Preparation and properties of electrodeposited Hg1-xCdxSe films

Hg_1-xCd_xSe (0.87 < x < 1) films were electrodeposited potentiostatically onto ITO glass substrates by varying the Hg content in the aqueous electrolyte with the pH adjusted to 1.5–2 at room temperature. The as-deposited and the as-grown and annealed films showed hexagonal structure with c-axes perpendicular to the substrate. The band gap (direct allowed transition) energies as determined from the optical absorption measurements, varied between 1.7 eV for x=1, and 1.2 eV for x=0.88. All of the samples showed n-type conductivity. The carrier concentration increased and the resistivity decreased with the increase of the Hg content in the solution. The resistivity activation energy of the sample, E, decreased with increasing mole fraction of Hg, and E became 0 at higher Hg concentrations (x < 0.88).     

Electrical conductivity and optical properties of ZnO films annealed in hydrogen atmosphere after chemical vapor deposition

The d.c. electrical conductivity and optical properties of polycrystalline zinc oxide films (220–450 nm thick) annealed in hydrogen after chemical vapor deposition are investigated. A minimum film resistivity after the annealing gives 0.31 cm for the film as-deposited at a substrate temperature of 823 K. From the temperature dependence of conductivity, band conduction is confirmed for the films at temperatures above 250 K. The effect of grain-boundary scattering is due to thermionic emission of electrons over grain boundary barriers. At temperatures below 250 K, variable-range hopping transport is found to be dominant. The films are transparent in the wavelength range 400 to 1000 nm and have sharp ultraviolet absorption edges at 380 nm. The absorption edge analysis reveals the optical band gap energy for the films to be 3.18–3.23 eV. The Urbach tail analysis gives the width of localized states E_e=0.06-0.14eV.     

Annealing effects on the structural and optical characteristics of electron beam deposited Ge-Se-Bi thin films

Measurements of absorbance and transmittance in Ge₂₀Se_80-xBi_x (0 x 10 at %) chalcogenide thin films in the visible range at room temperature were carried out. The dependence of the optical absorption on the photon energy is described by the relation hv=B(hv-E₀)². It was found that the optical energy gap E₀ decreases gradually from 1.93 to 1.205 eV with increasing Bi content up to 10 at %. The rate of the change decreases with increasing Bi content. The composition dependence of the optical energy gap is discussed on the basis of the concentration of covalent bonds formed in the chalcogenide film. The decrease of optical energy gap with increasing Bi content is related to the increase of Bi–Se bonds and the decrease of Se-Se bonds. The effect of thermal annealing for different periods of time on the behavior of optical absorption of the as-deposited films was investigated. The optical gap increases with increasing annealing time. The rate of change decreases with increasing annealing time, then E₀ reaches a steady state. The increase in the values of the optical gap of the amorphous films with heat treatment is interpreted in terms of the density of states model. The structure of the as-prepared and thermally annealed films were investigated using transmission electron microscopy, energy dispersive analysis and X-ray diffraction. It was confirmed that the as-prepared films were in the amorphous state. Phase separation was observed after thermal annealing. The separated crystalline phases were identified.     

Transport properties of CdGa2Se4 thin films

The d.c. conductivity was determined for CdGa₂Se₄ thin films in the temperature range 300–625 K for as-deposited and heat-treated films. The conductivity at room temperature of films of thickness 326 nm prepared at a temperature of 573 K was found to be 10^–12 ( cm)^–1. The dependence of the electrical conductivity on the annealing temperature in a vacuum of 1 Pa for a thin film of thickness 140 nm, shows that the electrical conductivity increases with increasing annealing temperature. However, the activation energies E and E decrease with increasing annealing temperature. The data of these annealed films are in agreement with the Meyer–Neldel rule. The thermoelectric power measurements indicate p-type conduction in the as-deposited films as well as for the heat-treated films. The p-type conduction is due to the cadmium deficiency as indicated by EDX. The difference between the value of the activation energy calculated from the thermoelectric power E _S and that obtained from the conductivity E indicates the presence of long-range potential fluctuations.     

Thin and superthin photoconductive CdSe films deposited at room substrate temperature

Low conductive and photosensitive CdSe films were deposited on substrates at room temperature. X-ray diffraction spectra showed a microcrystalline cubic structure for film thicknesses greater than 20 nm and an amorphous structure for film thicknesses below 10 nm. An optical band gap, E _g ⁰ , of 1.6 eV was determined using the Tauc-dependence usually employed for amorphous semiconductors. The dark conductivity, , of 10^{–9 –1} cm^–1 was measured in the as-deposited state, but an increase of five orders of magnitude was observed after heating the layer above 450 K. High photosensitivity was observed under illumination with white light as well as with monochromatic light over a wide spectral region (400–750 nm). A conclusion is reached concerning the existence of compensated donor and acceptor defects in the as-deposited state.     

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.     

Formation and characteristics of lead lanthanum zirconium titanate thin films formed by using the r.f. sputtering method

PLZT (lead lanthanum zirconium titanate) thin films were prepared by using the r.f. magnetron sputtering method and post-annealing for crystallization at 650 C. The films which were annealed at 650 C for 10 min consisted of a metastable phase and a stable phase. However, another film which was annealed at 650 C for 20 min had only stable perfect perovskite phase. The stability of the post-annealed thin film and substrate interfaces was observed by using scanning electron microscopy. The longer the annealing time, the more unstable were the interfaces. By analysing the EDX data, the composition difference between the sputtering target and thin films, and the composition variation between as-deposited and post-annealed PLZT were studied. The films annealed at 650 C for 20 min showed good ferroelectric and electrical properties with a remanent polarization (P _r) of 11.5 C cm^–2, and a coercive field (E _c) of 164 kV cm^–1.     

Structural and optical properties of CdGa2Se4 thin films

CdGa₂Se₄ thin films were prepared by vapour deposition onto either room temperature or preheated quartz and glass substrates (T _S) or they were deposited at room temperature and then annealed at about (T _A) 623 K. The films thus prepared were crystalline with a thiogallate tetragonal structure. The optical constants (the refractive index n and the absorption index, k) were determined for CdGa₂Se₄ films deposited onto quartz substrates held at either room temperature or at T _S = 573 K. These constants were also determined for preannealed films (T _A = 623 K). Plots of (hv)² = f(hv) and (hv)^1/2 = g(hv) were linear, indicating the existence of both direct and indirect optical transitions. It was found that the values of E _g ^d and E _g ^ind for as-deposited CdGa₂Se₄ films were 2.46 and 1.91 eV, respectively. The corresponding values for the annealed films and the films deposited at T _S = 573 K were 2.56 and 2.06 eV, respectively.     

Effect of heat treatment on the structural and optical properties of CuInTe2 thin films

The CuInTe2 thin films were prepared by thermal vacuum evaporation of the bulk compound. The structural and optical properties in the temperature range 300–47 K of thin films grown on glass substrates and annealed in vacuum were studied. The films were investigated by X-ray diffraction and electron microscope techniques. The calculated lattice constants for CuInTe₂ powder were found to bea=0.619 nm andc –1,234 nm. From the reflection and transmission data, the optical constants, refractive indexn, absorption index,k, and the absorption coefficient, , werw computed. The optical energy gap was determined for CuInTe₂ thin films heat treated at different temparatures for different periods of time. It was found thatE _g increases with both increasing temperature and time of annealing.     

Electrical and Optical Properties of Cu2CdGeS4 Single Crystals

Cu₂CdGeS₄ single crystals are grown by chemical vapor transport and directional solidification, and their properties are studied. Independent of the growth technique, the crystals are p-type, with a 290-K Hall mobility of holes in the range 10–15 cm²/(V s). The room-temperature band gap of Cu₂CdGeS₄ evaluated from the position of its fundamental absorption edge is E _g 2.05 eV. The nature of the defects responsible for the near-IR absorption in Cu₂CdGeS₄ is assessed by analyzing the absorption spectra of stoichiometric crystals prepared by different techniques and melt-grown nonstoichiometric crystals. Some of the crystals studied have a thermoelectric power as high as 1800 mV/K. Based on the experimental data, a model is proposed for the electronic processes in Cu₂CdGeS₄, which accounts for the observed electrical and optical properties of this semiconductor.     

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.     

Inelastic Interaction of an Electron with a C60 Cluster

The method of intersecting beams of C₆₀ fullerene clusters and of electrons is used to investigate the production of C ₆₀ ⁺ and C ₆₀ ^– ions and the radiation in the wavelength range from 300 to 800 nm for the electron energy E _e ranging from zero to 100 eV. The absolute values of the ionization and electron-attachment cross sections [⁺(E _e ) and ^–(E _e ), respectively] are determined. A maximum of ⁺(E _e ) of 0.53 nm²is observed at E _e = 52 eV. For a C₆₀ cluster excited by electron impact, delayed (150 s) ionization initiated by collision with the surface and ionization due to thermionic emission (its characteristic time at E _e = 60 eV is 6 s) is observed. It is found that, for E _e < 0.4 eV, the formation of C ₆₀ ^– is defined by the polarization capture of an s-electron (^– E _e ^–1 ), and a formula is suggested for determining ^–(E _e ). In the region of E _e from 1 to 6 eV, the cross section ^–(E _e ) shows only slight variations about ^–(E _e ) = 0.36 ± 0.03 nm². For E _e > 7.5 eV, C ₆₀ ^– ions proved to be unstable to electron autodetachment. In the region of intersection of C₆₀ and electron beams, radiation of a quasicontinuous spectrum described by a modified Planck formula for the thermal emission of spherical particles of diameter d is recorded. For E _e > 47 eV, the brightness temperature proved to be 3150 ± 50 K. It is found that this radiation is emitted predominantly by hot C ₆₀ ^+* ions produced as a result of thermionic emission from C ₆₀ ^* . The rate of radiation loss of energy by a C ₆₀ ^+* ion and the cross section for the formation of radiating C ₆₀ ^+* ions are found to be, respectively, 5.5 × 10⁵ eV/s at T = 3150 K and 2 × 10^–2 nm² at E _e = 60 eV.     

Space charge limited current, variable range hopping and mobility gap in thermally evaporated amorphous InSe thin films

We have analyzed the properties of as-deposited InSe thin films, deposited onto well cleaned glass substrates under a vacuum of 10^–5 Torr, using X-ray diffraction, Rutherford back scattering, energy dispersive analysis of X-rays, optical transmittance and current–voltage (120–390 K) measurements. Allowed and indirect transition was identified and the mobility gap was determined as 1.44 eV. Under low field (<1×10⁵ V cm^–1) and in the temperature range of 130–200 K, the conductivity in the films was behaving like that of Mott's variable-range hopping (VRH) type. Mott's parameters such as characteristics temperature (T ₀), hopping range (R _hop), hopping energy (W _hop), values of localized states density N (E _F), and activation energy (E _a) were estimated. In the temperature range 210–290 K, thermionic conduction mechanism plays a dominant role and its activation energy was calculated. At high field (>2×10⁵ V cm^–1) and in the temperature range of 300–390 K, space charge limited conduction currents (SCLC) mechanism was observed and the related parameters, such as electron density (n ₀), trap density (n _t), the ratio between free electron density to the total electron density (), mobility () and the effective mobility (_eff) of the InSe film of typical thickness 265 nm were calculated and the results are discussed.     

Current–Voltage Characteristics of EuGa2S4<Co> Single Crystals

The current–voltage characteristics of EuGa₂S₄ single crystals were measured from 80 to 300 K with the aim of elucidating the mechanism of electrical transport. The results, interpreted as evidence of space-charge-limited currents with an exponential distribution of trap energies, were used to determine the trap depth (E _t = 0.30 eV), trap concentration (N _t = 4 × 10¹³ cm^–3), Fermi energy (E _F = 0.51 eV), equilibrium carrier concentration (p ₀ = 9 × 10⁹ cm^–3), contact potential difference ( = 0.43 eV), and shift of the Fermi level (E _F = 0.09 eV).     

Correlation Between the Morphology of Ag and the Contact Resistivity of the Ag/YBa2Cu3O7?δ Thin Film Contact

The morphology of the silver films deposited and annealed on laser ablated YBa₂Cu₃O_7– thin films and the corresponding contact resistivity have been systematically investigated. A minimum contact resistivity of 6 × 10^–8 cm² was reached at 77 K by annealing Ag/YBa₂Cu₃O_7– contact at the optimum temperature. The effect of the annealing temperature on the contact resistivity was explained by considering the morphology of the silver films and the diffusion of silver into YBa₂Cu₃O_7– film, etc. The difference of the contact resistivity for Ag contact to polycrystalline, single crystal and thin film of YBa₂Cu₃O_7– were also explained.     

Aqueous precipitation and electrical properties of In2S3: characterization of the In2S3/polyaniline and In2S3/polypyrrole heterojunctions

The stability of indium sulphide aqueous supersaturated solutions at pH 2.50 and 25°C was investigated. Spontaneous precipitation proceeded at rates proportional to the solution supersaturation via a polynuclear mechanism and the phase formed was identified as In₂S₃. The absorption spectrum of In₂S₃ was measured from 190–800 nm and from the absorption threshold the optical energy gap was estimated to be E ₀ = (1.8 ± 0.1) eV. The thermal energy gap E _t = (1.6 ± 0.2)eV was determined from resistivity against temperature measurements and a thermopower coefficient S = –100VK^–1 at room temperature was found. Finally, In₂S₃/polyaniline and In₂S₃/polypyrrole heterojunctions were prepared and from the investigation of their I–V characteristics, the values of the ideality factor, n, the saturation current density, J ₀, and the effective barrier height, _B, were determined to be n = (15 ± 2), J ₀ = (38 ± 7) A m^–2 and _B = 0.56 eV for the polyaniline and n = (64 ± 8), J ₀ = (13 ± 2) A m^–2 and _B = 0.59 eV for the polypyrrole heterojunction.     

Characterization of (Pb,La) TiO3 thin films prepared by the sol-gel method

Pb_(1–1.5x)La_xTiO₃ thin films were synthesized by the sol-gel spin-coating technique. The films became crystallized when the spin-coated films were annealed at 600 °C and at higher temperature, and became amorphous when annealed at 550 °C. The breakdown voltage, V _B, was recorded at around 30 V for 600–650 °C annealed films and varied only slightly with the composition. The V _B value of the amorphous films was observed to be higher than that of the crystalline films. The ferroelectric properties of both the amorphous and crystalline films were found to be similar. The dielectric constant, charge storage density and optical index of refraction of the films were _r =5–20, Q _c=0.12–0.54 C cm^–2 and n=1.6–2.3, respectively. They all increased moderately with La³⁺ content in the films. One possible reason why the ferroelectric properties are not modified as the amorphous films crystallize, may be that the octahedra are equilateral, whether the films are amorphous or crystalline. Additionally, a possible cause which lowers the breakdown voltage in crystalline film, is the formation of lead vacancies due to lead loss. The electrical properties of films coated on bare silicon become significantly lowered due to interdiffusion between films and substrate. The diffusion of Si⁴⁺ ions into-the films can be prevented by coating SrTiO₃ on the silicon substrate as a buffer layer. The charge storage capacity consequently becomes substantially enhanced.     

Thermal and optical properties of Ge5Bi18Se77 films

Amorphous films of Ge₅Bi₁₈Se₇₇ deposited by vacuum evaporation have been studied for their thermal and optical properties. Differential scanning calorimetry (DSC) has been used to perform the thermal analysis to estimate the activation energy for crystallization (E_c) and the order of crystallization (m) of this material. The high value of (E_c), 1.672 eV, indicates good stability of the amorphous phase. The optical constants of the as-deposited, amorphous and the thermally annealed crystalline films indicate semiconducting behavior and the band gap (E_g) determined from Tauc's plot are 0.92 eV and 0.8 eV for the amorphous and crystalline films, respectively. The value of the absorption coefficient () is of the order of 10⁴cm^-1 in the optical range for both amorphous and crystalline films. The studies on optical and thermal properties confirm the suitability of these firms in phase change optical recording.     

Electrical and optical properties of annealed gallium arsenide thin films on glass substrates

The effects of annealing in vacuum on the electrical and optical properties of GaAs thin films deposited by the flash evaporation method were studied. Thin films of compound GaAs deposited upon glass substrates at room temperature were annealed in a vacuum of 2×10^–6 torr at different temperatures up to 350° C. The properties of the films depended strongly on annealing temperature. The lowest resistivity measured was about 1.6 × 10⁴ cm at an annealing temperature of about 240° C. The activation energy of as-deposited and annealed films were measured and compared. Optical absorption measurements of the asdeposited samples and the samples annealed at a temperature of 240° C were made as a function of photon energy.     

Properties of r.f. sputtered cadmium telluride thin films

CdTe thin films were prepared using r.f. magnetron sputtering in an Ar atmosphere. Substrate temperatures in the range 100–320 °C were used. XRD results showed that the films are amorphous below 200 °C while above 200 °C the firms were polycrystalline with cubic structure and grains preferentially oriented along the [1 1 1] crystallographic direction. SEM measurements showed significant enhancement of crystallite size with increase of T _s or with post-preparation annealing above 400 °C. The 5 K photoluminescence spectrum showed a broad (FWHM=80 meV) band with a maximum at 1.538 eV. This band showed significant narrowing after annealing above 400 °C suggesting that it originates from transitions involving grain boundary defects. The refractive index n was determined from the interference pattern of the optical transmission. The results agree with the values of n calculated using the Jensen theory. The absorption coefficient was determined for photon energies hE _g (the energy bandgap) from the optical transmission spectra in the absorption region using the Swanepoel theory. Several direct and indirect allowed optical transitions were identified. It was found that the transitions can be grouped into four main allowed transitions (two direct; E _o, E ₃ and two indirect; E ₁, E ₂) whose energy values vary from one sample to another due the quantum size effect associated with small grain size. The main transitions are: E _o (1.50–1.77 eV) assigned to ₈ valence band (VB)₆ conduction band (CB) transition, E ₁ (1.84–2.05 eV) assigned to L_4,5(VB) _i transition where _i is an impurity level at 1.2 eV above the ₈ (VB), E ₂ (2.37–2.49 eV) assigned to L_4,5 (VB)₆ (CB) transition and E ₃ (2.25–2.55 eV) assigned to ₇ (VB) _i transition. The impurity is attributed to native centers or grain-boundary-related defects.