The dependence of the structural and optical properties on the Te mole fraction in ZnS1−xTex/GaAs heterostructures

Lattice-mismatched ZnS_1−xTe_x epilayers with various Te mole fractions on GaAs (100) substrates were grown by double well temperature gradient vapor deposition. X-ray diffraction patterns showed that the grown ZnS_1−xTe_x layers were epitaxial films. The photoluminescence spectra showed that the peak position of the acceptor-bound exciton (A⁰, X) varied dramatically with changing the Te mole fraction and that the behavior of the (A⁰, X) peak position of the ZnS_1−xTe_x epilayers with a small amount of the Te mole fraction was attributed to a bowing effect. The reflectivity and ellipsometry spectra showed that the absorption energy peak was significantly affected due to the Stoke's effect. These results provide important information on the structural and optical properties of ZnS_1−xTe_x/GaAs heterostructures for improving optoelectronic device efficiencies operating in the spectral range between near ultraviolet and visible regions.     

Electrodeposition of n-type Bi2Te3−ySey thermoelectric thin films on stainless steel and gold substrates

Thermoelectric films of n-Bi₂Te_3−ySe_y were prepared by potentiostatic electrodeposition technique onto stainless steel and gold substrates at room temperature. These films were used for morphological, compositional and structural analysis by environment scanning electron microscope (ESEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The effect of different substrates on the structure and morphology of Bi₂Te_3−ySe_y films and relation between Se content in the electrodepositing solutions and in the films were also investigated. These studies revealed that Bi, Te and Se could be co-deposited to form Bi₂Te_3−ySe_y semiconductor compound in the solution containing Bi³⁺, HTeO₂⁺ and H₂SeO₃. The morphology and structure of the films are sensitive to the substrate material. The doped content of Se element in the Bi₂Te_3−ySe_y compound can be controlled by adjusting the Se⁴⁺ concentration in the electrodepositing solution. X-ray diffraction analysis indicates that the films prepared at −40 mV versus saturated calomel electrode (SCE) exhibit strong (1 1 0) orientation with rhombohedral structure.     

Interdot carrier transfer in semimagnetic Pb1−xMnxSe nanocrystals embedded in oxide glass

Temperature- and excitation-intensity-dependent photoluminescence (PL) spectra of semimagnetic Pb_1−xMn_xSe nanocrystals embedded in glass matrix have been studied. Two types of dot families with different sizes and dispersions were identified by spectral deconvolution in Gaussian components with different full widths at half maxima values. Temperature induced carrier-transfer interdots are responsible for the sigmoidal temperature dependence of the higher PL peak energy and for anomalous enhanced photoluminescence emission efficiency, at low temperatures. The activation energy of nonradiative channel responsible for a strong thermal quenching, at T>80 K, is deduced from an Arrhenius plot of integrated PL intensity.     

Electronic structure and optical properties of CdSexTe1−x mixed crystals

The band structure and optical properties of the CdSe_xTe_1−x ternary mixed crystals have been studied using the pseudopotential formalism under an improved virtual crystal approximation approach. Quantities such as, energy gaps, band-gap bowing parameters, electron effective mass and dielectric constants are calculated. Our results agree well with the available data in the literature. The composition dependence of all studied quantities has been expressed by quadratic polynomial forms.     

Composition-dependent structural and electrical properties of PbSe1−xTex thin films

Polycrystalline PbSe_1−xTe_x ingots were prepared by solid-state microwave synthesis. Thin films of PbSe_1−xTe_x were then deposited onto clean glass substrates using vacuum evaporation technique. Their nanostructure morphologies and stoichiometric ratio were examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectra (EDX). X-ray diffraction (XRD) patterns indicated that the lattice constants of PbSe_1−xTe_x powders and thin films increased with the increasing amount of Te. From the electrical property measurements, the thin films were characterized by n-type behavior.     

Syntheses of optically efficient (La1−x−yCexTby)F3 nanocrystals via a hydrothermal method

Optically efficient cerium and terbium doped lanthanide fluoride (La_1−x−yCe_xTb_y)F₃ nanocrystals with different doping concentrations have been synthesized by a hydrothermal route in the presence of ethylenediamine tetraacetic acid disodium salt (EDTA). The results showed that the formation of nanocrystals with different morphologies depends on terbium ion Tb³⁺ doping concentration, but independent of cerium ion Ce³⁺ doping concentration. With increase in Tb³⁺ doping concentration, the morphologies of nanocrystals evolved from a spherical shape to a plated-like one. In addition, both the photoluminescence quantum yield (PL QY) and the fluorescence lifetime of nanocrystals increased with the increase in Ce³⁺ doping concentration in cerium and terbium co-doped system. The PL QY reached up to 55%, and the lifetime up to 7.3 ms. Transmission electron microscopy (TEM), X-ray diffraction (XRD), selected area electron diffraction (SAED), X-ray fluorescence (XRF), energy dispersive spectroscopy (EDS), ultraviolet-visible (UV-vis) absorption, photoluminescence (PL) and infrared (IR) spectroscopies were employed to characterize the properties of nanocrystals. The growth mechanism of nanocrystals with different morphologies and optical properties of nanocrystals with different doping concentrations were investigated.     

The structural, electronic, elastic and optical properties of AlCu(Se1−xTex)2 compounds from first-principle calculations

The structural, electronic structure, elastic and optical properties of the AlCu(Se_1−xTe_x)₂ compounds have been investigated by using a first-principles method based on density functional theory. The lattice constants of the quaternary compounds AlCu(Se_1−xTe_x)₂ increase with the increasing of Te composition. The calculated lattice constants for the ternary compounds i.e. AlCuSe₂ and AlCuTe₂ are in good agreement with the experimental data. The band structures show that the compounds have direct band gap and the band gaps are found to vary nonlinearly with composition. The total and part density of states of the quaternary AlCu(Se_1−xTe_x)₂ compounds are discussed. The calculated elastic constants indicate that all of the AlCu(Se_1−xTe_x)₂ compounds are mechanically stable. The bulk modulus B, shear modulus G, Young’s modulus E and Poisson’s ratio ν can be obtained by using the Voigt-Reuss-Hill averaging scheme. The B/G ratios of the AlCu(Se_1−xTe_x)₂ compounds indicate that AlCu(Se_0.8Te_0.2)₂ is ductile and the others are brittle. The Debye temperature of the AlCu(Se_1−xTe_x)₂ compounds decreases a little with increasing Te content except the compound with x = 0.4. The dielectric functions, refractive index, extinction coefficient, absorption spectrums and energy-loss function of the AlCuSe₂ and AlCuTe₂ are also calculated and discussed in this work.     

The effect of Te doping on the electronic structure and thermoelectric properties of SnSe

SnSe_1−xTe_x (x=0, 0.0625) bulk materials were fabricated by melting Sn, Se and Te powders and then hot pressing them at various temperatures. The phase compositions of the materials were determined by X-ray diffraction (XRD) and the crystal lattice parameters were refined by the Rietveld method performed with DBWS. XRD analysis revealed that the grains in the materials preferentially grew along the (l 0 0) directions. The structural behavior of SnSe_1−xTe_x (x=0, 0.0625) was calculated using CASTEP package provided by Materials Studio. We found that the band gap of SnSe reduced from 0.643 to 0.608 eV after Te doping. The calculated results were in good agreement with experimental results. The electrical conductivity and the Seebeck coefficient of the as-prepared materials were measured from room temperature to 673 K. The maximum power factor of SnSe is ∼0.7 μW cm⁻¹K⁻² at 673 K.     

Ab-initio calculations of Co-based diluted magnetic semiconductors Cd1−xCoxX (X=S, Se, Te)

Ab-initio calculations are performed to investigate the structural, electronic and magnetic properties of spin-polarized diluted magnetic semiconductors composed of II-VI compounds Cd_1−xCo_xX (X=S, Se, Te) at x=0.25. From the calculated results of band structure and density of states, the half-metallic character and stability of ferromagnetic state for Cd_1−xCo_xS, Cd_1−xCo_xSe and Cd_1−xCo_xTe alloys are determined. It is found that the tetrahedral crystal field gives rise to triple degeneracy t_2g and double degeneracy e_g. Furthermore, we predict the values of spin-exchange splitting energies Δ_x(d) and Δ_x(p−d) and exchange constants N₀α and N₀β produced by the Co 3d states. Calculated total magnetic moments and the robustness of half-metallicity of Cd_1−xCo_xX (X=S, Se, Te) with respect to the variation in lattice parameters are also discussed. We also extend our calculations to x=0.50, 0.75 for S compounds in order to observe the change due to increase in Co.     

Structural and electrical properties of CdSexTe1−x thin films

The ternary semiconductors compounds are found to be very useful in the fabrication of thin film devices. This paper reports the preparation of CdSe_xTe_1?x films (1 ? x ? 0) in the thickness range 1000–3000 Å by vacuum evaporation technique onto glass and mica substrates held at temperatures, 303 to 623 K in a vacuum better than 5 × 10^?6 torr. The films were characterised by determining their composition and structure. The structure of the films, examined using XRD and TEM techniques, was found to be cubic (zincblende) in the entire composition range. The electrical resistivity and Hall mobilities have been determined as a function of film composition and deposition temperature.     

Study of polycrystalline bulk CuIn1-xGaxTe2

Polycrystalline CuIn_1−xGa_xTe₂ bulk films were synthesized by reacting, in stoichiometric proportions, high purity Cu, In, Ga and Te in a vacuum sealed quartz ampoule. The phase structure and composition of the bulk films were analysed by X-ray diffraction and energy-dispersive X-ray analysis, respectively. The bulk samples, of p-type conductivity, are found to be near-stoichiometric, polycrystalline, with tetragonal chalcopyrite structure, predominantly oriented along a direction perpendicular to the (1 1 2) plane. Photoluminescence spectra were recorded at 7 K and 700 mW to characterize the defects and the structural quality. The main peak as a function of composition has been studied.     

Nanoscale semiconductor Pb1−xSnxSe (x = 0.2) thin films synthesized by electrochemical atomic layer deposition

In this paper the fabrication and characterization of IV-VI semiconductor Pb_1−xSn_xSe (x = 0.2) thin films on gold substrate by electrochemical atomic layer deposition (EC-ALD) method at room temperature are reported. Cyclic voltammetry (CV) is used to determine approximate deposition potentials for each element. The amperometric I-t technique is used to fabricate the semiconductor alloy. The elements are deposited in the following sequence: (Se/Pb/Se/Pb/Se/Pb/Se/Pb/Se/Sn …), each period is formed using four ALD cycles of PbSe followed by one cycle of SnSe. Then the deposition manner above is cyclic repeated till a satisfactory film with expected thickness of Pb_1−xSn_xSe is obtained. The morphology of the deposit is observed by field emission scanning electron microscopy (FE-SEM). X-ray diffraction (XRD) pattern is used to study its crystalline structure; X-ray photoelectron spectroscopy (XPS) of the deposit indicates an approximate ratio 1.0:0.8:0.2 of Se, Pb and Sn, as the expected stoichiometry for the deposit. Open-circuit potential (OCP) studies indicate a good p-type property, and the good optical activity makes it suitable for fabricating a photoelectric switch.     

Properties of optical phonons in Zn1−x−yMgyBexSe quaternary mixed crystal

Long wavelength optical lattice vibration and dielectric constants of the quaternary mixed crystal Zn_1−x−yMg_yBe_xSe are investigated based on the pseudo-unit-cell mode and Born-Huang procedure. It is found that this material shows a three-mode behavior and the oscillator strength of each mode is mainly controlled by only one component. The theoretical results also show that the linear interpretation method for dielectric constants is reliable. The vibrational frequencies and the oscillator strengths of the ternary mixed crystals Be_xZn_1−xSe, Be_xMg_1−xSe and Mg_xZn_1−xSe are also calculated as special cases of the quaternary mixed crystal for comparing with experiments. The calculation shows agreement with the experimental results.     

Electrodeposition of BixSb2−xTey thermoelectric thin films from nitric acid and hydrochloric acid systems

The electrochemical behaviors of Bi^III, Te^IV and Sb^III single ions and their mixtures were investigated in nitric acid and hydrochloric acid system separately. Based on which, Bi_xSb_2−xTe_y thermoelectric films were prepared by potentiostatic electrodeposition from the solutions with different concentrations of Bi^III, Te^IV and Sb^III in the two acid systems. The morphologies, compositions, structures, Seebeck coefficients and resistivities of the deposited thin films were characterized and compared by ESEM (or FESEM), EDS, XRD, Seebeck coefficient measurement system and four-probe resistivity measuring device respectively. The results show that although Bi_xSb_2−xTe_y thermoelectric thin film which structure is consistent with the standard pattern of Bi_0.5Sb_1.5Te₃ can be gained in both of the two acid solutions by adjusting the deposition potential, their morphologies and thermoelectric properties have big differences in different acid solutions.     

Resonant photoemission study of Eu1−xGdxTe layers

Resonant photoemission study of electronic structure of molecular beam epitaxy grown Eu_1−xGd_xTe layers without and with cover protected layer of Te were performed using synchrotron radiation. The analysis of the valence band and shallow core levels spectra of the clean surface of Eu_1−xGd_xTe obtained in situ under UHV conditions showed the existence of Eu²⁺ and Eu³⁺ ions in the layers. The trivalent europium ions mostly are located at the surface and its amount strongly depends on sample surface preparation conditions. The prolonged annealing of Eu_1−xGd_xTe layers covered with protected layer of Te leads to formation of clean surface of the sample not changing the stoichiometry of it and without the accumulation of Eu³⁺ ions at the surface region.     

NiP:Mn as a potential magnetic contacting material to Cd1−xMnxTe

Injection of spin-polarized current into spintronic devices is a challenge to the semiconductor physicists and technologists. II-VI compound semiconductors can act as the spin aligner on the top of GaAs light emitting diode. However, II-VI compound semiconductor like Cd_1−xMn_xTe is still suffering from contacting problem. Application of electroless deposited magnetic NiP:Mn contact would enhance efficient current injection into Cd_1−xMn_xTe than the standard gold contact. A technique for electroless deposition of NiP:Mn on Cd_1−xMn_xTe have been described here. The electronic and magnetic properties of the contact material NiP:Mn and the contact performance of NiP:Mn relative to evaporated gold have been evaluated. The contact fulfills the requirements of resistivity and ferromagnetism for application to Cd_1−xMn_xTe.     

X-ray photoelectron study of Sn1−xMnxTe semimagnetic semiconductors

X-ray photoelectron (XPS) studies of core-levels in Sn_1−xMn_xTe (x < 0.1) semimagnetic semiconductors have been performed. The spectra were acquired under UHV conditions from the clean (as-cleaved or in-situ scraped) crystal surface. The single-phase NaCl structure of the alloys studied was verified by X-ray diffraction (XRD). The structure of Sn 3d and Te 3d core-levels in SnMnTe was found fully consistent with that of SnTe. Remarkable qualitative similarity of the Mn 2p spectrum of Sn_1−xMn_xTe (x = 0.09) with the case of zinc-blende MnTe [R.J. Iwanowski, M.H. Heinonen, E. Janik, Chem. Phys. Lett. 387 (2004) 110] has been shown: (1) the same binding energies (BEs) of the main contributions to the Mn 2p_3/2 line, related to Mn²⁺ state of the bulk MnTe bond; (2) occurrence of low BE component in the Mn 2p spectrum, indicative of clean-surface species containing reduced-valence Mn ions (i.e. Mn^q+, where 0 < q < 2); (3) strong satellites of the 2p_3/2 (Mn²⁺ related) parent lines. In SnMnTe, the highest intensity ratio of the satellite to main peak (ever reported for Mn 2p photoelectron spectrum) was revealed; this was interpreted in terms of the so-called charge-transfer model.     

Optical properties and ferromagnetism of ternary Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te nanocrystals

Cd_1−x Mn _x Te (x = 0, 0.1, 0.2) nanocrystals have been synthesized by mechanical alloying (MA) Cd, Mn, and Se elemental powders. XRD patterns and HRTEM images confirmed the formation of cubic Cd_1−x Mn _x Te nanocrystals. All the diffraction peaks from elemental Cd, Mn, and Te powders disappeared completely in those XRD patterns of as-milled Cd_1−x Mn _x Te nanocrystals for more than 20 h. When the MA process was carried out for 40 h, typical zinc blende structure diffraction mode was exhibited in the XRD pattern. Subsequently, capping the surface of as-milled Cd_1−x Mn _x Te nanocrystals with long chain trioctylphosphine/trioctylphosphine oxide/nitric acid (TOP/TOPO/NA) molecules has achieved colorful dispersion solution, which shows similar optical properties to those CdTe nanocrystals prepared by wet chemical process. The grain size is within the range of 2–8 nm for the capped Cd_1−x Mn _x Te nanocrystals being ball milled for 40 h. The PL excitation peak red shifts to longer wavelength side with increasing Mn concentration. Pure CdTe nanocrystals show ferromagnetism behavior at room temperature, the saturation magnetization value and magnetic hysteresis loop increase with the content of substituting Mn ions within the Cd_1−x Mn _x Te nanocrystals.     

Nanoscratch behavior of Zn1−xCdxSe heteroepitaxial layers

This paper describes the nanoscratch behavior of Zn_1−xCd_xSe epilayers grown using molecular beam epitaxy (MBE). Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Hysitron Triboscope nanoindenter techniques were employed to determine the microstructures, morphologies, friction coefficients (μ), and hardnesses (H) of these materials, and thereby propose an explanation for their properties in terms of nanotribological behavior. Nanoscratch analysis revealed that the coefficient of friction of the Zn_1−xCd_xSe epilayer system decreased from 0.172 to 0.139 upon increasing the Cd content (x) from 0.07 to 0.34. Furthermore, studies of the scratch wear depth under a ramping load indicated that a higher Cd content provided the Zn_1−xCd_xSe epilayers with a higher shear resistance, which enhanced the strength of the CdSe bonds. These findings suggest that the greater stiffness of the CdSe bond, relative to that of the ZnSe bond, enhances the hardness of the epilayers. Indeed, the effect of the Cd content on the growth of the Zn_1−xCd_xSe epilayers is manifested in the resulting nanotribological behavior.     

Characterization and properties of ZnO1−xSx alloy films fabricated by radio-frequency magnetron sputtering

A series of ZnO_1−xS_x alloy films (0 ≤ x ≤ 1) were grown on quartz substrates by radio-frequency (rf) magnetron sputtering of ZnS ceramic target, using oxygen and argon as working gas. X-ray diffraction measurement shows that the ZnO_1−xS_x films have wurtzite structure with (0 0 2) preferential orientation in O-rich side (0 ≤ x ≤ 0.23) and zinc blende structure with (1 1 1) preferential orientation in S-rich side (0.77 ≤ x ≤ 1). However, when the S content is in the range of 0.23 < x < 0.77, the ZnO_1−xS_x film consists of two phases of wurtzite and zinc blende or amorphous ZnO_1−xS_x phase. The band gap energy of the films shows non-linear dependence on the S content, with an optical bowing parameter of about 2.9 eV. The photoluminescence (PL) measurement reveals that the PL spectrum of the wurtzite ZnO_1−xS_x is dominated by visible band and its PL intensity and intensity ratio of UV to visible band decrease greatly compared with undoped ZnO. All as-grown ZnO_1−xS_x films behave insulating, but show n-type conductivity for w-ZnO_1−xS_x and maintain insulating properties for β-ZnO_1−xS_x after annealed. Mechanisms of effects of S on optical and electrical properties of the ZnO_1−xS_x alloy are discussed in the present work.