Film-thickness effects on the optical and electrical properties of Cu-GeO2 thin cermet films

The effect of film thickness on the optical and electrical properties of Cu-30 wt % GeO₂-70 wt % thin cermet films prepared by electron-beam deposition at about 10^–3 Pa and at a substrate temperature of 300 K is reported. The ultraviolet, visible and direct current (d.c.) conductivity results are analysed with the aim of determining the optical band gap,E _opt, the width of the band tails,E _e, and the d.c. thermal activation energy,E _a. It was found that the optical energy gap increases with increasing thickness and that the absorption was due to indirect transitions ink-space. The general feature of the absorption edge remains similar for both unannealed and annealed films, but annealing has the effect of decreasingE _opt. The d.c. conductivity results show thatE _a decreases with increasing thickness. From a knowledge ofE _opt andE _a, a probable model of the electronic band structure in Cu-GeO₂ thin films has been suggested.     

Thickness dependence of refractive index and optical gap of PMMA layers prepared under electrical field

Optical parameters (refractive index, dispersion energy, optical gap) of polymethylmethacrylate (PMMA) layers prepared by spin coating and modified by electric field have been studied. Refractive index was measured using a refractometer, internal structure was investigated as a structural parameter (E _d) within the One Oscillator Model. Optical gap width (E _g^opt) was assessed using Tauc Approximation from UV-Vis spectra. Surface morphology and roughness was investigated using an AFM. The electric field imposed during preparation of layers increases their refractive index. The highest increase in n (Δn = 0.042) was found for the thinnest PMMA layer (70 nm). Oriented layers have produced higher E _g^opt than non-oriented ones for all studied values of thickness. The electrical field applied at preparation of the oriented layer will not change its surface morphology and roughness.     

Annealing dependence of optical properties of Ga20S75Sb5 and Ga20S40Sb40 thin films

Amorphous Ga₂₀S₇₅Sb₅ and Ga₂₀S₄₀Sb₄₀ thin films were prepared onto glass substrates by using thermal evaporation method. The effect of annealing (under vacuum) at different temperatures on the optical parameters was investigated in the temperature range 373-593 K. The optical absorption coefficient (α) for the as-deposited and annealed films were calculated from the reflectance and transmittance measurements in the range 190-900 nm. X-Ray diffraction indicates that the as-deposited films and those annealed up to the glass transition temperature (T_g) exhibit amorphous state. On annealing above the glass transition temperature these films show a polycrystalline structure. Analysis of the optical absorption data indicates that the optical band gap E_g^opt of these films obeys Tauc's relation for the allowed non-direct transition. It was found that the optical band gap E_g^opt increases with annealing temperature up to T_g, whereas above T_g there is a remarkable decrease. The obtained results were interpreted on the basis of amorphous- crystalline transformation.     

Some optical properties of Se-Ge-As amorphous chalcogenide glasses

The effects of composition, film thickness, substrate temperature, and annealing of amorphous thin films of Se₇₅Ge_25−x As _x (5⩽x⩽20) on their optical properties have been investigated. X-ray diffraction revealed the formation of amorphous films. The absorbance and transmission of vacuum-evaporated thin films were used to determine the band gap and refractive index. Optical absorption measurements showed that the fundamental absorption edge is a function of glass composition and the optical absorption is due to indirect transition. The energy gap increases linearly with increasing arsenic content. The optical band gap,E _opt, was found to be almost thickness independent. The shapes of the absorption edge of annealed samples displayed roughly the same characteristic as those of the unannealed films, but were shifted towards shorter wavelengths; as a result,E _opt increased andE _e, the width of the band tails, decreases. The increase inE _opt is believed to be associated with void removal and microstructural re-arrangement during annealing. The influence of substrate temperature on the optical parameters is discussed.     

Optical and electrical properties of carbon nitride films deposited by cathode electrodeposition

Carbon nitride (CN _x ) films were prepared on silicon (100) wafers and ITO conductive glasses by cathode electrodeposition, using dicyandiamide (C₂H₄N₄) in acetone as precursors. The composition ratios (N/C) were approximated to or larger than 1 from XPS. The optical properties and electrical resistivities of the films were investigated. Intense PL with two bands in the range 2.5–3.5 eV was observed on the CN _x films. The band gaps (E _opt) deduced from measurements of the optical absorption coefficients in the UV-VIS spectra were found to be in the range of 1.1–1.6 eV. From the PL and UV-VIS spectra, the nitrogen content has a large effect on the PL band gap and E _opt. The electrical resistivities of the films on Si wafers are in the 10⁹–10¹⁰ · cm range.     

EPR and optical absorption studies of Cu ions in alkaline earth alumino borate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cu²⁺ ions in alkaline earth alumino borate glasses doped with different concentrations of CuO have been studied. The EPR spectra of all the glasses exhibit the resonance signals, characteristic of Cu²⁺ ions present in axially elongated octahedral sites. The number of spins participating in the resonance has been calculated as a function of temperature for calcium alumino borate (CaAB) glass doped with 0.1 mol% of CuO. From the EPR data, the paramagnetic susceptibility (χ) was calculated at different temperatures (T) and from the 1/χ-T graph, the Curie temperature of the glass has been evaluated. The optical absorption spectra of all the glasses show a single broad band, which has been assigned to the ²B_1g → ²B_2g transition of the Cu²⁺ ions. The variation in the intensity of optical absorption with the ionic radius of the alkaline earth ion has been explained based on the Coulombic forces. By correlating the EPR and optical absorption spectral data, the nature of the in-plane σ bonding between Cu²⁺ ion and the ligands is estimated. From the fundamental ultraviolet absorption edges of the glasses, the optical energy gap (E_opt) and the Urbach energy (ΔE) are evaluated. The variation in E_opt and ΔE is explained based on the number of defect centers in the glass.     

Study of hydrogenated nanoamorphous silicon(na-Si:H) thin film prepared by RF magnetron sputtering for graded optical band gap (<Emphasis Type="Italic">E</Emphasis><Superscript>opt</Superscript><Subscript>g</Subscript>)

The schematic of the energy band gap figure of the graded optical band gap (E_g^opt) in p-i-n layer in na-Si:H solar cells was given in the paper. The intrinsic hydrogenated nanoamorphous silicon(na-Si:H) thin films with the graded band gap as a function of depth through the films were prepared by varying the processing power, gas pressure, gas composition, and etc., We have carried out a investigation of the relationships between the E_g^opt with the crystallization ratio (Xc) and the E_g^opt with the nanocrystalline grain size (D) in na-Si:H thin films grown by PECVD on glass substrates through XRD, Raman scattering, transmission. The E_g^opt increase with the decreases of the crystallization ratio (Xc) and the nanocrystalline grain size (D). The hydrogen dilution ratio is found to increase basically both the crystallization ratio (Xc) and the nanocrystalline grain size (D). Two relationships in na-Si:H are discussed by the etching effect of atomic hydrogen in the framework of the growth mechanism and the quantum size effect (QSE).     

Synthesis and Optical Studies of Superconducting MgB<Subscript>2</Subscript> Thin Films

Synthesis and optical transmission of MgB₂ thin films on optically transparent glass are reported. In the 400–1000 nm regime as deposited films show high metallic reflectivity and very little transmission. After deposition, the films were annealedex situ and rendered superconducting withT _c of 38 K, approaching that of the bulk material. The reaction conditions where quite soft ∼ 10 min at 550°C. The optical absorption coefficient,α and photon energy,E followed a Tauc-type behavior, (αE)^1/2=β _T(E −E _g). The band gap (E _g) was observed to peak at 2.5 eV; but, the slope parameterβ _Tbehaved monotonically with reaction temperature. Our results indicate that an intermediate semiconducting phase is produced before the formation of the superconducting phase; also optical measurements provide valuable information in monitoring the synthesis of MgB₂ from its metallic constituents. In addition these films have interesting optical properties that may be integrated into optoelectronics.     

Optical and structural properties of amorphous and heat treated In0.4Se0.6 film of different thicknesses

The optical gap (E _g ^opt ) of as-deposited In_0.4Se_0.6 film is found to increase with the thickness of the film. The optical gap attains a steady value after several heat treatments of the films at elevated temperatures. The values of optical gaps of In_0.4Se_0.6 films are found to depend on the temperature of the heat treatment. The effect of thickness and temperature of heat treatment on the optical gap of the film is interpreted in terms of creation or elimination of defects in the amorphous structure of In_0.4Se_0.6 film. X-ray diffraction spectra are taken for both as deposited and heat treated In_0.4Se_0.6 films. From the results related to the radial distribution functions of the specimens it is noted that in the case of In_0.4Se_0.6 a transition from amorphous to crystalline states takes place at 523 K and above. The results also show that both the optical gap and the coordination number decrease with increasing temperature of heat treatment. Therefore it is evident that the heat treatment improves the long range order of In_0.4Se_0.6 but reduces the coordination in the short range.     

Cd(S<Subscript>(1 − <Emphasis Type="Italic">x</Emphasis>)</Subscript> + CO<Subscript>3(<Emphasis Type="Italic">x</Emphasis>)</Subscript>) thin films by chemical synthesis

A microcrystalline mixture of cadmium carbonate (CdCO₃) and cadmium sulfide (CdS) were grown in the thin film format onto glass substrates by means of chemical bath. The temperature of the bath (T_d) was selected in the interval 23–80^∘C. At low temperatures, CdCO₃ is the compound predominant in the layers. At high temperatures CdS is the compound deposited on the substrate. At intermediate T_d-values a mixture of both materials are present, i.e., the gradual transition from an insulator (CdCO₃) to a semiconductor (CdS) growth occurs when T_d increases. Physical properties of films were studied by means of X-ray diffraction and optical absorption. The forbidden energy band gap of direct electronic transitions (E_g) was calculated by applying the α² ∝ (hν − E_g) relation to the optical absorption spectra.     

Cu2+ ions in sodium fluoride-sodium borate glasses studied by EPR and optical absorption techniques

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cu²⁺ ions in 80Na₂B₄O₇-(20 – x)NaF – xCuO (NFNB) glass system with 0 x 6 mol% have been studied. EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu²⁺ ions. The values of spin-Hamiltonian parameters indicate that the Cu²⁺ ions in sodium fluoride-sodium borate (NFNB) glasses were present in octahedral sites with tetragonal distortion. The number of spins (N) participating in resonance was calculated as a function of temperature for NFNB glass sample containing 1 mol% of Cu²⁺ ions and the activation energy was calculated. From the EPR data, the paramagnetic susceptibility () was calculated at various temperatures and the Curie constant was calculated from the 1/ – T graph. The optical absorption spectra of these samples show a broad absorption band centered at 13280 cm^–1 which is assigned to the ² B _1g ² B _2g transition of Cu²⁺ ions in distorted octahedral sites. The optical band gap energy (E _opt) and Urbach energy (E) are calculated from their ultraviolet edges. It is observed that as the copper ion concentration increases, E _opt decreases while E increases. This has been explained as due to the creation of additional localized states by CuO, which overlap and extend in the mobility gap of the matrix. By correlating the EPR and optical data, the molecular orbital coefficients have been evaluated.     

Optical constants of thermally evaporated amorphous GeSe3 thin films

The optical transmission spectra of amorphous Ge-Se films of chemical composition GeSe₃, prepared by thermal evaporation, are measured overthe 300 nm to 2500 nm spectral region. A simple, straightforward procedure suggested by Swanepoel, which is based on the use of interference fringes, has been applied in order to derive the real and imaginary parts of the complex refractive index, and also the film thickness. Furthermore, thickness measurements made by a surface-profiling stylus are also carried out to cross-check the results obtained by the present optical method, employing only T(). The dispersion of n is discussed in terms of the single-oscillator Wemple and DiDomenico model, and the optical band gap E _g ^opt has been determined from the absorption coefficient values, using the Tauc procedure.     

Optical properties of CulnSe2 thin films

The optical constants of vacuum-deposited CulnSe₂ films were determined from the measured transmittance and reflectance at normal incidence of light in the wavelength range 500 to 2000 nm. The analysis of the experimental points of the absorption coefficient revealed the existence of two optical transition processes: an allowed direct transition withE _g=1.03±0.01 eV and a forbidden direct transition withE _f=1.254±0.001 eV. The optical constants of the films were independent of the substrate temperature.On leave to the Kingdom of Saudi Arabia.     

A crystallization study of the Ge25Sb20S55 glassy alloy

Differential scanning calorimetry analysis was used to gain insight on the thermal stability and calorimetric behaviour on heating of the Ge₂₅Sb₂₀S₅₅ glassy alloy. The as-quenched glass shows a glass transition atT _g=570 K. From the variation ofT _g with the heating rate a value of h=240 kJ mol^–1 was obtained for the activation enthalpy of the mean relaxation time. On further heating an exothermic process appears ranging from 725 to 800 K giving rise to glassy GeS₂ and needle-like crystals of antimony. The microstructure of the crystallization product was obtained from X-ray diffraction and scanning electron microscopy. The resulting material shows two glass transitions. The crystallization process is explained correctly by the Johnson-Mehl-Avrami-Erofe'ev equation with a kinetic exponent ofn=1.6. The apparent activation energy of crystallization is evaluated asE=286 kJ mol^–1.     

4.6 Some optical properties of thin Cd3As2 films

The transmission of vacuum deposited Cd₃As₂ films has been measured in the spectral range 0.6–15 ωm. The absorption coefficient α has been calculated from the data for crystalline films obtained at substrate temperature T_s = 440 K and also for amorphous films obtained on substrates at T_s = 300?393 K. The position of the low-energy absorption edge in the spectral range 0.1–0.3 eV is discussed and the value Δ_g^opt = 0.06 eV for indirect transion Γ → Γ₁₅ is obtained for crystalline films. For amorphous films, a shift of the minimum absorption edge to $\text{Δ}\text{E}{}_{\mathrm{g}}{}^{\mathrm{opt}}\text{? 0.4?0.5}\text{eV}$ was observed. For higher energies two direct transitions have been established: the first at E₁ = 0.9 eV in the photon energy range 0.9–1.2 eV.and the second at E₂ = 1.2?1.3 eV in the photon energy range 1.3?1.6 eV. The last value is in good agreement with the value calculated by Lin Chung and with reflectivity data; it corresponds to the transition Γ₁₅ → X₁ in the hypothetical band model of Lin Chung. Differences in the reflective index values for crystalline and amorphous Cd₃ As₂ are also discussed.     

Determination of optical constants of CdS x Se y electrodeposits

The optical constantsn(λ) and α(λ) have been determined for CdS_xSe _y films electrodeposited on to glass doped with SnO₂. From a (αhv)² versus photon energy plot, a value for the direct band gap of the electrodeposited polycrystalline semiconductor ofE _g=1.83 eV was obtained, which indicates a low proportion of S²⁻ in the thin films formed.     

Temperature shift of the optical gap in some PbO-ZnO-P2O5 glasses

The optical gap (E_g) between 4.54 eV-4.88 eV at room temperature was determined for PbO-ZnO-P₂O₅ glasses with the formal content of P₂O₅ in the region of 30 to 50 mol% and with the formal content of PbO in the region of 50 to 45 mol%, respectively. The temperature (T) dependence of the optical gap (E_g(T)) in the region 80 < T[K] < 600 was determined, and an electron-phonon interaction is suggested to be a major contribution to the temperature shift of the optical gap. In the temperature region of 300-600 K, the E_g(T) dependence can be approximated by a simple linear relation with the temperature coefficient (γ) of the optical gap in the region 6.04 ≤ γ × 10⁴ [eV/K] ≤ 7.39.     

Structural, optical and electrical characterization of selenium sulphide nanostructured thin film

Thin film of selenium sulphide (Se₇₅S₂₅) has been prepared using inert-gas consolidation (IGC) method and micro-structural, optical and electrical measurements were carried out on the film. Scanning electron microscopy (SEM) studies show that the deposited film is well adherent and grains are uniformly distributed over the surface of the substrate. X-ray diffraction (XRD) analysis shows that the film is polycrystalline nature with single phase and crystallizes in the orthorhombic structure. The field emission transmission electron microscope (FETEM) revealed the uniform dispersion and an average particle size of 20 nm. Analysis of the optical absorption data indicates that the optical band gap E_opt of this film obeys Tauc's relation for the allowed non-direct transition with energy gap is 2.48 eV. Electrical conduction measurements also show the presence of two distinct phases of the materials and characteristic changes in transport properties due to the nanosize of the materials.     

Optical bandgap of Sb0.2Bi1.8Te3 thin films

Sb_0.2Bi_1.8Te₃ Thin Films were grown using the thermal evaporation technique on a (001) face of NaCl crystal as a substrate at room temperature. The optical absorption was measured in the wave number range 500 cm^–1 to 4000 cm^–1. From the optical absorption data the bandgap has been evaluated and studied as a function of the film thickness and deposition temperature. The data indicate absorption through direct interband transition with a bandgap around 0.21 eV. The detailed results are reported.     

ZnO as dielectric for optically transparent non-volatile memory

This paper discusses the application of a DC sputtered ZnO thin film as a dielectric in an optically transparent non-volatile memory. The main motivation for using ZnO as a dielectric is due to its optical transparency and mechanical flexibility. We have established the relationship between the electrical resistivity (ρ) and the activation energy (E_a) of the electron transport in the conduction band of the ZnO film. The ρ of 2 × 10⁴–5 × 10⁷ Ω-cm corresponds to E_a of 0.36–0.76 eV, respectively. The k-value and optical band-gap for films sputtered with Ar:O₂ ratio of 4:1 are 53 ± 3.6 and 3.23 eV, respectively. In this paper, the basic charge storage element for a non-volatile memory is a triple layer dielectric structure in which a 50 nm thick ZnO film is sandwiched between two layers of methyl silsesquioxane sol–gel dielectric of varying thickness. A pronounced clockwise capacitance–voltage (C–V) hysteresis was observed with a memory window of 6 V. The integration with a solution-processable pentacene, 13,6-N-Sulfinylacetamodipentacene resulted in an optically transparent organic field effect transistor non-volatile memory (OFET-NVM). We have demonstrated that this OFET-NVM can be electrically programmed and erased at low voltage (± 10 V) with a threshold voltage shift of 4.0 V.