Optical, electrical and thermal studies on (As2Se3)3−x(As2Te3)x glasses

Optical properties and conductivity of glassy (As₂Se₃)_3−x(As₂Te₃)_x were studied for 0 ≤ x ≤ 3. The films of the above mentioned compound were prepared by thermal evaporation with thickness of about 250 nm. The optical-absorption edge is described and calculated using the non-direct transition model and the optical band gap is found to be in the range of 0.92 to 1.84 eV. While, the width of the band gap tail exhibits opposite behaviour and is found to be in the range of 0.157 to 0.061 eV, this behaviour is believed to be associated with cohesive energy and average coordination number. The conductivity measurement on the thin films is reported in the temperature range from 280 to 190 K. The conduction that occurs in this low-temperature range is due to variable range hopping in the band tails of localized states, which is in reasonable agreement with Mott's condition of variable range hopping conduction. Some parameters such as coordination number, molar volume and theoretical glass transition temperature were calculated and discussed in the light of the topological bonding structure.     

Conduction à l'obscurité dans les couches minces de CdTe,type N

In order to determine the trap levels in the band gap of n-type CdTe thin films, we have studied the dark conduction in high electric fields. Our study enables us to propose values for the density of a uniform distribution of traps located in the band gap, η_t ≈ 5 × 10¹³ eV^-1, and for the position of the Fermi level, E_c - E_F0 ≈ 0,666 eV. We have also determined the width and the position of this distribution in the band gap to be ΔE = E₂ - E₁ ≈ 0,107 eV and E_c - E₁ ≈ 0,5 eV respectively. These values enable us to construct a possible model for the band gap.     

The optical properties of AgInTe2 thin films

The optical absorption in electron-beam-evaporated AgInTe₂ thin films was studied in the energy range 0.5–2 eV. AgInTe₂ was found to be a direct gap semiconductor with a room temperature gap of 1.03±0.01 eV. Another direct transition observed at 1.04±0.01 eV was ascribed to an optical transition from the crystal-field-split valence band to the conduction band minimum. A third direct allowed transition from the spin-orbit-split valence band to the conduction band was identified at 1.77±0.03 eV. An estimate of the p-d hybridization of the uppermost valence bands yields a value of about 15%.     

Optical,DC and AC electrical investigations of 4-hydroxy coumarin molecule as an organic Schottky diode

The current–voltage (I–V) and optical characteristics of 4-hydroxy coumarin Schottky diode were investigated. The conventional methods related with device were used to extract the various diode parameters. From dielectric study low dielectric constant and loss was observed. From its optical study, an indirect allowed transition is shown by this compound. The optical band gap (E _g ) was found to be around 3.78 eV. The observed properties shown by this molecule give a bright opportunity to explore its application for different organic devices.     

Optical properties of praseodymium oxide films

The refractive index n_f, absorption index k_f and absorption coefficient α of vacuum-deposited films of praseodymium oxide prepared under different conditions have been evaluated from the transmittance T data at different wavelengths in the visible region. The optical energy gap was estimated to be approximately 3.40 eV.     

Growth and optical properties of ZnIn2Se4 films

Thin films of ZnIn₂Se₄ were deposited on quartz substrates at 297 K by the conventional thermal evaporation technique. The as-deposited films were amorphous. On annealing at 623 K under vacuum for 3 h, the films crystallized with a preferred (1 1 2) orientation corresponding to the chalcopyrite-type structure. Films deposited on a quartz substrate heated to 573 K were also crystalline. The optical constants were computed from the measured transmittance and reflectance at normal incidence of light in the wavelength range 400 to 2000 nm. The analysis of the data gave a direct gap of 2.2 and 2.06 eV for the amorphous and crystallized films, respectively. The dispersion curve exhibited a peak above the absorption edge. An indirect gap of 1.8 eV for the crystallized films and a direct forbidden gap of 1.75 eV for the amorphous films were also deduced. A direct allowed transition with a gap of 2.065 eV and an indirect transition with a gap of 1.69 eV were deduced for the crystalline films deposited on the heated substrate.     

A Preliminary Analysis of the Superconductivity of Ba1−x K x BiO by the Generalized Hartree–Fock Theory

The gap and the renormalization functions of Ba_1?x K _x BiO have been numerically analyzed by means of the simplified equations of the generalized Hartree–Fock (GHF) theory. Measured functions and parameters have been used as inputs to the GHF integral equations, and all the relevant functions have been iterated self-consistently over the large energy range of 9 eV. The results show a reasonably large gap function which does not reverse its sign below 2 eV, despite static Coulomb repulsion and despite the low density of states at the Fermi level. It is also shown that these results are inconsistent with the conventional Eliashberg equations.     

Formation and properties of AgInSe2 thin films by co-evaporation

The electrical and optical properties of silver indium selenide thin films prepared by co-evaporation have been studied. X-ray diffraction indicates that the as prepared films were polycrystalline in nature. The lattice parameters were calculated to be a=0.6137 and b=1.1816 nm. Composition was determined from energy dispersive analysis of X-ray. Silver indium selenide thin films were also prepared by bulk evaporation of powdered sample for comparative study. They have an optical band gap (E_g) of 1.25 eV and it is a direct allowed transition. Refractive index (n) and extinction coefficient (k) were calculated from absorption and reflection spectra. Steady-state photoconductivity was measured from 300 to 400 K. Carrier lifetime was calculated from transient photoconductivity measurements at room temperature at different intensities of illumination.     

Synthesis and characterization of a novel poly(p-phenylenevinylene) derivative containing dibenzothiophene-5,5-dioxide moiety in the main chain with high electron affinity

We synthesized poly(2,8-dibenzothiophene-5,5-dioxide-vinylene-alt-1,4-phenylene-vinylene) (DD-PPV) (oligomer) by the traditional Wittig condensation and characterized it with IR, NMR, UV-vis etc. It is thermally stable and soluble in high polar solvents. Its UV-vis absorption and photoluminescence spectra in N,N-dimethyl-formamide solution show peaks at 369 and 457 nm, respectively. Its fluorescence quantum efficiency in solution is 75%. Its electron affinity is 3.17 eV. The band gap is 2.88 eV. Single-layer light-emitting diode device indium-tin oxide glass/DD-PPV/Aluminum emits greenish blue and the turn-on voltage is approximately 12 V.     

The optical properties of CuInS2 thin films

The optical absorption in flash-evaporated CuInS₂ thin films was studied in the photon energy range from 0.5 to about 4.2 eV. CuInS₂ was found to be a direct gap semiconductor with a gap energy of 1.524±0.005 eV at room temperature. The ground state energy of the free exciton was found to be about 8 meV. An indirect allowed transition was observed at 1.565±0.005 eV and was ascribed to an optical transition from the valence band maxima at the boundary of the Brillouin zone to the lowest conduction band minimum at the zone centre. Three further optical transitions which were probably due to the copper d states in the valence band were found at energies well above the fundamental edge.     

High-resolution X-ray photoelectron spectroscopy study of InTe thin film in structural phase transition from amorphous to crystalline phase

We investigated the chemical states of InTe thin film in the structural phase transition from the amorphous to the crystalline phase, using high-resolution X-ray photoelectron spectroscopy with synchrotron radiation. We confirmed the structural phase transition by transmission electron microscopy. Clean amorphous InTe (a-InTe) free of oxygen impurity was obtained after Ne⁺ ion sputtering at the ion beam energy of 1 kV for 1 h. Additionally, we obtained crystalline InTe (c-InTe) from clean a-InTe by annealing at 250 °C in an ultra-high vacuum. During the transition to the crystalline phase, the binding energy of the Te 4d core-level was unchanged, but the peak width was somewhat wider than in the amorphous phase. In the case of the In 4d core-level, the chemical shift was 0.1 eV at the higher binding energy between the amorphous and crystalline phases. The valence band maximum was shifted at the higher binding energy of 0.34 eV. We assumed that the Te atom was almost fixed and that the In atoms moved in the tight binding energy state to the center of the 4-Te atoms.     

Optical properties of β-FeSi2 single crystals grown from solutions

We studied the optical absorption, polarized reflectance (PR) and photoluminescence (PL) of β-FeSi₂ single crystals grown from solution. In low-absorption measurements, we found a phonon emission and absorption structure, which suggests an indirect transition. The exciton energy gap of 0.814 eV was determined from the absorption spectrum at 3.5 K. We also found a direct transition with the gap energy of 0.939 eV. PR measurements for Ea, Eb and Ec revealed the anisotropy of reflectivity of β-FeSi₂. We observed the PL with a peak wavelength of about 1.56 μm at 20 K.     

Effect of ion bombardment on the optical properties of LDPE/EPDM polymer blends

Ion bombardment is a suitable tool to improve the physical properties of polymers. In the present study, the effect of ion bombardment on the optical properties of low density polyethylene (LDPE)/Ethylene propylene diene monomer (EPDM) blend (LDPE/EPDM) was studied. Polymer samples was bombarded with 130 keV He and 320 keV Ar ions at fluencies levels ranging from 1 × 10¹³ to 2 × 10¹⁶ ions/cm². The untreated and ion beam bombarded samples were investigated using ultraviolet-visible (UV-Vis) spectrophotometry. The optical band gap (E_g), was decreased from ∼2.9 eV for the pristine sample down to 1.7 eV for the samples bombarded with He and Ar ions at the highest fluences. Change in the optical gap indicates the presence of a gradual phase transition for the polymer blends. Activation energy has been investigated as a function of the ion fluences. With increasing ion fluence, a decrease in both the energy gap and the activation energy was observed. The number of carbon atoms (N) in a formed cluster is determined according to the modified Tauc's equation.     

Effect of NiO content on the optical band gap, refractive index, and density of TeO2–V2O5–NiO glasses

Amorphous layers and bulk glasses of 40TeO₂–(60 ? x)V₂O₅–xNiO compositions with 0 ≤ x ≤ 30 (in mol%) have been prepared using the usual blowing technique and press-melt quenching method, respectively. The optical absorption spectra of the layers have been recorded in the wavelength range 400–800 nm. The fundamental absorption edge has been identified from the optical absorption spectra. The optical band gap, width of the tail of the localized states, and refractive index have been evaluated using available theories. Results show that the values of optical band gap decrease from 2.02 to 1.64 eV as the contribution of NiO increases. The refractive index dispersion is fitted to the single oscillator model, and results show that the static refractive index increase from 1.309 to 1.673 as the NiO content increases. The glass transition temperature, density, and molar volume have been studied, indicating act of NiO as network modifier. Values of theoretical optical basicity are also reported.     

Single crystal growth and characterization of nearly stoichiometric LiVO2

LiVO₂ undergoes an imperfectly understood orbital ordering transition near 500 K resulting in a loss of magnetic moment below the transition. Studies of the transition have been hampered by a lack of high-quality stoichiometric single crystals. Here we report the growth and basic characterization of large, nearly stoichiometric LiVO₂ single crystals. The crystals were characterized by magnetic susceptibility, electrical resistivity, differential scanning calorimetry, and specific heat measurements over a temperature range from 2 to 650 K. A first-order phase transition with large hysteresis near T_t≈500 K was observed in all measurements. An anisotropy of the order of 100 was observed in the in-plane versus out-of-plane resistivity, and the inferred semiconducting energy gap was 0.18 eV for T<T_t and 0.14 eV for T<T_t. Electron diffraction experiments were performed on LiVO₂ single crystals at temperatures below and above T_t. Superlattice reflections were observed below T_t and disappeared upon heating above the phase transition temperature. Upon cooling below T_t, the supperlattice spots reappeared. Bright field electron micrographs indicate that the crystals develop a roughly hexagonal network of cracks.     

Optical,structural and electrical properties of co-evaporated CuIn2Se3.5 thin films

Polycrystalline CuIn₂Se_3.5 thin films have been prepared by a three-source co-evaporation technique. The optical band gap, structural and electrical properties of these co-evaporated CuIn₂Se_3.5 thin films deposited on glass substrate held at 673 K and in situ annealed at 723 K in selenium atmosphere are reported here. Optical band gap of the films, determined from spectral transmission data, is found to be 1.22 eV. Powder X-ray diffraction (XRD) studies reveal the CuIn₂Se_3.5 films to be polycrystalline in nature with tetragonal structure. The lattice parameters are found to be a=0.576 nm and c=1.151 nm. The average grain size of the films is 1 μm. The films are n-type with room temperature resistivity of 180 Ω cm. The activation energies, determined from the temperature dependence of electrical conductivity, are found to be 0.34 eV (320–415 K) and 0.10 eV (223–320 K) and are attributed to In_Cu and V_Se, respectively.     

Electronic and optical properties of monoclinic and rutile vanadium dioxide

The electronic and optical properties of vanadium dioxide are investigated in the frameworks of density functional theory and GGA+U, in detail. It is found that, the metal–insulator transition in VO₂ is induced by the on-site correlation effects, accompanied with a distinct charge-transfer. Unlike that in rutile phase, the energy gap in the monoclinic phase opens suddenly and abruptly, which is consistent with the experimental observation. The calculated indirect energy gap (0.32 eV) and the direct energy gap (0.58 eV) can be used to theoretically interpret the experimental optical transmission at 0.31 eV and the optical energy gap 0.6 eV, respectively. Consequently, both of them are confirmed by our optical calculation. Furthermore, our calculated optical absorption peaks agree with the experiment very well.     

Determination and analysis of optical constants for thermally evaporated PbSe thin films

PbSe films have been deposited on glass and quartz substrates at room temperature by thermal evaporation technique. X-ray diffraction patterns of the obtained films showed that they have polycrystalline texture and exhibit cubic FCC structure. The optical constants, the refractive index n and absorption index k were calculated in the spectral range of 400-4000 nm from transmittance and reflectance data using Murmann’s exact equations. Both n and k are practically independent on the film thickness in the range 28 nm to 210 nm. From the analysis of absorption index data, an indirect allowed energy gap of 0.16 eV and direct allowed energy gap of 0.277 eV were obtained. Other direct allowed optical transitions were obtained with energy gap of 0.49 eV and may be due to the splitting of valence band at the Γ point due to the effect of spin-orbit interaction.     

Physical characteristics of thermally evaporated Bismuth thin films

Thin films of bismuth with different thicknesses were produced by thermal evaporation from a molybdenum boat source onto cleaned glass substrates at room temperature. The material has been characterized using X-ray diffraction, electrical and optical measurements. A polycrystalline transition phase was observed. The resistivity was calculated for different film thicknesses and found to vary with thickness and temperature. An anomalous dependence of resistivity on temperature was observed during heating. The optical constants were determined from the transmission and reflection data of these thin films for normal incidence. The absorption coefficient revealed the existence of an allowed direct transition with energy gap (E_g) values ranging from 3.45 to 3.6 eV.     

Étude de l'absorption optique de couches minces de fluorure de magnésium et de fluorure d'aluminium en fonction de leurs conditions de préparation

An investigation of the optical absorption of thin films of magnesium fluoride and of aluminium fluoride obtained by evaporation was performed for the energy range 1.5–12 eV. A previous study by nuclear analysis had shown the influence of the conditions of preparation on the composition of the layers; in particular the content of Na in parts per thoudand of the basic metal varied from 10^-1 to 20.For these two fluorides, we note a first optical gap below 10.95 eV which is strongly dependent on the degree of crystallisation and on the composition of the layers, and which corresponds to transition states between F^- and Na⁺. For a high sodium concentration an absorption band appears at 6.5 eV; it can be interpreted in terms of transitions between OH^- and Na⁺ in inclusions of NaMgF_3?x(OH)_x (or NaAlF_4?x(OH)_x). If tantalum crucibles are used the presence of TaO₂F in the layers can be observed and the absorption edge in this case is at 5.6 eV. It was also noted that for a disordered structure or for a large concentration of impurities the absorption presents a long exponential tail that can be interpreted in terms of a superposition of different mechanisms.