparently derives from intracenter transitions in the Mn²⁺ ions in the regions where the crystal lattice has the rock-salt rather than the conventional zinc blende structure.     

Lattice IR reflection spectra of ZnSe/Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript>x</Subscript>Se strained superlattices grown on a GaAs substrate by molecular beam epitaxy

This paper reports the results of measurements of the lattice IR reflection spectra for the ZnSe/Zn_1?xCd_xSe (x=0.20, 0.40, and 0.47) superlattices grown on a GaAs(001) substrate by molecular beam epitaxy. Mathematical analysis of the experimental spectra has revealed the presence of only one lattice mode rather than the two expected IR-active lattice modes of the ZnSe and Zn_1?xCd_xSe layers. This result is explained by the fact that the frequencies of the lattice modes of these layers become closer in magnitude under the effect of internal elastic stresses.     

Magnetic dynamics of phase separation domains in GdMn<Subscript>2</Subscript>O<Subscript>5</Subscript> and Gd<Subscript>0.8</Subscript>Ce<Subscript>0.2</Subscript>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

Specific features of the magnetic properties and magnetic dynamics of isolated phase separation domains in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ have been investigated. These domains represent 1D superlattices consisting of dielectric and conducting layers with the ferromagnetic orientation of their spins. A set of ferromagnetic resonances of separate superlattice layers has been studied. The properties of the 1D superlattices in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ are compared with the properties of the previously investigated RMn₂O₅ (R = Eu, Tb, Er, and Bi) series. The similarity of the properties for all the RMn₂O₅ compounds with different R ion types is established. Based on the concepts of the magnetic dynamics of ferromagnetic multilayers and properties of semiconductor superlattices, a 1D model of the superlattices in RMn₂O₅ is built.     

CdTe中Zn的定域模和准定域模晶格振动

本文报道4.2—300K和20—350cm^-1范围内低组份Cd_1-xZn_xTe混晶远红外吸收与反射光谱。实验观察到Zn及ZnTe在CdTe中诱发的定域模和准定域模以及类CdTe2TA双声子吸收过程。用质量缺陷模型结合格林函数方法估计了模式频率。用等位移模型计算了混晶的双模行为并拟合反射光谱,对类CdTe和类ZnTe模的弛豫与声子吸收强度的温度效应也作了计算和解释。 关键词：     

Signs of phase transitions and a thermooptic memory effect in absorption spectra of (N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>)<Subscript>2</Subscript>Zn<Subscript>0.8</Subscript>Ni<Subscript>0.2</Subscript>Cl<Subscript>4</Subscript> solid solutions

We have studied the signs of phase transitions and spatial modulation of the structure in the absorption spectra of an (NCH₃)₄)₂Zn_0.8Ni_0.2Cl₄ crystal. We have observed the existence of phase transitions in the given solid solution at temperatures of 155 K, 168 K, 275 K, 280 K, and 296 K. We have established that the thermooptic memory effect observed in the absorption spectra is completely consistent with a model of defect ordering in the sample in the field of the modulated structure. According to this model, stabilization of the sample in an incommensurable phase leads to fixing of a certain symmetry in the crystal (usually a lower symmetry than the average symmetry of the incommensurable phase) and a metal-halogen complex corresponding to the defect wave. As a result, we observe an appreciable shift of the intra-ionic absorption bands and an increase in their intensity. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 5, pp. 717–723, September–October, 2008.     

NMR study of topological insulator Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> in a wide temperature range

NMR studies of ¹²⁵Te in the topological insulator bismuth telluride Bi₂Te₃ in a wide temperature range from room temperature to 12.5 K are performed. The pulsed NMR spectrometer Bruker Avance 400 is applied. The NMR spectra are obtained for the powder from Bi2Te3 single crystal and monocrystalline plates with the orientations c || B and c ⊥ B. At room temperature, the spectra consist of two lines related to two nonequivalent positions of tellurium nuclei Te1 and Te2. The parameters of the NMR frequency shift tensor are found from the powder spectrum. The temperature dependences of the spectra for the powder and plates with the orientation c ⊥ B agree with each other. The line shift with decreasing temperature is explained by the reduction of the Knight shift. The thermal activation energy of charge carriers is estimated. The spectra for the plates with the orientation c || B demonstrate peculiar behavior below 91 K. The spin-lattice relaxation time for the powder and monocrystalline plates with both orientations at room temperature is measured.     

Exciton and intracenter luminescence in Cd<Subscript>0.6</Subscript>Mn<Subscript>0.4</Subscript>Te/Cd<Subscript>0.5</Subscript>Mg<Subscript>0.5</Subscript>Te quantum-well structures

Exciton luminescence and intracenter luminescence (IL) of Mn²⁺ ions in Cd_0.6Mn_0.4Te/Cd_0.5Mg_0.5Te structures with quantum wells (QWs) 7, 13, and 26 monolayers thick were studied. It was established that in QWs the intensity of exciton luminescence with respect to that of IL is a few orders of magnitude higher than that in bulk crystals. The spectral position of manganese IL profile changes noticeably in going from a bulk crystal to a QW of the same composition. The nonexponential parts of the IL decay curves are determined by excitation migration and the cooperative upconversion process, whose contribution is high under strong excitation and efficient migration. At 77 K, the IL decay constant τ within the exponential region increases with decreasing QW thickness. The decay constant τ in a QW, unlike in a bulk Cd_0.5Mn_0.5Te crystal, decreases substantially under cooling from 77 to 4 K.     

Lattice IR spectroscopy of Zn<Subscript>1 −<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se epitaxial layers grown on a GaAs substrate by molecular-beam epitaxy

Based on the results from comparison of the data on lattice reflection for Zn_{1 ?x} Cd _x Se bulk crystals (x = 0.08, 0.22) and Zn_{1 ?x} Cd _x Se thin layers of the same compositions grown on GaAs, a more adequate interpretation is given to the lattice IR reflection spectra of Zn_{1 ?x} Cd _x Se alloy films (x = 0–0.55) grown on a GaAs substrate by molecular-beam epitaxy. In this composition region, two ZnSe-and CdSe-like lattice modes are observed corresponding to a double-mode type of rearrangement of the vibration spectrum with varying x. A weak (with oscillator strength smaller than 0.25) high-frequency mode (with respect to the dominant modes) at 220 cm^?1 is also observed.     

Anti-stokes luminescence of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed organic dye molecules and few-atom silver clusters

For microcrystals of Zn_0.6Cd_0.4S with adsorbed molecules of a number of organic dyes, we have observed sensitized anti-Stokes luminescence excited by radiation with wavelengths in the range 610–750 nm and flux density 10¹⁴–10¹⁵ photons/cm²·sec. The positions of the bands in the excitation spectra for such luminescence match those of the absorption spectra for the adsorbed dye molecules, which is evidence in favor of a cooperative mechanism for its appearance. We have shown that enhancement of the anti-Stokes luminescence is possible when silver atoms and few-atom clusters appear on the Zn_0.6Cd_0.4S surface in addition to the dye molecules. We hypothesize that its excitation in the latter case occurs as a result of two-photon optical transitions. These transitions occur sequentially, with transfer of an electron or the electronic excitation energy from the dye molecules to silver atoms and few-atom clusters adsorbed on the surface of Zn_0.6Cd_0.4S, creating deep localized states in the bandgap with photoionization energies 1.80–2.00 eV. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 617–621, September–October, 2007.     

Optical and vibrational properties of CdxHg1-x-y ZnyTe solid solutions

The lattice dynamics of Cd_xHg_1-x-y Zn_yTe solid solutions is studied theoretically and experimentally. The frequencies of the basic optical phonons of Cd_xHg_1-x-y Zn_yTe are calculated in terms of a modified random-element isodisplacement model. As a result, all basic vibrations of the crystal lattice that substantially affect the optical properties of this material in the spectral region corresponding to one-phonon resonance are identified. The optical properties of epitaxial Cd_xHg_1-x-y Zn_yTe layers grown by liquid-phase epitaxy on Cd_1-x Zn_xTe substrates are studied. The calculated and experimental spectral dependences of the dielectric function of Cd_xHg_1-x-y Zn_yTe solid solutions of various compositions are compared at 295 and 78 K, and good agreement between them is reached. The additional lattice vibrations whose frequencies in the phonon density of states are lower than that of the HgTe mode are shown to be caused by the lattice defects of the Cd_xHg_1-x-y Zn_yTe solid solutions.     

Effect of the phase transition on the infrared spectra of improper ferroelastic Hg<Subscript>2</Subscript>Cl<Subscript>2</Subscript>

The effect of the improper ferroelastic transition on the IR spectra of the Hg₂Cl₂ crystal is considered. It is shown that, due to lattice anharmonicity, the distortions caused by the phase transition give rise to mixing of the normal modes of the higher symmetry phase of the crystal, which results in the appearance of extra poles in the frequency dependence of the lattice permittivity. Experimentally, such extra poles manifest themselves in the appearance of extra lines in the IR absorption spectra. Symmetry analysis shows that this effect in Hg₂Cl₂ is strongly anisotropic in the plane normal to the tetragonal axis of the crystal.     

Zn_(1-x)Mn_xTe/ZnTe超晶格的多声子拉曼谱

Ｚｎ１－ｘＭｎｘＴｅ／ＺｎＴｅ超晶格的多声子拉曼谱陈辰嘉李海涛王学忠李晓莅孙允希王晶晶周赫田（北京大学物理系北京１００８７１）Ｍｕｌｔｉ┐ｐｈｏｎｏｎＲａｍａｎＳｐｅｃｔｒａｏｆＺｎ１－ｘＭｎｘＴｅ／ＺｎＴｅＳｕｐｅｒｌａｔｉｃｅＣｈｅｎＣｈｅｎｊ...     

Multilayer metallization of ferrite Mg<Subscript>0.54</Subscript>Zn<Subscript>0.46</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript>

The character of interatomic interaction in ferrite Mg_0.54Zn_0.46Fe₂O₄ was studied using the x-ray spectroscopy technique and theoretically. It was found that the electronic structure of samples is rearranged during annealing at high temperatures (1280°C, 0.5–0.8 h). The electronic structure rearrangement was shown to be associated with multilayered ferrite metallization in which alternating layers with metallic and ionic-covalent bonds form.     

Manganese doping effects on interband electronic transitions,lattice vibrations,and dielectric functions of perovskite-type Ba<Subscript>0.4</Subscript>Sr<Subscript>0.6</Subscript>TiO<Subscript>3</Subscript> ferroelectric ceramics

The Ba_0.4Sr_0.6−x Mn _x TiO₃ (BSMT) ceramics with different Mn composition (from 1% to 10%) have been prepared via the conventional solid-state reaction sintering. The X-ray diffraction analysis shows that the ceramics are polycrystalline with the single perovskite phase. The lattice vibrations and optical properties have been investigated using Raman scattering, spectroscopic ellipsometry (SE), and infrared reflectance spectra. It was found that the optical bandgap for the BSMT ceramics is varied between 3.40 and 3.65 eV. The three first-order Raman-active phonon modes can be observed, and the frequency of the A ₁(LO₃)/E(LO) mode shows a blue shift of 8 cm⁻¹ with the Mn composition, which can be attributed to the distortion of the TiO₆ octahedron. With increasing Mn composition, the frequency of the infrared-active TO₄ mode decreases from 532 to 520 cm⁻¹, owing to the local variation of the lattice constant induced by the Mn incorporation. Moreover, the optical functions of the ceramics from the far-infrared to ultraviolet region are obtained based on the SE and reflectance spectra, which is useful for the potential applications in ferroelectric-based optoelectronic devices.     

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.     

Sensitized anti-Stokes luminescence centers in microcrystals of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed dye molecules and few-atomic silver clusters

The sensitized anti-Stokes luminescence excited by radiation with wavelengths from 610 to 750 nm and flux densities of 10¹⁴–10¹⁵ quanta/(cm²·s) is detected for microcrystals of Zn _0.6 Cd _0.4 S solid solutions with adsorbed organic malachite green and methylene blue dye molecules. The position of its excitation spectra coincides with that of the absorption spectra of adsorbed dye molecules, which suggests the cooperative mechanism of its occurrence. The possibility of amplification of the anti-Stokes luminescence by means of adsorption of silver atoms and few-atomic silver clusters, in addition to the dye molecules, on the Zn _0.6 Cd _0.4 S surface is investigated. It is assumed that in the latter case, the anti-Stokes luminescence is excited as a result of two-quantum optical transitions with electron or electron excitation energy transfer from the dye molecules adsorbed on the Zn _0.6 Cd _0.4 S surface to silver atoms and few-atomic silver clusters creating deep local states with photoionization energies of 1.8–2.0 eV in the gap. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 21–26, March, 2008.     

Fabrication and characterization of p-ZnO/n-Zn0.8Cd0.2O/n-ZnO heterojunction

We report the fabrication of p-ZnO/n- Zn_0.8Cd_0.2O/n-ZnO heterojunctions that contain Al–N codoped p-ZnO, undoped n- Zn_0.8Cd_0.2O, and Al-doped n-ZnO layers. An InZn alloy is used as the p- and n-ZnO Ohmic contact electrodes. This structure exhibits improved rectifying p–n junction behavior, with forward turn-on voltage in the range of 3–5 V. The reverse breakdown voltage can be as high as 15 V, with 10^?6-A reverse leakage current. Photoluminescence spectra show strong near band-edge emissions for both p- and n-ZnO at 368 nm and for undoped n- Zn_0.8Cd_0.2O, which is substantially red-shifted to 399 nm.     

P2-type Na<Subscript>0.67</Subscript>Mn<Subscript>0.6</Subscript>Fe<Subscript>0.4-x-y</Subscript>Zn<Subscript>x</Subscript>Ni<Subscript>y</Subscript>O<Subscript>2</Subscript> cathode material with high-capacity for sodium-ion battery

The P2-type Na_0.67Mn_0.6Fe_0.4O₂ (NaMnFe), Na_0.67Mn_0.6Fe_0.3Zn_0.1O₂ (NaMnFeZn), and Na_0.67Mn_0.6Fe_0.2Zn_0.1Ni_0.1O₂ (NaMnFeZnNi) are prepared using an acetate decomposition reaction and developed as promising cathode materials for high-capacity sodium-ion batteries. The XRD patterns show that Zn²⁺ and Ni²⁺ ions are successfully incorporated into the lattice of the Na-Mn-Fe-O system, and the P2-type structure remains unchanged after substitution. The charging/discharging tests exhibit that the Na_0.67Mn_0.6Fe_0.4O₂, Na_0.67Mn_0.6Fe_0.3Zn_0.1O₂, and Na_0.67Mn_0.6Fe_0.2Zn_0.1Ni_0.1O₂ electrodes have the capacities of 200.4, 182.0, and 202.2 mAhg^?1, respectively. The Na_0.67Mn_0.6Fe_0.4O₂ electrode has a higher initial capacity but faster capacity decay. When partially substituting Zn and Ni for Fe, the Na_0.67Mn_0.6Fe_0.3Zn_0.1O₂ and Na_0.67Mn_0.6Fe_0.2Zn_0.1Ni_0.1O₂ electrodes exhibit lower reversible capacity but improved cycling stability (88.3 and 93.4% capacity retention over 100 cycles). The greatly improved electrochemical performance of the Na_0.67Mn_0.6Fe_0.2Zn_0.1Ni_0.1O₂ electrode apparently belongs to the contribution of the Zn²⁺ and Ni²⁺ substitution, which facilitates to alleviate the Jahn-Teller distortion of Mn and suppresses the polarization.