Electronic properties of multi-quantum dot structures in Cd 1-xZn xS alloy semiconductors

In this paper, we present a theoretical study of the quantized electronic states in Cd_1-xZn_xS quantum dots. The shape of the confining potential, the subband energies and their eigen envelope wave functions are calculated by solving a one-dimensional Schr?dinger equation. Electrons and holes are assumed to be confined in dots having a flattened cylindrical geometry with a finite barrier height at the boundary. Optical absorption measurements are used to fit the bandgap edge of the Cd_1-xZn_xS nanocrystals. An analysis of the electron band parameters has been made as a function of Zn composition. Two main features were revealed: (i) a multiplicity in Cd_1-xZn_xS quantum dots with different crystalline sizes has been found to fit accurately experimental data in the composition range 0 ≤x ≤0.2; (ii) the fit did not, however, show a multiplicity for x higher than 0.4. On the other hand, we have calculated the energy level structure of coupled Cd_1-xZn_xS semiconductor quantum dots using the tight-binding approximation. As is found the Zn composition x = 0.4 is expected to be the most favorable to give rise a superlattice behavior for the Cd_1-xZn_xS quantum dots studied.     

Effects of Cd compensation on the photoluminescence of Cd1-xZnxTe single crystals grown by the modified Bridgman method

In order to deal with the phenomenon of Cd evaporation during the growth of Cd_1-xZn_xTe (x=0.1) crystals, Cd compensation was adopted during the growth by adding excess Cd into the raw materials. Photoluminescence (PL) spectra were used to investigate the effects of Cd compensation on the properties of Cd_1-xZn_xTe. A free exciton (FE) peak appeared in the near band-edge region after Cd compensation, which indicated that the concentration of Cd vacancies (V_Cd) was reduced in Cd_1-xZn_xTe crystals by Cd compensation. The donor–acceptor pair (DAP) peak became dominant in the PL spectrum and its first and second order phonon replica could also be easily identified after Cd compensation, which was only a weak hump in the case of Cd_1-xZn_xTe crystals without Cd compensation. It possibly meant that impurities of Al and In were released from the V_Cd-related complexes. In addition, the deep energy level transition D peak, decreased obviously after Cd compensation, which confirmed that Cd compensation could reduce the dislocation density effectively. PACS 71.20.Nr; 71.55.Gs; 78.55.-m     

Photoluminescent analysis of Zn1-xCdxO alloys

The photoluminescent (PL) spectra of Zn_1-xCd_xO (0≤x≤0.53) alloy films were obtained successfully. A new explanation from the viewpoint of band structure is brought forward to comprehend the PL nature of the alloy films. According to this explanation, the near-band-energy emissions of the Zn_1-xCd_xO (x>0) films are caused by the radiative transitions between the Zn4s–Cd5s hybrid level and the O2p level, and the broadenings of the two levels are responsible for the gradually increased line width of the PL peak of the film; Zn3d and Cd4d orbital levels have great effects on the band-gap variations of the alloys. In addition, a quadratic equation is put forward to depict the relationship between the band gaps E_g of the alloys and their Cd contents x, i.e. E_g(x)=3.30-1.22x+1.26x² (0≤x≤0.53). PACS 78.55.-m; 78.55.Et; 81.15.Cd     

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：     

1.8—4.2K GaAs1-xPx注N+,注Zn+光荧光谱

本文研究了1.8—4.2K下离子注N,注ZnGaAs_1-xP_x样品的光致发光行为。实验结果表明由N-Zn跃迁完全转变到N束缚激子复合的x值依赖N,Zn的浓度。利用Campbell局域化模型计算了N-Zn跃迁与N束缚激子复合的几率比。这一几率比是组分x和N-Zn浓度的函数。在1.8K,在同一样品上我们清晰地观测到对应N-Zn跃迁与N束缚激子复合的光谱。 关键词：     

Electronic and optical properties of Cd1-xZnxS nanocrystals

We report a numerical simulation of the conduction and valence band edges of Cd_1-xZn_xS nanocrystallites using a one — dimensional potential model. Electron — hole pairs are assumed to be confined in nanospheres of finite barrier heights. Optical absorption measurements are used to fit the bandgap of the Cd_1-xZn_xS nanocrystal material. A theoretical analysis is also made to calculate the energy location of bound excitons and the oscillator strength of interband transitions as a function of zinc composition. The aim of the latter study is to investigate the optical behavior of Cd_1-xZn_xS nanocrystals. An attempt to explain all the results is presented.     

Cd1-xZnxTe晶体退火条件的选择及Zn压对退火晶体质量的影响

采用传统Bridgman方法和加入accelerated crucible rotation technique的Bridgman(缩写为ACRT-B)方法生长的Cd_1-xZn_xTe(x=0.04)晶体中存在有点缺陷、位错、杂质和Te沉淀等缺陷.为了减少甚至消除这些缺陷,必须将生长后的CdZnTe晶片在Cd气氛下退火.从Cd-Te和Cd_0.96Zn_0.04Te的PT相图出发,详细讨论了CdZnTe晶体的气固平衡条件,并 关键词： 1-xZn_xTe')" href="#">Cd_1-xZn_xTe 退火 气-固平衡     

Studies on the synthesis and characterization of Zn1− x Cd x S and Zn1− x Cd x S:Mn2+ semiconductor quantum dots

Quantum dots (3–4?nm) of Zn_{1? x} Cd _x S (both free of Mn²⁺ and with Mn²⁺ incorporated) were synthesized through a novel solvothermal-microwave irradiation technique. Detailed structural analysis of the Zn_{1? x} Cd _x S and Zn_{1? x} Cd _x S:Mn²⁺ (x?=?0, 0.25, 0.5, 0.75 and 1) materials was carried out using powder X-ray diffraction technique. For all the compositions, the crystallite size was controlled to less than 1.5?nm. The optical energy gap for Zn_{1? x} Cd _x S was found to vary from 3.878 to 2.519?eV and for Zn_1?x Cd _x S:Mn²⁺ it varies from 3.830 to 2.442?eV when x is increased from 0 to 1. Overall, the optical energy gap could be tuned from a minimum of 2.442?eV to a maximum of 3.878?eV. DC conductivity analysis (from 40°C to 150°C) and electrical energy gap analysis for all the compositions were also performed. The dc conductivity for Zn_{1? x} Cd _x S solid solutions varies from 0.3840?×?10^?10 to 8.7782?×?10^?10?mho/m at 150°C and for Zn_{1? x} Cd _x S:Mn²⁺ it varies from 0.5751?×?10^?10 to 9.8078?×?10^?10 mho /m at 150°C (for x?=?0 to x?=?1). The method of synthesis and the results observed in this investigation may assist in the fabrication of optical devices when the required operational performance falls under the range observed in the study.     

Structural,electrical and optical properties of Cd1−xZnxO thin films and alloying effects on Kβ/Kα intensity ratios

Cd_1?xZn_xO thin films were prepared by spray pyrolysis in air atmosphere on a glass substrate at 250 °C. The Zn content in Cd_1?xZn_xO films was varied from x = 0 to 0.60. Structural, electrical and optical properties of Cd_1?xZn_xO films were investigated by x‐ray diffraction, electrical resistivity and optical transmittance spectra, respectively. As the Zn content in Cd_1?xZn_xO thin films increased, the preferred orientation of the films did not change, only the peak intensity of the planes decreased. In addition to the peaks of CdO, peaks of ZnO were observed in the film with x = 0.6. The resistivity of Cd_1?xZn_xO thin films increased with increasing Zn content. Transmittance spectra studies of films were carried out in the 190‐1100 nm wavelength range and the results showed that the bandgap energy range varied from 2.42 to 3.25 eV. In addition, alloying effect on the Kβ/Kα intensity ratio in Cd_1?xZn_xO semiconductor thin films was studied. It was found that the Kβ/Kα intensity ratio is changed by alloying effects in Cd_1?xZn_xO semiconductor thin films for different composition of x. The results were compared with the theoretical values. Copyright © 2006 John Wiley & Sons, Ltd.     

Band gaps and charge distribution in quasi-binary (GaSb) (InAs) crystals

Pseudopotential investigation of energy band gaps and charge distribution in quasi-binary (GaSb)_1-x(InAs)_x crystals has been reported. To the best of our knowledge, there had been no reported theoretical work on these materials. In agreement with experiment, the quasi-binary crystals of interest showed a significant narrowing of the optical band gap compared to the conventional Ga_xIn_1-xAs_ySb_1-y quaternary alloys (with x = 1 - y). Moreover, the absorption at the optical gaps indicated that (GaSb)_1-x(InAs)_x is a direct Γ to Γ band-gap semiconductor within a whole range of the x composition. The information derived from the present study predicts that the band gaps cross very important technological spectral regions and could be useful for thermophotovoltaic applications. Received 30 August 2002 Published online 1st April 2003 RID="a" ID="a"Present address: Physics Department, University of M'sila, 28000 M'sila, Algeria e-mail: N_Bouarissa@yahoo.fr     

Ground-state properties of LiV2O4 and Li1-xZnx(V1-yTiy)2O4

We present susceptibility, microwave resistivity, NMR and heat-capacity results for Li_1-xZn_x(V_1-yTi_y)₂O₄ with 0 ? x ? 0.3 and 0 ? y ? 0.3. For all doping levels the susceptibility curves can be fitted with a Curie-Weiss law. The paramagnetic Curie-Weiss temperatures remain negative with an average value close to that of the pure compound Θ≈ - 36 K. Spin-glass anomalies are observed in the susceptibility, heat-capacity and NMR measurements for both type of dopants. From the temperature dependence of the spin-lattice relaxation rate we found critical-dynamic behavior in the Zn doped compounds at the freezing temperatures. For the Ti-doped samples two successive freezing transitions into disordered low-temperature states can be detected. The temperature dependence of the heat capacity for Zn-doped compounds does not resemble that of canonical spin glasses and only a small fraction of the total vanadium entropy is frozen at the spin-glass transitions. For pure LiV₂O₄ the spin-glass transition is completely suppressed. The temperature dependence of the heat capacity for LiV₂O₄ can be described using a nuclear Schottky contribution and the non-Fermi liquid model, appropriate for a system close to a spin-glass quantum critical point. Finally an ( x / y , T )-phase diagram for the low-doping regime is presented. Received 16 March 2001 and Received in final form 30 October 2001     

Optical investigations of ZnxCd1-xS nanostructures

Zn_xCd_1-xS nanostructures with (x = 0, 0.25, 0.5, 0.75, 1) have been grown on glass substrates using spray pyrolysis technique. X-ray diffraction results have showed that Zn_xCd_1-xS nanostructures were formed with hexagonal and cubic structures. The structural parameters have been evaluated as a function of concentration (x). Also, the optical properties that depend on the mole fraction (x) are investigated for Zn_xCd_1-xS nanostructures.     

First-principles study of structrural and corrected band properties of wurtzite Zn1-xCdxO and Zn1-xMgxO systems

A first-principles method based on density functional theory(DFT),a generalized gradient approximation(GGA),and a projector-augmented wave(PAW) are used to study the structual and band properties of wurtzite Zn1-xCdxO and Zn1-xMgxO(0 ≤x≤1) ternary alloys.By taking into account all of the possible structures,the band gaps of Zn1-xCdxO and Zn1-xMgxO alloys are corrected and compared with experimental data.     

Absorption spectra of thin layers of solid solutions Cs<Subscript>2</Subscript>(Cd<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)I<Subscript>4</Subscript>

The excitonic absorption spectra of thin films of ferroelectric Cs₂CdI₄ and Cs₂ZnI₄ solid solutions are studied for the first time. It is found that, within the whole range of molar concentrations x, the spectra of Cs₂(Cd_{1 − x} Zn _x )I₄ comprise two bands that originate from the bands of the two compounds. It is shown that the exciton transfer occurs most efficiently between the tetrahedrons CdI₄ and ZnI₄ along the b axis of the crystal. Unusual behavior of the concentration with regards to the maxima of long-wavelength excitonic bands E _m (x) agrees well with the developed theory that takes into account dependence on x of the matrix element of the intertetrahedron exciton transfer and that is similar to the theory of Davydov’s splitting in molecular crystals.     

Doped Graphene Quantum Dots Modified Zn0.9Cd0.1Se for Improved Photoelectric Properties

S-graphene quantum dots (GQDs), N-GQDs, P-GQDs, and Cl-GQDs are prepared by a solution chemistry method and further incorporated with Zn_xCd_1−xSe by one-step hydrothermal method. In the previous study, Zn_xCd_1−xSe reached the optimal photoelectric performances at the Zn/Cd ratio of 0.9:0.1, so the Zn_0.9Cd_0.1Se were combined with doped GQDs (D-GQDs) to form Zn_0.9Cd_0.1Se/doped-GQDs. The influence of GQDs doped with different elements on the photoelectric properties of Zn_0.9Cd_0.1Se composites is discussed. Compared with pristine Zn_0.9Cd_0.1Se, Zn_0.9Cd_0.1Se/Cl-GQDs, and Zn_0.9Cd_0.1Se/P-GQDs can improve the photocurrent response and current intensity, therein, Zn_0.9Cd_0.1Se/Cl-GQDs reaches the lowest interfacial charge transfer resistance and the highest photocurrent response of 5.48 × 10⁻⁶ A cm⁻². Mott–Schottky analysis shows that the fitting slope of Zn_0.9Cd_0.1Se/Cl-GQDs composites is significantly lower than that of Zn_0.9Cd_0.1Se/GQDs with other doped elements. The results indicate that Zn_0.9Cd_0.1Se/Cl-GQDs composites has the largest carrier density, which is beneficial to charge conduction.     

The determination of Li+ mobility in solid electrolyte Li1.3Al0.1Zn0.1Ti1.8P3O12 in view of ionic diffusivity and conductivity

Nasicon-type solid electrolyte Li_1.3Al_0.1Zn_0.1Ti_1.8P₃O₁₂ was prepared by citric acid-assisted acrylamide polymerisation gel method. X-ray diffraction pattern showed that the introduction of Zn²⁺ in the parent matrix Li_1+x Al _x Ti_2?x P₃O₁₂ made it easier to get high-purity rhombohedral structure (space group $ R\overline 3 C $ ) Li_1.3Al_0.1Zn_0.1Ti_1.8P₃O₁₂ without the evidence of impurity secondary phase. The Li⁺ kinetics were investigated by complex impedance in bulk pellet and ionic conductivity in battery-type composite cathode, respectively. Grain-interior resistance measured by galvanostatic intermittent titration technique, potential step chronoamperometry, and AC impedance spectroscopy at 20 °C varies in the range 1.2–1.95?×?10^?4?S?cm^?1, which is in good agreement with that obtained by complex impedance method 1.5?×?10^?4?S?cm^?1.     

The discovery of a 2H–6H solid state transformation in ZnxCd1−xS single crystals

Single crystals of Zn_xCd_1?xS have been grown from the vapour phase at 1100°C in the presence of H₂S gas. X-ray diffraction studies of the as-grown crystals show that polytypism and stscking faults occur in Zn_xCd_1?xS crystals for x ? 0.94. It is observed that for 0.92 < x < 0.98 the 2H structure of Zn_xCd_1?xS crystals transforms to a disordered 6H structure on annealing in vacuum around 600°C. For 0.95 < x < 0.98 this 6H structure finally transforms to a disordered 3C structure on annealing further at higher temperatures around 800°C. The structural transformations occur through a non-random insertion of stacking faults, as revealed by the diffuse streak joining the X-ray diffraction maxima along the 10.L reciprocal lattice row. Experimental investigation of the diffuse intensity distribution, as recorded on a single crystal diffractometer from partially transformed single crystals, reveals that the mechanism of the transformation is very different from that reported for the same transformation in silicon carbide and cannot be described in terms of a single-parameter model of non-random deformation faulting.     

Cu对Zn1－xFexO稀磁半导体磁性的影响

采用水热法，在温度430 ℃，填充度35%，矿化剂为3 mol·L^-1KOH，前驱物为添加适量的FeCl₂·6H₂O的Zn(OH)₂，反应时间24h，合成了Zn_1-xFe_xO和Zn_1-xFe_xO：Cu稀磁半导体晶体.当在Zn(OH)₂中添加一定量的FeCl₂·6H₂O为前驱物，水热反应产物为掺杂Fe的Zn_1-xFe_xO多种形态晶体混合物，其个体较大的晶体中的Fe原子百分比含量为0.49%—0.52%.采用超导量子干涉磁强计测量了材料的磁性，晶体的磁化强度随温度下降而减小.在前驱物中同时加入适量比例的Cu化合物，合成了共掺杂Cu的Zn_1-xFe_xO：Cu，和Zn_1-xFe_xO相比，其室温下的磁化强度有明显的提高，且在室温下具有铁磁性. 关键词： 氧化锌 水热 稀磁半导体 晶体     

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.     

Optical properties and recombination processes in (Zn,Cd)Se Graded Index Separate Confinement Heterostructures

Graded Index Separate Confinement Heterostructures have been grown by MOVPE using (Zn,Cd)Se and ZnSe wide bandgap semiconductors. These structures are composed of a deep Zn_1-xCd_xSe (x<0.23%) central well embedded between two thick (Zn,Cd)Se graded layers within which the cadmium composition varies from 0 up to 10% on one side of the well, and from 10 down to 0% on the other side. Both carriers and photon confinement occur in such structures making them suitable for blue-green stimulated emission.Reflectance and photoreflectance data, taken at 2K on a series of samples of different designs allowed us to observe excitonic transitions up to the third quantum number. The structures exhibit a strong photoluminescence line due to the recombination of the carriers in the quantum well. The photoluminescence intensity is analyzed as a function of the temperature and displays a strong thermal quenching. This quenching is due to an increase of the non-radiative recombinations through defects at low temperature and to the thermal escape of the carriers above 40-60K. The value of the activation energy for thermal escape shows that this mechanism is unipolar and concerns the heavy holes.