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.     

Effect of sulfur substitutions on optical, electrical and structural properties of Ge(SxSe1−x)2 system

Glasses in the system Ge-Se-S were prepared with different Se/S ratios in order to investigate the compositional dependence of selected physical properties. We report the results of a systematic study examining the UV-vis transmission, dc electrical conductivity and X-ray diffraction of the system Ge(S_xSe_1−x)₂ with x=0, 0.1, 0.4 and 1.0 where replacement of S by Se was made. The changes in the optical energy gap, E_g, (from 1.95 to 2.43 eV) and band tail width, E_e, (from 103 to 243 meV) behave contrarily to the change in refractive index, n, (from 2.3 to 2) with the progressive replacement of S by Se. This behavior was discussed and interpreted with the changes in cohesive energy. The analysis of defects in the prepared films was carried out by the examination of activation energies obtained from dc electrical conductivity. The analysis of the X-ray diffraction pattern revealed a remarkable reduction in the intensity of the first and second diffraction peaks with the progressive replacement of S content, which confirms a change in the intermediate range order structure: reorganization of the structural properties.     

Annealing of thin Zr films on Si1−xGex(0≤x≤1): X-ray diffraction and Raman studies

X-ray diffraction experiments have been combined with Raman scattering and transmission electron microscopy data to analyze the result of rapid thermal annealing (RTA) applied to Zr films, 16 or 80 nm thick, sputtered on Si_1−xGe_x epilayers (0≤x≤1). The C49 Zr(Si_1−xGe_x)₂ is the unique phase obtained after complete reaction. ZrSi_1−xGe_x is formed as an intermediate phase. The C49 formation temperature T_f is lowered by the addition of Ge in the structure. Above a critical Ge composition close to x=0.33, a film microstructure change was observed. Films annealed at temperatures close to T_f are continuous and relaxed. Annealing at T>T_f leads to discontinuous films: surface roughening resulting from SiGe diffusion at film grain boundaries occurred. Grains are ultimately partially embedded in a SiGe matrix. A reduction in the lattice parameters as well as a shift of Raman lines are observed as T exceeds T_f. Both Ge non-stoichiometry and residual stress have been considered as possible origins for these changes. However, as Ge segregation has never been detected, even by using very efficient techniques, it is thought that the changes originate merely from residual stress. The C49 grains are expected to be strained under the SiGe matrix effect and shift of the Raman lines would indicate the stress is compressive. Some simple evaluations of the stress values indicate that it varies between −0.3 and −3.5 GPa for 0≤x≤1 which corresponds to a strain in the range (−0.11, −1.15%). X-ray and Raman determinations are in good agreement.     

The influence of microscopic parameters on the ionic conductivity of SrBi2(Nb1−xVx)2O9−δ(0≤x≤0.3) ceramics

Layered SrBi₂(Nb_1−xV_x)₂O_9−δ (SBVN) ceramics with x lying in the range 0-0.3 (30 mol%) were fabricated by the conventional sintering technique. The microstructural studies confirmed the truncating effect of V₂O₅ on the abnormal platy growth of SBN grains. The electrical conductivity studies were centred in the 573-823 K as the Curie temperature lies in this range. The concentration of mobile charge carriers (n), the diffusion constant (D₀) and the mean free path (a) were calculated by using Rice and Roth formalism. The conductivity parameters such as ion-hopping rate (ω_p) and the charge carrier concentration (K′) term have been calculated using Almond and West formalism. The aforementioned microscopic parameters were found to be V₂O₅ content dependent on SrBi₂(Nb_1−xV_x)₂O_9−δ ceramics.     

Structure, thermally and optically induced effects in amorphous As2Se3 films prepared by pulsed laser deposition

Thin amorphous As-Se films were prepared by pulsed laser deposition (PLD) and by classical thermal evaporation techniques. Raman spectra and optical properties (optical gap, E_g^opt, index of refraction, n, third-order non-linear susceptibility, χ⁽³⁾) of prepared films and their photo- and thermally induced changes were studied. The structure of laser deposited films was close to the corresponding bulk glasses contrary to thermal evaporated films. The composition of PLD films was practically unchanged during the process of deposition. The optically and thermally induced changes of n and of E_g^opt in PLD films are different from the changes in thermally deposited films. The differences are discussed.     

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.     

Chemical spray pyrolysis deposition and characterization of p-type CuCr1−xMgxO2 transparent oxide semiconductor thin films

A chemical spray pyrolysis technique for deposition of p-type Mg-doped CuCrO₂ transparent oxide semiconductor thin films using metaloorganic precursors is described. As-deposited films contain mixed spinel CuCr₂O₄ and delafossite CuCrO₂ structural phases. Reduction in spinel CuCr₂O₄ fraction and formation of highly crystalline films with single phase delafossite CuCrO₂ structure is realized by annealing at temperatures ?700 °C in argon. A mechanism of synthesis of CuCrO₂ films involving precursor decomposition, oxidation and reaction between constituent oxides in the spray deposition process is presented. Post-annealed CuCr_0.93Mg_0.07O₂ thin films show high (?80%) visible transmittance and sharp absorption at band gap energy with direct and indirect optical band gaps 3.11 and 2.58 eV, respectively. Lower (∼450 °C) substrate temperature formed films are amorphous and yield lower direct (2.96 eV) and indirect (2.23 eV) band gaps after crystallization. Electrical conductivity of CuCr_0.93 Mg_0.07O₂ thin films ranged 0.6-1 S cm⁻¹ and hole concentration ∼2×10¹⁹ cm⁻³ determined from Seebeck analysis. Temperature dependence of conductivity exhibit activation energies ∼0.11 eV in 300-470 K and ∼0.23 eV in ?470 K region ascribed to activated conduction and grain boundary trap assisted conduction, respectively. Heterojunction diodes of the structure Au/n-(ZnO)/p-(CuCr_0.93Mg_0.07O₂)/SnO₂ (TCO) were fabricated which show potential for transparent wide band gap junction device.     

Substitutional sites of Co ions in CuGa1−xAlxSe2:Co crystals

The substitutional sites of Co²⁺ ions in Co²⁺-doped CuG_1−xAl_xSe₂ (including CuGaSe₂ where x=0 and CuAlSe₂ where x=1) semiconductors are studied by analyzing the composition x dependence of optical spectral parameters reported in the previous literature for these materials. From the studies, we suggest that Co²⁺ occupy I-group cation site rather than III-group cation site. The suggestion is discussed.     

Effective medium theory for the inhomogeneous PdMnxFe1−x alloys

Ternary PdMn_xFe_1−x alloys are known to form a microinhomogeneous random mixture of PdMn and PdFe phases. The unconventional ρ(x) dependence of dc resistivity and singularities in low frequency optical conductivity spectra of alloys are described footing within the effective medium approach. The essential point of the model proposed is the anomalous role of insulating interfaces, whose proliferation at intermediate x gives rise to the observed maximum of resistivity near x?0.8.     

Effect of indium doping in CdO thin films prepared by spray pyrolysis technique

Preparation of transparent and conducting indium doped CdO thin films by spray pyrolysis on glass substrate is reported for various concentration of indium (2-8 wt%) in the spray solution. The electrical, optical and structural properties of indium doped CdO films were investigated using different techniques such as Hall measurement, optical transmission, X-ray diffraction and scanning electron microscope. X-ray analysis shows that the undoped CdO films are preferentially orientated along (2 0 0) crystallographic direction. Increase of indium doping concentration increases the films packing density and reorient the crystallites along (1 1 1) plane. A minimum resistivity of 4.843×10⁻⁴ Ω cm and carrier concentration of 3.73×10²⁰ cm⁻³ with high transmittance in the range 300-1100 nm were achieved for 6 wt% indium doping. The band gap value increases with doping concentration and reaches a maximum of 2.72 eV for 6 wt% indium doping from 2.36 eV of that of undoped film. The minimum resistivity achieved in the present study is found to be the lowest among the reported values for In-doped CdO films prepared by spray pyrolysis method.     

Electroresistance effect in La1−xCaxMnO3 (0<x<1) ceramics

Electroresistance (ER) effects were investigated for a full series of manganite ceramics La_1−xCa_xMnO₃ (0<x<1), synthesized by solid state reaction. The results indicate that while the ER effects are large only in the presence of electrically active, high E-field boundaries, the equilibrium or metastable electronic-magnetic states in the adjoining domains are also significant, as a large ER occurs only at x=0.51 and x=0.17; those compositions are both near a two-phase coexistence region, i.e. close to a compositional regime where equilibrium insulating/metallic phase domains and interfaces would occur spontaneously.     

Low-temperature thermoelectric properties of Bi85Sb15−xNbx alloys prepared by high-press sintering

The nanocrystalline materials with the general formula Bi₈₅Sb_15−xNb_x (x=0, 0.5, 1, 2, 3) were prepared by mechanical alloying and subsequent high-pressure sintering. Their transport properties involving electrical conductivity, Seebeck coefficient and thermal conductivity have been investigated in the temperature range of 80-300 K. The absolute value of Seebeck coefficient of Bi₈₅Sb₁₃Nb₂ reaches a maximum of 161 μV/K at 105 K, which is 69% larger than that of Bi₈₅Sb₁₅ at the same temperature. The power factor and figure-of-merit are 4.45×10⁻³ WK⁻²m⁻¹ at 220 K and 1.79×10⁻³ K⁻¹ at 196 K, respectively. These results suggest that thermoelectric properties of Bi₈₅Sb₁₅ based material can be improved by Nb doping.     

Effects of uniaxial pressure and annealing on the resistivity of Ba(Fe1−xCox)2As2

Single crystals of underdoped Ba(Fe_1−xCo_x)₂As₂ were detwinned by applying uniaxial pressure. The anisotropic in-plane resistivity was measured using the Montgomery method without releasing pressure. The resistivity along the a-axis shows metallic behavior down to 5 K, while the resistivity along the b-axis shows an insulator-like behavior in some temperature range. Annealing the sample radically reduces the residual resistivity for x=0, and at the same time the anisotropy becomes much smaller at low temperatures.     

Increase of Tc in the 1:2:3 superconductors YBCO and (CaxLa1−x)(LauBa1−u)2Cu3Oy after slow cooling or low temperature annealing

The well known phenomenon of the increase of T_c of YBCO after slow cooling or low temperature annealing without change of the oxygen content, was found also for the YBCO like tetragonal superconductors of (Ca_xLa_1−x)(La_uBa_1−u)₂Cu₃O_y (this compound has been previously denoted as CLBLCO, CLBCO or CaLaBaCuO). It has been observed at 150 and 100 °C for oxygen underdoped, optimally- and overdoped ceramics. The products retain their tetragonal unit cells. The possible reasons of this phenomenon are discussed.     

Effect of Cu deficiency on the optical properties and electronic structure of CuIn1−xGaxSe2

Spectroscopic ellipsometry measurements of CuInSe₂ (CIS) and CuIn_1−xGa_xSe₂ (CIGS) over a range of Cu compositions reveal that there are important differences in electronic and optical properties between α-phase CIS/CIGS and Cu-poor CIS/CIGS. We find a reduction in the imaginary part of the dielectric function ?₂ in the spectral region, 1-3 eV. This reduction can be explained in terms of the Cu-3d density of states. An increase in band gap is found for Cu-poor CIS and CIGS due to the reduction in repulsive interaction between Cu-3d and Se-4p states. We also characterize the dielectric functions of polycrystalline thin-film α-phase CuIn_1−xGa_xSe₂ (x=0.18 and 0.36) to determine their optical properties and compare them with similar compositions of bulk polycrystalline CuIn_1−xGa_xSe₂. The experimental results have important implications for understanding the functioning of polycrystalline optoelectronic devices.     

The suppression of structural phase transformation in LaVO3 and La1−xSrxVO3 thin films fabricated by pulsed laser deposition

Highly oriented (100) thin films of LaVO₃ and La_1−xSr_xVO₃ have been fabricated by pulsed laser deposition in a reducing atmosphere. The films show a transition from insulating to metallic behaviour in the composition region of x, 0.175<x<0.200. In the single crystals of the antiferromagnetic insulating phase, a first-order structural phase transition is observed few degrees below the magnetic transition, which manifests itself as a kink in the temperature dependence of resistivity. In the highly oriented thin films of LaVO₃ and La_1−xSr_xVO₃ fabricated on lattice matched substrates in this study, the structural phase transformation in the insulating phase has been suppressed. The electrical conduction is found to take place via hopping through localized states at low temperatures. The metallic compositions show a non-linear (T^1.5) behaviour in the temperature dependence of resistivity. V (2p) core level spectra of these films show a gradual change in the relative intensities of V³⁺ and V⁴⁺ ions as the value of x increases.     

Raman spectra of Ba6−3xSm8+2xTi18O54 solid solution

Raman spectra of Ba_6−3xSm_8+2xTi₁₈O₅₄ solid solution were investigated as the function of x and sintering time. Reasonable explanations were provided about the Raman shifts and their intensities at 1013, 590, 751, 280, 232 cm⁻¹. 1013 cm⁻¹ demonstrates the existence of BaCO₃ phase in solid solution, 590 cm⁻¹ is the symmetric stretching mode of the basal oxygens of the octahedral; 280 and 232 cm⁻¹ are the symmetric stretching modes resulted from the tilt of octahedral when large cation sites are Sm³⁺ and Ba²⁺. The shoulder peak appearing around 302 cm⁻¹ is related to the vacancy produced by the unequal valence of Sm³⁺ and Ba²⁺.     

Temperature-dependent Fe Mössbauer spectroscopy and local structure of the mullite-type Bi2(FexGa1−x)4O9 (0.1≤x≤1) solid solution

The Bi₂(Fe_xGa_1−x)₄O₉ oxide solid solution possessing a mullite-type structure has been investigated by ⁵⁷Fe Mössbauer spectroscopy in dependence of composition (0.1≤x≤1) and temperature (293≤T/K≤1073). The spectra have been fitted with two doublets for tetrahedrally and octahedrally coordinated high-spin Fe³⁺ ions, respectively. The experimental areas of the subspectra were used to determine the distribution of iron on the two inequivalent structural sites. The fraction of iron cations occupying the octahedral site is found to increase with decreasing Fe content and the cation distribution is almost independent of temperature. The unusual temperature dependence of the quadrupolar splitting, QS, observed for the octahedral site with dQS/dT>0 is discussed in connexion with structural data for Bi₂Fe₄O₉. The temperature dependence of Mössbauer isomer shifts and signal intensities is examined in the context of local vibrational properties of iron on the two inequivalent sites of the mullite-type lattice structure.     

Magnetic, electric, and optical functionalities of (PLZT)x(BiFeO3)1−x ferroelectric-ferromagnetic thin films

We have prepared a series of (PLZT)_x(BiFeO₃)_1−x transparent thin films with thickness of 300 nm by a thermal pyrolysis method. Only films with x≦0.10 formed a single phase of perovskite structure. The film where x=0.10 exhibited both ferromagnetic and ferroelectric properties at room temperature with spontaneous magnetization and coercive magnetic fields of 0.0027μ_B and 5500 G, respectively. The remanent electric polarization and coercive electric field for the film where x=0.10 were 3.0 μC/cm² and 24 kV/cm, respectively. Additionally, films with 0.02≦x≦0.10 showed both magneto-optical effects and the second harmonic generation of transmitted light.