Effect of composition and thermal annealing on the absorption and electrical conduction of Se85−xTe15Sbx glasses

The optical absorption of the as-prepared and thermally annealed Se_85−xTe₁₅Sb_x (0≤x≤9) thin films was measured. The mechanism of the optical absorption follows the rule of non-direct transition. The optical energy gap (E₀) decreased from 1.12 to 0.84 eV with increasing Sb content of the as-prepared films from 0 to 9 at.%. The as-prepared Se₇₆Te₁₅Sb₉ films showed an increase in (E₀) with increasing the temperature of annealing in the range above T_g (363 K). The electrical conductivity of the as-prepared and annealed films was found to be of Arrhenius type with temperature in the range 300-360 K. The activation energy for conduction was found to decrease with increasing both the Sb content and temperature of annealing. The results were discussed on the basis of the lone-pair electron effect and of amorphous crystalline transformation.     

Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films

Ion beam sputtering process was used to deposit n-type fine-grained Bi₂Te₃ thin films on BK7 glass substrates at room temperature. In order to enhance the thermoelectric properties, thin films are annealed at the temperatures ranging from 100 to 400 °C. X-ray diffraction (XRD) shows that the films have preferred orientations in the c-axis direction. It is confirmed that grain growth and crystallization along the c-axis are enhanced as the annealing temperature increased. However, broad impurity peaks related to some oxygen traces increase when the annealing temperature reached 400 °C. Thermoelectric properties of Bi₂Te₃ thin films were investigated at room temperature. The Bi₂Te₃ thin films, including as-deposited, exhibit the Seebeck coefficients of −90 to −168 μV K⁻¹ and the electrical conductivities of 3.92×10²-7.20×10² S cm⁻¹ after annealing. The Bi₂Te₃ film with a maximum power factor of 1.10×10⁻³ Wm⁻¹ K⁻² is achieved when annealed at 300 °C. As a result, both structural and transport properties have been found to be strongly affected by annealing treatment. It was considered that the annealing conditions reduce the number of potential scattering sites at grain boundaries and defects, thus improving the thermoelectric properties.     

Structural, electrical and optical properties of AgInS2 thin films grown by thermal evaporation method

Structural electrical and optical properties of AgInS₂ (AIS) thin films grown by the single-source thermal evaporation method were studied. The X-ray diffraction spectra indicated that the AIS single phase was successful grown by annealing above 400 °C in air. The AIS grain sizes became large with increasing the annealing temperatures. All polycrystalline AIS thin films were sulfur-poor from the electron probe microanalysis and indicated n-type conduction by the Van der Pauw technique. It was deduced that the sulfur vacancies were dominant in the films and enhanced n-type conduction.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

The effect of the Bi source on optical properties of Bi2Te3 nanostructures

nanostructures were synthesized by using different Bi sources via a simple solvothermal process, in which and BiCl₃ were used as the Bi sources. Optical properties of nanostructures prepared with and BiCl₃ as the Bi sources were investigated by micro-Raman spectroscopy. The Raman scattering spectrum of hexagonal nanoplates prepared by using as the Bi source shows that the infrared (IR) active mode A_1u, which must be odd parity and is Raman forbidden for bulk crystal due to its inversion symmetry, is greatly activated and shows up clearly in the Raman scattering spectrum. We attribute the appearance of the infrared active A_1u mode in the Raman spectrum to crystal symmetry breaking of hexagonal nanoplates. However, the Raman scattering spectrum of nanostructures with irregular shape prepared by using as the Bi source only exhibits the two characteristic Raman modes of crystals. Micro-Raman measurements on nanostructures with different morphologies offer us a potential way to tailor optical properties of nanostructures by controlling the morphologies of the nanostructures, which is very important for practical applications of nanostructures in thermoelectric devices.     

The role of As2O3 on the stability and some physical properties of PbO-Sb2O3 glasses

PbO-Sb₂O₃ glasses added with different concentrations of As₂O₃ (10-55 mol%) were prepared to understand their IR spectra, elastic properties (Young's modulus E, Shear modulus G, microhardness H), optical absorption and dielectric properties (constant ε, loss tan δ, ac conductivity σ_ac over a moderately wide range of frequency and temperature and breakdown strength in air medium at room temperature). Results have indicated that the structure of the PbO-Sb₂O₃-As₂O₃ glass is more rigid when the concentration of As₂O₃ is around 40 mol%.     

Structural,thermal and electrical properties of the semiconductor system Ag(1−x)CuxInSe2

This paper presents a study of bulk samples synthesized of the Ag_1−xCu_xInSe₂ semiconductor system. Structural, thermal and electrical properties, as a function of the nominal composition (Cu content) x=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 were studied. The influence of x on parameters such as melting temperature, solid phase transition temperature, lattice parameters, bond lengths, crystallite size t (coherent domain), electrical resistivity, electrical mobility and majority carrier concentration was analyzed. The electrical parameters are analyzed at room temperature. In general, it is observed that the properties of the Ag_1−xCu_xInSe₂ system for x≤0.4 are dominated by n-AgInSe₂, while for x>0.4, these are in the domain of p-CuInSe₂. The crystallite size t in the whole composition range (x) is of the order of the nanoparticles. Secondary phases (CuSe, Ag₂Se and InSe) in small proportion were identified by XRD and DTA.     

Preparation and properties of spray-deposited ZnIn2Se4 nanocrystalline thin films

Zinc indium selenide (ZnIn₂Se₄) thin films have been deposited onto amorphous and fluorine doped tin oxide (FTO)-coated glass substrates using a spray pyrolysis technique. Aqueous solution containing precursors of Zn, In, and Se has been used to obtain good quality deposits at different substrate temperatures. The preparative parameters such as substrate temperature and concentration of precursors solution have been optimized by photoelectrochemical technique and are found to be 325 °C and 0.025 M, respectively. The X-ray diffraction patterns show that the films are nanocrystalline with rhombohedral crystal structure having lattice parameter a=4.05 Å. The scanning electron microscopy (SEM) studies reveal the compact morphology with large number of single crystals on the surface. From optical absorption data the indirect band gap energy of ZnIn₂Se₄ thin film is found to be 1.41 eV.     

Glass formation in the As2Se3-As2Te3-Sb2Te3 system

Chalcogenide glasses from the As₂Se₃-As₂Te₃-Sb₂Te₃ system were synthesized for the first time. The glass-forming region was determined by X-ray diffraction and electron microscopic analyses.The basic physicochemical parameters such as density (d), microhardness (HV) and temperatures of phase transformations (glass transition T_g, crystallization T_cr and melting T_m) were measured. Compactness and some thermomechanical characteristics such as volume (V_h) and formation energy (E_h) of micro-voids in the glassy network as well as the elasticity module (E) were calculated. The glass-forming ability was evaluated according to Hruby's criteria (K_G). The correlation between composition and properties of the (As₂Se₃)_x(As₂Te₃)_y(Sb₂Te₃)_z glasses was established and comprehensively discussed.     

Exciton spectra, valence band splitting, and energy band structure of CuGaXIn1−XS2 and CuGaXIn1−XSe2 crystals

Exciton spectra are studied in CuGa_XIn_1−XS₂ solid solutions by means of photoreflectivity and wavelength modulation spectroscopy at liquid nitrogen temperature. The exciton parameters, dielectric constants, and free carrier effective masses are deduced from experimental spectra by calculations in the framework of a model taking into account the spatial dispersion and the presence of a dead-layer. The crystal field and spin orbit valence band splitting is calculated as a function of X taking into account the energy position of excitonic lines. The energy band structure of CuGa_XIn_1−XS₂ and CuGa_XIn_1−XSe₂ compounds is derived from optical spectra at photon energies higher than the fundamental band gap. The energies of optical transitions are tabulated for X values from 0 to 1.     

Optical, electrical and photovoltaic properties of thermally annealed PPHT:DDE blend thin films

We have investigated the optical, electrical and photovoltaic properties of devices based on 1,2-diazoamino diphenyl ethane (DDE) and poly(3-phenyl hydrazone thiophene) (PPHT):DDE blend. It is observed from the J-V characteristics of the Al/DDE/ITO (ITO—indium tin oxide) device that the electron current injected from Al contact was shown to be space charge limited (SCL), indicating that Al forms nearly ohmic contact for electron injection into lowest unoccupied molecular orbital (LUMO) of DDE. The effect of thermal annealing and composition, on the optical, electrical and photovoltaic response of blend of PPHT and DDE sandwiched between a transparent ITO electrode and an Al back contact are investigated. The observed absorption quenching in the PPHT:DDE blend is attributed to the disordering of PPHT chains and charge transfer between PPHT and DDE as evidenced by FTIR spectra. The observed red shift in the absorption peak on thermal annealing is due to the improvement in the ordering and increases in conjugation length in PPHT. The observed dark current-voltage curves agree well with trap-controlled SCL transport theory. The photophysics of the blend material and influence of thermal annealing on the performance and morphology of these devices were discussed. Annealing process results in the formation of PPHT:DDE complex and increase in the ordering of polymer chain, that increases the incident photon to current efficiency (IPCE) and power conversion efficiency of the photovoltaic devices.     

Effect of Mn-doping on the electrical properties of Cu2GeSe3

In this work we report the temperature dependence of the resistivity ρ of p-Cu₂GeSe₃ and manganese-doped p-Cu₂GeSe₃ at low temperature. It was found that for a intrinsic sample ρ obeys the Shklovskii-Efros-type variable-range hopping resistivity law in the temperature range from 4 to 63 K. This behaviour is governed by generation of a Coulomb gap Δ=78 meV in the density of localized states. We find a low activation term T₀=0.24 K, which is an indication of a large localization length ξ. For Mn-doped sample a metal-insulator transition (MIT) is observed at T=65 K. On the basis of the Mott criterion for metal-insulator transition, the critical carrier density n_c is determined. From the analysis of resistivity data it is concluded that Mn acts as acceptor impurity.     

Third-order nonlinear optical properties of Bi2S3 and Sb2S3 nanorods studied by the Z-scan technique

Bismuth sulfide (Bi₂S₃) and antimony sulfide (Sb₂S₃) nanorods were synthesized by hydrothermal method. The products were characterized by UV-vis spectrophotometer, X-ray powder diffraction (XRD) and transmission electron microscope (TEM). Bi₂S₃ and Sb₂S₃ nanorods were measured by Z-scan technique to investigate the third-order nonlinear optical (NLO) properties. The result of NLO measurements shows that the Bi₂S₃ and Sb₂S₃ nanorods have the behaviors of the third-order NLO properties of both NLO absorption and NLO refraction with self-focusing effects. The third-order NLO coefficient χ⁽³⁾ of the Bi₂S₃ and Sb₂S₃ nanorods are 6.25×10⁻¹¹ esu and 4.55×10⁻¹¹ esu, respectively. The Sb₂S₃ and Bi₂S₃ nanorods with large third-order NLO coefficient are promising materials for applications in optical devices.     

Preparation of Cu2ZnSnS4 thin films by hybrid sputtering

In order to fabricate Cu₂ZnSnS₄ thin films, hybrid sputtering system with two sputter sources and two effusion cells is used. The Cu₂ZnSnS₄ films are fabricated by the sequential deposition of metal elements and annealing in S flux, varying the substrate temperature. The Cu₂ZnSnS₄ films with stoichiometric composition are obtained at the substrate temperature up to 400 °C, whereas the film composition becomes quite Zn-pool at the substrate temperature above 450 °C. The Cu₂ZnSnS₄ film shows p-type conductivity, and the optical absorption coefficient and the band gap of the Cu₂ZnSnS₄ film prepared in this experiment are suitable for fabricating a thin film solar cell.     

Effect of ZnO/CdO on the structure and electrical conductivity in Li2O·MO·Bi2O3·B2O3 glasses (M=Zn, Cd)

Studies of structural and electrical properties have been carried out on a number of glasses with wide ranging compositions in the glass systems Li₂O·MO·Bi₂O₃·B₂O₃ (where M=Zn or Cd), in order to understand the effect of transition metal (TM) ions on the structure of these glasses. The density and molar volume measurements have also been made to understand the structural changes occurring in these glasses. The dc conductivity measured in the temperature range 423-623 K obeys Arrhenius law. It increases with increase in Li₂O/MO ratio. The results of infrared spectra indicate that TM ions (Zn²⁺ or Cd²⁺) behave as network former in the present system. Boron exists in both tri- and tetra-hedral units in these glasses and no boroxol ring formation takes place in the glass structure. Values of theoretical optical basicity have also been reported.     

Electronic structure of a thermoelectric material: CsBi4Te6

We have calculated the electronic structure of CsBi₄Te₆ by means of first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital method. From our calculated electronic structure we have calculated the frequency dependent dielectric function. Our calculations shows that CsBi₄Te₆ a semiconductor with a band gap of 0.3 eV. The calculated dielectric function is very anisotropic. Our calculated density of state support the recent experiment of Chung et al. [Science 287 (2000) 1024] that CsBi₄Te₆ is a high performance thermoelectric material for low temperature applications.     

Effect of substrate and annealing on the structural and optical properties of ZnO:Al films

ZnO:Al thin films with c-axis preferred orientation were deposited on glass and Si substrates using RF magnetron sputtering technique. The effect of substrate on the structural and optical properties of ZnO:Al films were investigated. The results showed a strong blue peak from glass-substrate ZnO:Al film whose intensity became weak when deposited on Si substrate. However, the full width at half maxima (FWHM) of the Si-substrate ZnO:Al (0 0 2) peaks decreased evidently and the grain size increased. Finally, we discussed the influence of annealing temperature on the structural and optical properties of Si-substrate ZnO:Al films. After annealing, the crystal quality of Si-substrate ZnO:Al thin films was markedly improved and the intensity of blue peak (∼445 nm) increased noticeably. This observation may indicate that the visible emission properties of the ZnO:Al films are dependent more on the film crystallinity than on the film stoichiometry.     

Structural and optical properties of In35Sb45Se20−xTex phase-change thin films

Physical properties of In₃₅Sb₄₅Se_20−xTe_x thin films with different compositions (x=2.5, 5, 7.5, 10, 12.5 and 15 at %) prepared by electron beam evaporation method are studied. X-ray diffraction results indicate that the as-evaporated films depend on the Te content and the crystallized compounds consist mainly of Sb₂Se₃ with small amount of Sb₂SeTe₂. Transmittance and reflectance of the films are found to be thickness dependent. Optical-absorption data indicate that the absorption mechanism is direct transition. Optical band gap values decrease with increase in Te content as well as with increase in film thickness.     

Epitaxial Cu(In,Ga)S2 thin film solar cells

Epitaxial layers of the quaternary compound Cu(In,Ga)S₂ and the ternary compound CuInS₂ were grown on Si(111) substrates via Molecular Beam Epitaxy. The layers were investigated for their morphological and structural properties using Rutherford backscattering spectroscopy, atomic force microscopy, reflection high-energy electron diffraction and X-Ray diffraction. Furthermore, complete solar cell devices were processed from these layers and their photovoltaic properties were investigated by means of I(U)-curves under illumination. Thus, efficiencies up to η=3.2% were achieved. The comparatively low performance of the solar cell devices is attributed to certain heterogeneities of the samples as a result of the growth process.     

Thermally stimulated current measurements in undoped Ga3InSe4 single crystals

The trap levels in nominally undoped Ga₃InSe₄ crystals were investigated in the temperature range of 10-300 K using the thermally stimulated currents technique. The study of trap levels was accomplished by the measurements of current flowing along the c-axis of the crystal. During the experiments we utilized a constant heating rate of 0.8 K/s. Experimental evidence is found for one hole trapping center in the crystal with activation energy of 62 meV. The analysis of the experimental TSC curve gave reasonable results under the model that assumes slow retrapping. The capture cross-section of the trap was determined as 1.0×10⁻²⁵ cm² with concentration of 1.4×10¹⁷ cm⁻³.