xSex Alloys Research Articles

Thermoelectric properties of PbTe1−xSex alloys prepared by high pressure

N-type thermoelectric alloys PbTe1−xSex were prepared by high pressure technique and the composition-dependent thermoelectric properties were studied at room temperature. The measurement results indicate that the electrical properties could be tuned effectively through chemical partial substitution of Se with Te. Thermal conductivity of PbTe was depressed largely by alloying with PbSe. The maximum figure of merit, Z=1.04×10−1K−1, was obtained for the sample of PbTe0.75Se0.25, which is about two times higher than that of unalloyed PbTe. These results indicate that alloying with PbSe under high pressure provides a viable and controllable way of tuning the thermoelectric properties for PbTe.

Tuning structural, electrical, and optical properties of oxide alloys: ZnO1−xSex

Previously we showed that it is possible to narrow the band gap of zinc oxide from 3.3 to ∼2 eV through the addition of Se. Here, we use thin film samples of ZnO1−xSex grown by pulsed laser deposition to describe in detail the effect of growth parameters (temperature, pressure, and fluence) on the chemistry, structure, and optoelectronic properties of oxide alloys. We analyze the influences of temperature, laser fluence, and pressure during growth on the structure and composition of the films and define the parameter space in which homogeneous ZnO1−xSex alloy films can in fact be synthesized. Electronic transport in films grown under different conditions was characterized by resistivity, thermopower, and Hall effect measurements. We discuss how the electron affinity and native defects in polycrystalline oxide alloys enable reasonable mobilities (∼15 cm2/Vs) relative to their single crystalline counterparts. Finally, we elaborate on the model of optical structure in ZnO1−xSex and discuss the dependence of optical properties on growth temperature and fluence.

Structural, optical, FTIR and photoluminescence studies of CdS1−xSex thin films by chemical bath deposition method

The ternary CdS1−xSex alloy thin films with the nominal composition of x = 0.2, 0.4, 0.6 and 0.8 have been synthesized on glass substrate by chemical bath deposition (CBD) method at 80°C from aqueous solution. Crystalline phases and optical absorption of the films have been studied by X-ray diffraction and UV–visible spectrophotometer. Elemental composition of the CdS1−xSex films was studied by the energy dispersive X-ray (EDX) analysis. The optical absorption and transmission studies revealed that CdS1−xSex films had direct allowed transition with band gap energy decreased from 2.28 to 1.92 eV as thickness varied from 762.4 to 621.2 nm. The average crystalline size was calculated from X-ray line broadening and it is increased from 12.71 to 14.67 nm for x = 0.2–0.8 which was confirmed by SEM studies. The substitution of Se concentrations into the Cd–S and Cd–S–Se lattice is confirmed by the increase of lattice parameters, FTIR and photoluminescence studies. The broad variation in the band gap of CdS1−xSex thin films have potential applications in the field of optoelectronic devices.

<i>Ab Initio</i> Study of Structural and Electronic Properties of Barium Chalcogenide Alloys

First-principles calculations have been used to study the structural and electronic properties of BaS1–xSex ternary alloy using full-potential muffin-tin orbital’s (FP-LMTO) method within density functional theory (DFT). In this approach, the local-density approximation (LDA) and generalized gradient approximation (GGA) are used for the exchange-correlation (XC) potential. The effect of composition on lattice parameter, bulk modulus, band gap and effective mass was investigated. The deviations of the lattice constant from Vegard’s law and the bulk modulus from linear concentration depend- ence were observed for BaS1–xSex alloy. The microscopic origins of bowing parameter were explained using approach of Zunger and co-workers. Accordance is found from the comparison of our results with other experimental and theo- retical calculations.

Spatial Bandgap Engineering along Single Alloy Nanowires

Bandgap engineering of semiconductor nanowires is important in designing nanoscale multifunctional optoelectronic devices. Here, we report a facile thermal evaporation method, and realize the spatial bandgap engineering in single CdS(1-x)Se(x) alloy nanowires. Along the length of these achieved nanowires, the composition can be continuously tuned from x = 0 (CdS) at one end to x = 1 (CdSe) at the other end, resulting in the corresponding bandgap (light emission wavelength) being modulated gradually from 2.44 eV (507 nm, green light) to 1.74 eV (710 nm, red light). In spite of the existing composition (crystal lattice) transition along the length, these multicolor nanowires still possess high-quality crystallization. These bandgap engineered nanowires will have promising applications in such as multicolor display and lighting, high-efficiency solar cells, ultrabroadly spectral detectors, and biotechnology.

Highly efficient orange emission in ZnO:Se nanorods

The origin of the visible emissions in ZnO nanostructures is a long-standing issue. In this work, a strong orange emission around 2.1 eV in ZnO:Se nanorods is reported. Temperature-dependent photoluminescence (PL) and PL decay results indicate that radiative recombination dominates the overall decay process, leading to highly efficient orange emission at room temperature. It is proposed that Se acts as isoelectronic centers and no evidence of ZnO1−xSex alloy formation is found. Based on the PL excitation results, we propose a Se-induced impurity band model to describe the photophysics of the orange emission.

Epitaxial Growth and Composition-Dependent Optical Properties of Vertically Aligned ZnS1−xSex Alloy Nanowire Arrays

Vertically aligned ZnS1−xSex alloy nanowire arrays covering the entire compositional range were grown on GaAs (111)B substrates by metal organic chemical vapor deposition (MOCVD) using Ga nanoparticles as catalysts. The ZnS1−xSex nanowires obtained are predominantly zincblende in structure and preferentially grow along the [111] direction. They show a narrow distribution in composition, which is the result of good growth controls afforded by MOCVD. The relationship between composition of the nanowires and the precursors was studied and determined; it allows the predetermination of the composition of the alloy nanowires. In low temperature (77 K) cathodoluminescence spectra, the band edge emission of ZnS1−xSex nanowires is found to shift from 3.214 to 2.779 eV, and its line width significantly decreases from 213 to 44 meV, when the Se content increases from 0.45 to 1. This phenomenon is understood in terms of the chemical disorder in the alloy.

Band structure engineering of ZnO1−xSex alloys

ZnO 1 − x Se x alloys with Se substitutional composition x<0.12 were synthesized using pulsed laser deposition. Incorporation of small concentrations of Se results in a greater than 1 eV red shift in the ZnO optical absorption edge which is quantitatively explained in the framework of the band anticrossing model. The Se defect level is found to be located at 0.9 eV above the ZnO valence band and the band anticrossing coupling constant is determined to be 1.2 eV. These parameters allow prediction of the composition dependence of the band gap as well as the conduction and the valence band offsets in the full composition range of ZnO1−xSex alloys.

Thermal and electrical transport properties of VA-element doped Pb9.6M0.2Te10−xSex (M = Sb, Bi) thermoelectric materials

The VA-element doped Pb9.6M0.2Te10−xSex (M = Sb, Bi, x = 6, 7, 8) alloys have been synthesized and their thermoelectric (TE) properties were systematically investigated. The power factors of Sb- and Bi-doped samples reached ∼1.4 mW m−1 K−2 and 0.8–1.0 mW m−1 K−2, respectively, while the thermal conductivities decreased to 0.75 W m−1 K−1 and 0.8 W m−1 K−1 as the temperature was increased to 700 K. Due to the rather low thermal conductivity, a highest ZT of ∼1.05 has been attained for the Sb-doped sample, indicating that the VA element is another effective dopant besides the classical dopant iodine. The results show that the introduction of VA element is beneficial to improve TE performance of PbTe-based materials.

Studies on growth and characterization of CdS1−xSex (0.0≤x≤1.0) alloy thin films by spray pyrolysis

Abstract The n-CdS1−xSex thin films of variable composition have been deposited on amorphous glass and FTO-coated glass substrate by simple and cost effective spray pyrolysis technique. The various deposition parameters have been optimized by using photoelectrochemical technique. The structural, surface morphological, compositional, optical and electrical properties have been studied. The X-ray diffraction studies indicated that all the films are polycrystalline in nature with hexagonal structure irrespective of the composition. The lattice parameters ‘a’ and ‘c’ vary from 4.1034 to 4.2615 A and 6.6664 to 6.9243 A respectively with change in composition parameter from x = 0.0 to x = 1.0. Polycrystalline texture with nearly smooth surface and clearly defined grains has been observed for all samples from scanning electron microscopy (SEM). EDAX studies confirmed that CdS1−xSex films have approximately same stoichiometry both initially and finally. The absorption coefficient ‘α’ is of the order of 104 cm−1. The optical absorption studies reveal that direct allowed transition with band gap energy between 2.44 and 1.74 eV. It is found that resistivity of the films decreased with increase in ‘x’ up to 0.8 and further it increases for x = 1.0. Semiconducting behavior has been observed from resistivity measurements.

First principles calculations of structural, electronic and thermodynamic properties of SrS, SrSe, SrTe compounds and SrS1−xSex alloy

The ab initio full potential linearized augmented plane wave (FP-LAPW) method within density functional theory was applied to study the structural and electronic properties of the compounds SrS, SrSe, SrTe and their alloy SrS1−xSex in the NaCl structure. Results are obtained using both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation potentials. The ground-state properties, like lattice constant and bulk modulus obtained from our calculations agree very well with experimental and other theoretical calculations. We present the main features of electronic properties, where the electronic band structure shows that the fundamental energy gap is indirect (Γ→X). Moreover the alternative form of GGA proposed by Engel and Vosko (GGA-EV) is also used for band structure calculations. Results obtained with this approximation show that significant improvement over other theoretical work are closer to the experimental data. The effect of composition on lattice parameter and bulk modulus was investigated.Deviations of the lattice constant from Vegard’s law and the bulk modulus from linear concentration dependence were observed for the alloy. Moreover, the microscopic origins of the gap bowing were explained. In addition the thermodynamic stability of the alloy was investigated by calculating the critical temperature for SrS1−xSex alloy.

Optimum composition of a Bi2Te3 − x Se x alloy for the n-type leg of a thermoelectric generator

The reliability of determination of model parameters for the Bi2Te3 − xSex alloys is improved by extending the concentration and temperature ranges in experimental studies and, correspondingly, in calculations of kinetic coefficients based on the two-band model of the electronic spectrum. The obtained results served as a motivation for a study of the thermoelectric figure of merit for the above-mentioned alloys with x = 0.3, 0.45, and 0.6 and with the electron concentration varied in the range (1–50) × 1018 cm−3 at temperatures 300–550 K. Comparison of the results showed that the highest efficiency is exhibited by the Bi2Te2.7Se0.3 alloy with the absolute value of the thermoelectric power of about 165 μVK−1 at 300 K, and the dimensionless efficiency is equal to 1.2 at 410 K. An appreciable decrease in thermal conductivity in alloys with x = 0.6 at 410 K is related to a larger band gap and could beneficially affect the figure of merit. However, the magnitude of this effect is found to be too small to compensate a decrease in electrical conductivity due to a large fraction of heavy electrons in the concentration and to a high content of selenium.

Lattice thermal conductivity of nanostructured thermoelectric materials based on PbTe

We report the through-thickness lattice thermal conductivity Λl of (PbTe)1−x/(PbSe)x nanodot superlattices (NDSLs) over a wide range of periods 5 nm≤h≤50 nm, compositions 0.15≤x≤0.25, growth temperatures 550 K≤Tg≤620 K, and growth rates 1 μm h−1≤R≤4 μm h−1. All of our measurements approach Λl of bulk homogenous PbTe1−xSex alloys with the same average composition. For 5 nm≤h≤50 nm, Λl is independent of h; a result we attribute to short mean-free paths of phonons in PbTe and small acoustic impedance mismatch between PbTe/PbSe. We alloyed the PbTe layers of four NDSLs with SnTe up to a mole fraction y=18%; Λl is reduced by <25%.

First principles investigation of barium chalcogenide ternary alloys

The ab initio full potential linearized augmented plane wave (FP-LAPW) method within density functional theory was applied to study the effect of composition on the structural, electronic, thermodynamic and optical properties of BaS1� xOx, BaS1� xSex and BaS1� xTex ternary alloys. The effect of composition on lattice parameter, bulk modulus, band gap, refractive index and dielectric function was investigated. The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. The thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing DHm as well as the phase diagram. In addition to FP-LAPW method, the composition dependence of the refractive index and the dielectric constant was studied by different models.

Band anticrossing effects in MgyZn1−yTe1−xSex alloys

The electronic structures of MgyZn1−yTe1−xSex alloys were studied by optical absorption and photoluminescence techniques under applied hydrostatic pressure. In samples with both x and y≠0, the band gap exhibits a strongly nonlinear pressure dependence which is similar to the effects observed previously in ZnTe1−xSex and ZnTe1−xSx ternaries and that is well explained by the anticrossing interaction of the selenium localized electronic states with the conduction band of the matrix. In contrast, the pressure dependence of the band gap in MgyZn1−yTe (i.e., x=0) is not significantly changed in form from that of ZnTe; it is concluded that the effects of alloying MgTe with ZnTe can be well understood within the virtual crystal approximation.

Photoelectric properties of the n-SnSSe-p-InSe heterojunctions

Photoelectric properties of the n-SnSSe-p-InSe heterojunctions were investigated. A special feature of these structures is the use of an SnS2−x Sex alloy (x=0.5) as a wide-bandgap window material, which makes it possible to shift a short-wavelength threshold (lying in the 0.8–1.0 μm range) of the heterojunction photosensitivity band. It is demonstrated that high-quality p-n heterojunctions can be fabricated from layered crystals joined to make an optical contact.

Optical properties of ZnS1−xSex alloys fabricated by plasma-induced isoelectronic substitution

Nonequilibrium growth of thin-film ternary ZnS1−xSex semiconductor alloys was accomplished using physical vapor deposition with simultaneous electron cyclotron resonance H2S plasma activation. Substrate temperature, gas flow, and plasma power determine the ZnS1−xSex alloy composition and structure. Integrated optical transmission spectra for the ZnS1−xSex semiconductor alloys as a function of H2S plasma power are presented. Using the α2 vs hν plots for the various ZnS1−xSex films, the optical band gap Eg is extrapolated from each curve. This methodology yields the values of the band gap as a function of stoichiometry. We observe that the plasma induced isoelectronic substitution of S into the ZnSe lattice increases the band gap. This study shows that plasma-induced isoelectronic substitution is technologically feasible and useful for fabricating ternary II–VI alloys under nonequilibrium conditions.

Investigation on the effect of isoelectronic substitution in ZnS1−xSex alloys

The composition, structure, and optical properties of thin-film ZnS1−xSex semiconductor alloys were studied by x-ray photoemission spectroscopy, x-ray diffraction, and optical transmission. Nonequilibrium growth of the ternary II–VI semiconductor alloys was accomplished using molecular beam epitaxy with simultaneous electron cyclotron resonance H2S plasma activation to form the ZnS1−xSex alloys. Photoemission measurements were used to identify changes in bulk composition and valence band electronic structure as well as changes in the Zn 3d, Se 3d, and S 2p core lines. X-ray diffraction measurements revealed that these films contain the cubic phase and that the lattice constant decreases with sulfur content. Correlation of these results with growth parameters indicates that substrate temperature, gas flow, and plasma power determine the ZnS1−xSex alloy composition and structure. In addition, optical transmission measurements showed that substitution of S into the ZnSe lattice shifted the cutoff from 460 to 380 nm.

Critical phenomena and metal-nonmetal transition in the miscibility gap region of liquid Tl1−xSex (x < 1/3) alloys

The very first electroconductivity σ(T) and thermopower S(T) measurements of separated phases of liquid T11−xSex (x 8 × 10−3. Our experiments demonstrate a close correlation of the metal-nonmetal electron transition with phase separation. In particular, the influence of conductivity electrons on the formation of the critical region is discussed.

Exciton transfer between localized states in CdS1- xSex alloys: Time-resolved photoluminescence and theoretical models

Luminescence decay curves as well as time-resolved and time-integrated spectra obtained with selective excitation in the localized exciton band of CdS 1- x Se x are fitted using two theoretical models. In the first one, exciton transfer to lower energy states occurs through tunnel effect assisted by acoustical phonons. From the energy dependence of the lifetime measured with quasi-resonant excitation in the localized exciton band, we determine the characteristic energy of the tail density of states: E 0 = 2.2 meV. The radiative lifetime is set equal to 1.5 ns. We show that both piezoelectric and deformation potential coupling have to be taken into account to describe exciton transfer. The second model is phenomenological and was used to describe exciton transfer in GaAs 1- x P x alloys. We obtain a good agreement with experimental results using parameters close to the previous ones. We demonstrate that, for selective excitation in the low energy side of the luminescence band, not only the transfer process but also luminescence assisted by acoustical phonons must be taken into account in order to explain time-resolved luminescence results.