Characterization of PbSe1−xTex thin films

The lead salts and their alloys are extremely interesting semiconductors due to their technological importance. The fabrication of devices with alloys of these compounds possessing detecting and lasing capabilities has been an important recent technological development. The high quality polycrystalline thin films of PbSe_1−xTe_x with variable composition (0≤x≤1) have been deposited onto ultra clean glass substrates by vacuum evaporation technique. As deposited films were annealed in vacuum at 350 K. The optical, electrical and structural properties of PbSe_1−xTe_x thin films have been examined. The optical constants (absorption coefficient and bandgap) of the films were determined by absorbance measurements in the wavelength range 2500-5000 nm using Fourier transform infrared spectrophotometer. The dc conductivity and activation energy of the films were measured in the temperature range 300-380 K. The X-ray diffraction patterns were used to determine the sample quality, crystal structure and lattice parameter of the films.     

Characterization of Cu(In,Ga)Se2 thin films prepared by evaporationfrom ternary compounds

Thin films of Cu(In,Ga)Se₂ were fabricated by evaporation from ternary CuGaSe₂ and CuInSe₂ compounds for photovoltaic device applications and their properties were investigated. From XRF analysis, the Cu:(In+Ga):Se atomic ratio in all thin films was approximately 1:1:2. The Ga/(In+Ga) atomic ratio in the thin films changed linearly from 0 to 1.0 with increasing the [CGS]/([CGS]+[CIS]) mole ratio in the evaporating materials. However, for thin films prepared at the [CGS]/([CGS]+[CIS]) mole ratio above 0.4, the composition by EPMA analysis was not consistent with that by XRF analysis. The result of EPMA analysis showed that the surface of a thin film was Cu-rich. XRD studies demonstrated that the thin films prepared at the [CGS]/([CGS]+[CIS]) mole ratio under 0.2 had a chalcopyrite Cu(In,Ga)Se₂ structure and the preferred orientation to the 112 plane. On the other hand, XRD patterns of the thin films produced at the [CGS]/([CGS]+[CIS]) mole ratio above 0.6 showed the diffraction lines from a chalcopyrite Cu(In,Ga)Se₂ and a foreign phase. The separation of a peak was observed near 2θ=27°, indicative the graded Ga concentration in Cu(In,Ga)Se₂ thin film.     

Effect of sulfur additive on density of localized states in nanostructures chalcogenide Se95−xSxZn5 thin films

This paper reports the measurement of space charge limited conduction (SCLC) on the fabricated thin films of Se_95−xS_xZn₅ (0.2≤x≤10) in temperature range 313–353 K for the first time. At high electric fields (E∼10⁴ V/cm), the current could be fitted into the theory of space charge limited conduction, in case of uniform distribution of localized states in mobility gap. The homogeneity and surface morphology of thin films were assessed by scanning electron microscopy. The crystalline nature of the thin films was confirmed by powder XRD and the crystallite size was calculated using Scherer's formula. The crystallite size and density of localized states were found to increase with the increase of sulfur concentration. DC conductivity and activation energy were calculated and found to decrease and increase respectively, with the increase of sulfur concentration.     

Preparation and characterization of electrodeposited Sb2Se3 thin films from non-aqueous media

Semiconducting Sb₂Se₃ thin films have been prepared onto the stainless steel and fluorine doped tin oxide coated glass substrates from non-aqueous media using an electrodeposition technique. The electrodeposition potentials for different bath compositions and concentrations of solution have been estimated from the polarization curves. SbCl₃ and SeO₂ in the volumetric proportion as 1:1 with their equimolar solution concentration of 0.05 M form good quality films. The films are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and optical absorption techniques. The SEM studies show that the film covers the total substrate surface with uneven surface morphology. The XRD patterns of the films obtained by varying compositions and concentrations show that the as-deposited films are polycrystalline with relatively higher grain size for 1:1 composition and 0.05 M concentration. The optical band gap energy for indirect transition in Sb₂Se₃ thin films is found to be 1.195 eV.     

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.     

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.     

Photoluminescence of polycrystalline CuIn0.5Ga0.5Te2 thin films grown by flash evaporation

Polycrystalline CuIn_0.5Ga_0.5Te₂ films were deposited by flash evaporation from ingot prepared by reacting, in stoichiometric proportions, high purity Cu, In, Ga and Te elements in vacuum sealed quartz. The as-obtained films were characterized by X – ray diffraction (XRD), transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDS). XRD and TEM results showed that the layer has a chalcopyrite-type structure, predominantly oriented along (112) planes, with lattice parameters a?=?0.61?nm and c?=?1.22?nm. The optical properties in the near - infrared and visible range 600–2400?nm have been studied. The analysis of absorption coefficient yielded an energy gap value of 1.27?eV. Photoluminescence analysis of as-grown sample shows two main emission peaks located at 0.87 and 1.19?eV at 4?K.     

Photoelectrochemical properties of electrochemically deposited CdIn2S4 thin films

Thin films of CdIn₂S₄ have been deposited on to stainless steel and fluorine-doped tin oxide (FTO)-coated glass substrates from aqueous acidic bath using an electrodeposition technique. Ethylene diamine tetra-acetic acid (EDTA) disodium salt is used as complexing agent to obtain good-quality deposits by controlling the rate of the reaction. The different preparative parameters like concentration of bath, deposition time, bath temperature, pH of the bath have been optimized by the photoelectrochemical (PEC) technique in order to get good-quality photosensitive material. Different techniques have been used to characterize CdIn₂S₄ thin films. Optical absorption shows the presence of direct transition with band gap energy 2.17 eV. The X-ray diffraction (XRD) analysis of the as-deposited and annealed films showed the presence of polycrystalline nature. Energy-dispersive analysis by X-ray (EDAX) study for the sample deposited at optimized preparative parameters shows that the In-to-Cd ratio is almost 2 and S-to-Cd ratio is almost 4. Scanning electron microscopy (SEM) for samples deposited at optimized preparative parameters reveals that spherical grains are uniformly distributed over the surface of the substrate indicates the well-defined growth of polycrystalline CdIn₂S₄ thin film. PEC characterization of the films is carried out by studying photoresponse, spectral response and photovoltaic output characteristics. The fill factor (ff) and power conversion efficiency (η) of the cell are 69 and 2.94%, respectively.     

Orientation-dependent dielectric properties of Ba(Sn0.15Ti0.85)O3 thin films prepared by sol-gel method

The orientation-dependent dielectric properties of barium stannate titanate (Ba(Sn_0.15Ti_0.85)O₃, BTS) thin films grown on (1 0 0) LaAlO₃ single-crystal substrates through sol-gel process were investigated. The nonlinear dielectric properties of the BTS films were measured using an inter-digital capacitor (IDC). The results show that the in-plane dielectric properties of BTS films exhibited a strong sensitivity to orientation. The upward shift of Curie temperature (T_c) of the highly (1 0 0)-oriented BTS thin films is believed to be attributing to a tensile stress along the in-plane direction inside the film. A high tunability of 47.03% was obtained for the highly (1 0 0)-oriented BTS films, which is about three times larger than that of the BTS films with random orientation, measured at a frequency of 1 MHz and an applied electric field of 80 kV/cm. This work clearly reveals the highly promising potential of BTS films for application in tunable microwave devices.     

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.     

Synthesis and materials properties of transparent conducting In2O3 films prepared by sol-gel-spin coating technique

Transparent conducting indium oxide (In₂O₃) thin films have been prepared on glass substrates by the simple sol-gel-spin coating technique. These films have been characterized by X-ray diffraction, resistivity and Hall effect measurements, optical transmission, scanning electron microscopy and atomic force microscopy for their structural, electrical, optical and morphological properties. The influence of spin parameters, number of coating, process temperature on the quality of In₂O₃ films are studied. In the operating range of deposition, 400-475 °C, all the films showed predominant (2 2 2) orientation. Films deposited at optimum process conditions exhibited a resistivity of 2×10⁻² Ω cm along with the average transmittance of about 80% in the visible spectral range (400-700 nm).     

Effect of Ga incorporation on valence band splitting of OVC CuIn3Se5 thin films

Polycrystalline thin films of CuIn_2.95Ga_0.05Se₅ produced by the incorporation of Ga into the ordered vacancy compound CuIn₃Se₅ by a two-stage vacuum evaporation process were structurally, compositionally and optically characterized using X-ray diffraction, energy dispersive analysis of X-rays and optical absorbance measurements. From the X-ray diffraction data of the films, the structural parameters like lattice constants, tetragonal deformation, bond lengths and anion displacement were evaluated and their effect on the optical behavior of films was discussed. The Hopfields quasi-cubic model adapted for chalcopyrites with tetragonal deformation was used to elucidate the crystal field and spin orbital splitting parameters in the uppermost valence band of the compound, using the three energy gaps 1.649, 1.718 and 1.92 eV corresponding to the threefold absorption in the fundamental absorption region of the optical spectra of these films. The percentage contributions of Se p and Cu d orbitals to p-d hybridization in this compound were calculated using linear hybridization of orbitals model and the effects of p-d hybridization on the band gaps were studied.     

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.     

Characterization of interface between CuInSe2 and In2O3

CuInSe₂/In₂O₃ structures were formed by depositing CuInSe₂ films by stepwise flash evaporation onto In₂O₃ films, which were grown by DC reactive sputtering of In target in presence of (Ar+O₂) gas mixture. Phase purity of the CuInSe₂ and In₂O₃ films was confirmed by Transmission Electron Microscopy (TEM) studies. X-ray diffraction (XRD) results on CuInSe₂/In₂O₃/glass structures showed sharp peaks corresponding to (112) plane of CuInSe₂ and (222) plane of In₂O_3. Rutherford Backscattering Spectrometry (RBS) investigations were carried out on CuInSe₂/In₂O₃/Si structures in order to characterize the interface between In₂O₃ and CuInSe₂. The results show that the CuInSe₂ films were near stoichoimetric and In₂O₃ films had oxygen deficient composition. CuInSe₂/In₂O₃ interface was found to include a ∼20 nm thick region consisting of copper, indium and oxygen. Also, the In₂O₃/Si interface showed the formation of ∼20 nm thick region consisting of silicon, indium and oxygen. The results are explained on the basis of diffusion/reaction taking place at the respective interfaces.     

Characterization of SnO2 obtained from the thermal oxidation of vacuum evaporated Sn thin films

Tin oxide has been prepared by thermal oxidation of evaporated tin thin films onto pyrex glass substrates. Films oxidation was achieved in air at a temperature of 600 °C with varied duration from 20min to 3 h. Structural, optical and electrical properties of the films were characterized by means of X-ray diffraction, UV–vis spectroscopy and electrical resistivity measurements respectively. The X-ray analysis revealed the transformation of Sn into SnO₂ with preferential orientation along (101) plans. No intermediate phases such as SnO and Sn₃O₄ were evidenced. It was also found that the SnO₂ crystallites orientation changed with the annealing time due to the strain energy effect. Both band gap energy and electrical resistivity decrease with annealing time due to the crystalline quality improvement and films densification. We have noticed that oxidation at 600 °C for 3 h leads to transparent and conductive films with suitable properties for photovoltaic applications.     

Preparation of CuInGeSe4 thin films by selenization method using the Cu-In-Ge evaporated layer precursors

CuInGeSe₄ quaternary compounds are known to have a chalcopyrite-like structure and have band gaps of about 1.3 eV, suitable for optimum conversion efficiency for solar cells. We have prepared the CuInGeSe₄ thin films by the selenization method using the Cu-In-Ge evaporated layer precursors. The analyses of X-ray diffraction show that the single phase of CuInGeSe₄ is obtained by the selenization of precursors at 450-500 °C. The SEM observation of film surface shows that the grain sizes are in the order of 1-2 μm. The band gaps of selenized films close to 1.6 eV are wider than that of bulk crystals (about 1.3 eV). These films have p-type conduction and higher electrical resistivities than more 10⁵ Ω cm at room temperature.     

Ag/TiO2 sol prepared by a sol–gel method and its photocatalytic activity

Ag/TiO₂ sol with narrow particle size distribution was synthesized using TiCl₄ as the starting material. TiCl₄ was converted to Ti(OH)₄ gel. The Ag/TiO₂ sol was prepared by a process where H₂O₂ was added and then heated at 90–97 °C. After condensation reaction and crystallization, a transparent sol with suspended Ag/TiO₂ was formed. Ag/TiO₂ was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, contact angle analysis, and X-ray photoelectron spectroscopy. The photocatalytic properties of Ag/TiO₂ film were evaluated by degradation of methylene blue in aqueous solution under UV light irradiation. The suspended Ag/TiO₂ particles were rhombus primary particles with the major axis ca. 40 nm and the minor axis ca. 10 nm. Ag nanoparticles were well dispersed on TiO₂ and the particle size was only 1–2 nm. Ag could restrain the recombination of photo-generated electrons and holes effectively. Transparent thin films could be obtained through dip-coating glass substrate in the sol. The thin film had strong hydrophilicity after being illuminated by UV light. Ag/TiO₂ film showed a significant increase in photocatalytic activity compared to the TiO₂ film. The high amount of surface hydroxyls on Ag/TiO₂ film also played an important role in its photocatalytic activity.     

Mn1−xFexIn2Se4: a new layered semiconductor system

Mn_1−xFe_xIn₂Se₄ compounds (x=0.1; x=0.7) were grown by the chemical vapor transport method. X-ray diffraction analysis data show that these compositions crystallize as different polytypes that belong to the hexagonal structure. The crystal symmetry of the sample with x=0.1 belongs to the space group Rm and for the sample with x=0.7 the space group is P6₃mc.The magnetic behavior of both samples has been investigated in the temperature range between 5 and 300 K. Spin-glass-like behavior below the freezing temperature T_f=9 K has been found for the sample with x=0.7. The sample with Fe content x=0.1 behaves as a paramagnet down to the lowest experimental measured temperature. High-temperature susceptibility data follow the Curie-Weiss law with a negative paramagnetic temperature indicating predominant antiferromagnetic interactions.Optical studies reveal that both samples (x=0.1; 0.7) are direct band gap semiconductors. The temperature dependence of the energy gap fits Varshni relation quite well.     

Structural and optical properties of CuIn1−xAlxSe2 thin films prepared by four-source elemental evaporation

CuIn_1−xAl_xSe₂ (CIASe) thin films with x=0.25, 0.5 and 0.65 were prepared by four-source elemental evaporation. The structural and optical properties were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive analysis, and optical transmission. The results showed that these films contain chalcopyrite structure with preferred orientation along (112) direction. The morphology, grain distribution and composition of CIASe films were studied and compared for different Al content. The optical studies revealed that the films were highly absorbing and the energy band gap calculated from transmission spectra for x=0.25, 0.5 and 0.65 were 1.2, 1.51 and 1.73 eV, respectively. The variation of Al content in the CIASe composition offered a very effective change in the optical band gap.     

Properties of CuIn(SxSe1−x)2 polycrystalline thin films prepared by chemical spray pyrolysis

CuIn(S_xSe_1−x)₂ thin polycrystalline films were grown by the chemical spray pyrolysis method on the glass substrate at 280-400°C. The alloy composition in the film was studied with relation to that in the splay solution. Films were characterized by X-ray diffraction, optical absorption, Raman spectroscopy, resistivity and surface morphology. The CuInSe₂-rich alloy films grown at high substrate temperature had chalcopyrite structure, while, the CuInS₂-rich films grown at low substrate temperature exhibited sphalerite structure. Optical-gap energies were smaller than that of the bulk crystal by 0.1-0.2 eV for CuInS₂-rich films. Raman spectra exhibited both CuInSe₂-like and CuInS₂-like A₁ modes, and their relative changed systematically with alloy composition.