Effects of the sulfidation temperature on the structure,composition and optical properties of ZnS films prepared by sulfurizing ZnO films

Nanocrystalline ZnS films have been prepared by sulfidation of the reactive magnetron sputtered ZnO films. The structure, composition and optical properties of the sulfurized ZnO films as a function of the sulfidation temperature (T_S) have been systematically studied. It is found that at T_S ⩾ 400 °C ZnO is completely converted to ZnS with the hexagonal structure. The ZnS films have a strongly (0 0 2) preferred orientation and an optical transparency of about 80% in the visible region. In addition, at T_S < 444.6 °C (boiling point of sulfur), some residual sulfur decomposed from H₂S gas can adhere to the sulfurized film surface while at T_S = 580 °C a S/Zn ratio much higher than the ideal stoichiometric proportion of ZnS is obtained for the ZnS films. ZnS films with a minimum XRD FWHM value of 0.165° and a good S/Zn ratio of 0.99 are obtained at a temperature of 500 °C indicating the ZnS films to be suitable for use in the thin film solar cells.     

Influence of thermal annealing on optical properties and surface morphology of NiOx thin films

NiO_x thin films were deposited by reactive DC-magnetron sputtering from a nickel metal target in Ar + O₂ with the relative O₂ content of 5%. Thermal annealing effects on optical properties and surface morphology of NiO_x films were investigated by X-ray photoelectron spectroscopy, thermogravimetric analysis, scanning electron microscope and optical measurement. The results showed that the changes in optical properties and surface morphology depended on the temperature. The surface morphology of the films changed obviously as the annealing temperature increased due to the reaction NiO_x → NiO + O₂ releasing O₂. The surface morphology change was responsible for the variation of the optical properties of the films. The optical contrast between the as-deposited films and 400 °C annealed films was about 52%. In addition, the relationship of the optical energy band gap with the variation of annealing temperature was studied.     

Two-step growth of Cd0.96Zn0.04Te/Si(111) epilayers by HWE

High quality Cd_1−xZn_xTe, x = 0.04 epilayers are successfully grown directly on hydrogen-terminated Si(111) substrates by hot wall epitaxy method. Growth conditions are optimized in order to grow single crystal films with desired composition. It is found that surface morphology of the epilayers is dramatically affected by the growth temperature and the growth rate at the early stage of the crystal growth. Applying limited high substrate temperature of T_sub = 440 °C and low growth rate of 0.04 μm/h, the crystallinity is significantly improved and for the first time a pseudomorphic 2D growth is observed notwithstanding of the large lattice mismatch. Designing a suitable two-step growth process makes it possible that Cd_1−xZn_xTe/Si(111), x = 0.04 epilayers are fabricated with good surface morphology, which could be used as lattice matched substrates for MCT and MCZT epitaxy.     

New natively textured surface Al-doped ZnO-TCOs for thin film solar cells via magnetron sputtering

Natively textured surface aluminum doped zinc oxide (ZnO:Al) thin films were directly deposited via pulsed direct current (DC) reactive magnetron sputtering on glass substrates. During the reactive sputtering process, the oxygen gas flow rate was varied from 8.5 sccm to 11.0 sccm. The influences of oxygen flow rate on the structural, electrical and optical properties of naturally textured ZnO:Al TCO thin films with milky surface were investigated in detail. Gradual oxygen growth (GOG) technique was developed in the reactive sputtering process for textured ZnO:Al thin films. The light-scattering ability and optical transmittance of the natively textured ZnO:Al TCO thin films can be improved through gradual oxygen growth method while maintaining a low sheet resistance. Typical natively textured ZnO:Al TCO thin film with crater-like surface exhibits low sheet resistance (R_s ～ 4 Ω), high transmittance (T_a > 85%) in visible optical region and high haze value (12.1%).     

Properties of highly oriented K(Sr,Ba)2Nb5O15 thin films derived from a metal-alkoxide precursor solution

Highly oriented tungsten–bronze K(Sr,Ba)₂Nb₅O₁₅ (KSBN) thin films have been fabricated by a chemical solution deposition method. Alkoxy-derived K(Sr_0.5Ba_0.5)₂Nb₅O₁₅ (KSBN50) thin films directly crystallized into a tetragonal tungsten–bronze phase on fused silica, MgO(1 0 0), and Pt(1 0 0)/MgO(1 0 0) substrates with c-axis preferred orientation at 700 °C by optimizing the processing conditions. Ferroelectric KSBN50 thin films on Pt(1 0 0)/MgO(1 0 0) exhibited the diffuse-phase transition and typical relaxor-type dielectric behavior, which are characteristic properties along the c-axis of the tungsten–bronze (Sr,Ba)Nb₂O₆ crystal. The KSBN thin films synthesized on fused silica and MgO(1 0 0) showed high transparency over a wide wavelength range. The propagation modes of the synthesized KSBN thin films were characterized by the prism coupling method. The values of their refractive indices in TE and TM modes were 2.27 and 2.25, respectively.     

Effect of substrate temperature on the properties of pyrolytically deposited nitrogen-doped zinc oxide thin films

The effect of substrate temperature (T_s) on the properties of pyrolytically deposited nitrogen (N) doped zinc oxide (ZnO) thin films was investigated. The T_s was varied from 300 °C to 500 °C, with a step of 50 °C. The positive sign of Hall coefficient confirmed the p-type conductivity in the films deposited at 450 °C and 500 °C. X-ray diffraction studies confirmed the ZnO structure with a dominant peak from (1 0 0) crystal plane, irrespective of the variation in T_s. The presence of N in the ZnO structure was evidenced through X-ray photoelectron spectroscopy (XPS) analysis. The obtained high N concentration reveals that the 450 °C is the optimal T_s. Atomic force microscope (AFM) analysis showed that the surface roughness was increased with the increasing T_s until 400 °C but then decreased. It is found that the transmittance of the deposited films is increased with the increasing T_s. The optical band gap calculated from the absorption edge showed that the films deposited with T_s of 300 °C and 350 °C possess higher values than those deposited at higher T_s.     

Low temperature synthesis,crystal structure and thermal stability studies of nanocrystalline VN particles

The simultaneous thermal decomposition and nitridation of [VO(NH₂O)₂Gly]·H₂O complex in NH₃ atmosphere at 723–973 K gives the nanocrystalline vanadium nitride (VN) having crystallite size of 8–32 nm. It shows cubic NaCl structure with lattice parameter of a = 4.137 nm. XRD pattern Rietveld analysis program for crystal structure of VN shows the space group-Fm3m. The particle sizes measured by BET and SEM techniques are in the range of 26–100 nm. The particles are spherical and distributed homogeneously and found larger than XRD crystallite size because of agglomeration of crystallites. The fundamental IR absorption of VN material is found at 995 cm⁻¹ which gives the force constant of 634.3 Nm⁻¹. The electrical resistivity and magnetic studies show the superconducting to normal transition (T_c) at 9.2 K. Thermal decomposition of VN is carried out in O₂ atmosphere which goes through the formation of an oxynitride (V–N_p–O_q) intermediate phase up to 913 K. Finally, nanocrystalline V₂O₅ is formed at 973 K. The V₂O₅ has orthorhombic structure with lattice parameters of a = 11.537, b = 3.568 and c = 4.380 Å and the XRD crystallite size of 10 nm.     

Optical constants of thermally evaporated Se–Sb–Te films using only their transmission spectra

Amorphous Se_82 ? xTe₁₈Sb_x thin films with different compositions (x = 0, 3, 6 and 9 at.%) were deposited onto glass substrates by thermal evaporation. The transmission spectra, T(λ), of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. Based on the use of the maxima and minima of the interference fringes, a straightforward analysis proposed by Swanepoel has been applied to derive the optical constants and the film thickness. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. Tauc relation for the allowed non-direct transition describes the optical transition in the studied films. With increasing antimony content the refractive index increases while the optical band gap decreases. The optical band gap decreases from 1.62 to 1.26 eV with increasing antimony content from 0 to 9 at.%. The chemical-bond approach has been applied successfully to interpret the decrease of the optical gap with increasing antimony content.     

Preparation of calcium-doped boron nitride by pulsed laser deposition

Calcium-doped BN thin films Ca_xBN_y (x = 0.05–0.1, y = 0.7–0.9) were grown on α-Al₂O₃(0 0 1) substrates by pulsed laser deposition (PLD) using h-BN and Ca₃N₂ disks as the targets under nitrogen radical irradiation. Infrared ATR spectra demonstrated the formation of short range ordered structure of BN hexagonal sheets, while X-ray diffraction gave no peak indicating the absence of long-range order structure in the films. It was notable that Ca-doped film had 5.45–5.55 eV of optical band gap, while the band gap of Ca-free films was 5.80–5.85 eV. This change in the band gap is ascribed to interaction of Ca with the BN sheets; first principle calculations on h-BN structure indicated that variation of inter-plane distance between the BN layers did not affect the band gap. This study highlights that PLD could prepare BN having short-range structure of h-BN sheets and being doped with electropositive cation which varies the optical band gap of the films.     

Optical constants and band gap determination of Pb0.95La0.05Zr0.54Ti0.46O3 thin films using spectroscopic ellipsometry and UV–visible spectroscopy

We report the structural evolution and optical properties of lanthanum doped lead zirconate titanate (PLZT) thin films prepared on Pt/TiO₂/SiO₂/Si substrates by chemical solution deposition. X-ray diffraction demonstrates the post-deposition annealing induced crystallization for PLZT films annealed in a temperature (T_a) range of 550–750 °C. PLZT films annealed at higher temperature exhibit polycrystalline structure along with larger grain size. Optical band gap (E_g) values determined from UV–visible spectroscopy and spectroscopic ellipsometry (SE) for PLZT films were found to be in the range of 3.5–3.8 eV. E_g decreases with increasing T_a. The optical constants and their dispersion profiles for PLZT films were also determined from SE analyses. PLZT films show an index of refraction in the range of 2.46–2.50 (λ = 632.8 nm) with increase in T_a. The increase in refractive index at higher T_a is attributed to the improved packing density and crystallinity with the temperature.     

Thickness dependence of optical parameters for Ge–Se–Te thin films

The present work deals with thickness dependent study of the thin films of Ge₁₀Se_90 − xTe_x (x = 0, 10) chalcogenide glasses. Bulk samples of Ge₁₀Se₉₀ and Ge₁₀Se₈₀Te₁₀ have been prepared by melt quenching technique. Thin films (thickness d = 800 nm and 1100 nm) of the prepared samples have been deposited on glass substrate using vacuum evaporation technique. The optical parameters i.e. optical band gap (E_g^opt), absorption coefficient (α), refractive index (n) and extinction coefficient (k) are calculated from the transmission spectrum in the range 400–1500 nm. The optical band gap decreases with the increase of thickness from 1.87 ± 0.01 eV (d = 800 nm) to 1.80 ± 0.01 eV (d = 1100 nm) for Ge₁₀Se₉₀ and from 1.62 ± 0.01 eV (d = 800 nm) to 1.48 ± 0.01 eV (d = 1100 nm) for Ge₁₀Se₈₀Te₁₀ thin films.     

Spray pyrolysis deposition of nanostructured zirconia thin films

Polycrystalline zirconia thin films were obtained on silica substrates by the spray pyrolysis technique using a water/isopropanol solution of a precursor containing zirconium in the form of an anionic oxalate complex. The as-deposited products were amorphous. Crystallization with formation of homogeneous dense nanostructured cubic zirconia thin films occurred after heat-treatment in air at temperatures T=500–700 °C. In the range 700–1000 °C both cubic and monoclinic zirconia was present in the films, the C-ZrO₂ content decreasing with temperature rise. Penetration of SiO₂ from the silica substrate was registered in the films by X-ray photoelectron spectroscopy (XPS). A severe attack of the substrate by zirconia resulting in formation of ZrSiO₄ was observed after annealing of the film at 1100 °C.     

Spectroscopic ellipsometry and magneto-transport investigations of Mn-doped ZnO nanocrystalline films deposited by a non-vacuum sol–gel spin-coating method

Nanocrystalline Zn_1?xMn_xO films (x = 0, 0.05, 0.1, 0.15, and 0.2) were deposited onto corning glass substrates by a non-vacuum sol–gel spin coating method. All of the films were annealed at 450 °C for 2 h. The structural, optical and magneto-transport properties were investigated by X-ray diffraction, spectroscopic ellipsometry and a system for the measurement of the physical properties. X-ray diffraction analysis of the films reveals that the Mn-doped ZnO films crystallize in the form of a hexagonal wurtzite-type structure with a crystallite size decreases with an increase of the Mn concentration. It was also found that the microstrain increases with the increase of the Mn content. Evidence of nanocrystalline nature of the films was observed from the investigation of surface morphology using transmission, scanning electron microscopy and atomic force microscopy. The optical constants and film thicknesses of nanocrystalline Zn_1?xMn_xO films were obtained by fitting the spectroscopic ellipsometric data (ψ and Δ) using a three-layer model system in the wavelength range from 300 to 1200 nm. The refractive index was observed to increase with increasing Mn concentration. This increase in the refractive index with increasing Mn content may be attributed to the increase in the polarizability due to the large ionic radius of Mn²⁺ compared to the ionic radius of Zn²⁺. The optical band gap of the nanocrystalline Mn–ZnO films was determined by an analysis of the absorption coefficient. The direct transition of the series of films was observed to have energies increasing linearly from 3.17 eV (x = 0) to 3.55 eV (x = 0.2). Magnetoresistance (MR) was measured from 5 K to 300 K in a magnetic field of up to 6 T. Low-field positive MR and high-field negative MR were detected in Mn-doped ZnO at 5 K. Only negative MR was observed for temperatures above 200 K. The positive MR in Mn-doped ZnO films was observed to decrease drastically when the temperature increased from 5 K to 100 K. The isothermal MR of Zn_1?xMn_xO films with different Mn concentrations at 5 K reveals that the increase of the Mn content induces a giant positive MR above x = 0.05 and reaches up to 55% at an applied field of 30 kOe for x = 0.2.     

Growth of the Zn1−xMnxO alloy by the MOCVD technique

Single-phase thin films of the diluted magnetic semiconductor Zn_1−xMn_xO have been grown by the MOCVD technique. Depositions have been done at T = 450 °C on fused silica and (0 0 0 1) sapphire substrates. Layers on silica exhibit polycrystalline structure with [0 0 1] preferential orientation while Zn_1−xMn_xO films are (0 0 0 1) epitaxially grown on c-sapphire with the epitaxy relation: 30° rotation of the Zn_1−xMn_xO [1 0 0] direction with respect to the [1 0 0] of the substrate. The manganese content varies in the (0–30%) range and is always higher in samples grown on sapphire substrates under the same conditions. Variations of a and c lattice parameters, assessed by X-ray diffraction, follow Vegard's law and attest to the incorporation of substitutional Mn²⁺ ions. Hall effect measurements show a decrease of the mobility with the incorporation of manganese in ZnO, and optical transmission results present the shift of the absorption edge towards higher energies.     

Al2O3–TiO2 thin films on glass substrate by sol–gel technique

In the present research, self-cleaning Al₂O₃–TiO₂ thin films were successfully prepared on glass substrate using a sol–gel technique for photocatalytic applications. We investigated the phase structure, microstructure, adhesion and optical properties of the coatings by using XRD, SEM, scratch tester and UV/Vis spectrophotometer. Four different solutions were prepared by changing Al/Ti molar ratios such as 0, 0.07, 0.18 and 0.73. Glass substrates were coated by solutions of Ti-alkoxide, Al-chloride, glacial acetic acid and isopropanol. The obtained gel films were dried at 300 °C for 10 min and subsequently heat-treated at 500 °C for 5 min in air. The oxide thin films were annealed at 600 °C for 60 min in air. TiO₂, Ti₃O₅, TiO, Ti₂O, α-Al₂O₃ and AlTi phases were determined in the coatings. The microstructural observations demonstrated that Al₂O₃ content improved surface morphology of the films and the thickness of film and surface defects increased in accordance with number of dipping. It was found that the critical load values of the films with 0, 0.07, 0.18 and 0.73 Al/Ti molar ratios were found to be 11, 15, 22 and 28 mN, respectively. For the optical property, the absorption band of synthesized powders shifted from the UV region to the visible region according to the increase of the amount of Al dopant. The oxide films were found to be active for photocatalytic decomposition of methylene blue.     

Effects of high-temperature annealing on magnetic properties of (Zn0.91Mn0.09)O thin films grown by RF magnetron sputtering

The (Zn_0.91Mn_0.09)O thin film annealed at 1000 °C for 60 min in N₂ atmosphere showed two kinds of Curie temperature (T_C) values of T_C1 around 130 K and T_C2 above 300 K while the as-grown (Zn_0.91Mn_0.09)O thin film showed typical ferromagnetism with the T_C of 108 K. It is expected that the increase of T_C up to T_C1 might be attributed to the enhancement of crystal magnetic anisotropy because the increases of (0 0 0 l) peak for X-ray diffraction patterns and Mn²⁺-related emission for photoluminescence spectra were observed for annealed (Zn_0.91Mn_0.09)O thin films. The increase of T_C above T_C2 might be originated from the formation of nano-sized (Zn_0.91Mn_0.09)O islands because it was confirmed that the nano-sized (Zn_0.91Mn_0.09)O islands were formed after annealing treatment and that they revealed clearly the magnetic domains at 300 K for the measurements of atomic force microscopy and magnetic force microscopy, respectively. The room temperature ferromagnetism in the nano-sized (Zn_0.91Mn_0.09)O islands is considered to be originated from the inhomogeneous distribution of Mn ions.     

Structural and magnetic study of thin films based on anisotropic ternary alloys FeNiPt2

L1₀ ordered (Fe–Ni)₅₀Pt₅₀ alloy films with perpendicular magnetic anisotropy were successfully prepared by interdiffusing FePt(0 0 1) and NiPt(0 0 1) layers co-deposited on MgO(0 0 1) substrates by MBE. The [0 0 1] growth direction corresponds to the epitaxy of the alloy on the substrate and is the interesting growth orientation to get a perpendicular magnetization. The X-ray diffraction shows a high L1₀ chemical order (S = 0.7 ± 0.1). The easy magnetization direction is perpendicular for all samples. The MFM images display highly interconnected stripes corresponding to up and down orientations of the magnetization. Large uniaxial magnetic anisotropy (K_u = 9.10⁵ J/m³) and suitable magnetic transition temperature (T_C = 400 K) are obtained. The addition of Ni changes the spin–orbit interaction in the FePt compound system, hence causes a decrease of anisotropy, saturation magnetization and coercivity.     

Structural and optical properties of In doped Se–Te phase-change thin films: A material for optical data storage

Se_75−xTe₂₅In_x (x = 0, 3, 6, & 9) bulk glasses were obtained by melt quench technique. Thin films of thickness 400 nm were prepared by thermal evaporation technique at a base pressure of 10⁻⁶ Torr onto well cleaned glass substrate. a-Se_75−xTe₂₅In_x thin films were annealed at different temperatures for 2 h. As prepared and annealed films were characterized by X-ray diffraction and UV–Vis spectroscopy. The X-ray diffraction results show that the as-prepared films are of amorphous nature while it shows some poly-crystalline structure in amorphous phases after annealing. The optical absorption spectra of these films were measured in the wavelength range 400–1100 nm in order to derive the extinction and absorption coefficient of these films. It was found that the mechanism of optical absorption follows the rule of allowed non-direct transition. The optical band gap of as prepared and annealed films as a function of photon energy has been studied. The optical band gap is found to decrease with increase in annealing temperature in the present glassy system. It happens due to crystallization of amorphous films. The decrease in optical band gap due to annealing is an interesting behavior for a material to be used in optical storage. The optical band gap has been observed to decrease with the increase of In content in Se–Te glassy system.     

Synthesis and characteristics of nature-superlattice-structured Bi3TiNbO9–Bi4Ti3O12 ferroelectric thin films by MOD

Natural-superlattice-structured ferroelectric thin films, Bi₃TiNbO₉–Bi₄Ti₃O₁₂ (BTN–BIT), have been synthesized on Pt/Ti/SiO₂/Si by metal organic decomposition (MOD) using BTN–BIT (1 mol:1 mol) solution. BTN–BIT films show natural-superlattice peaks below 2θ = 20° in X-ray diffraction patterns, which indicate that the BTN–BIT films annealed at 700–800 °C in O₂ ambient are consisted of iteration of two unit cells of Bi₃TiNbO₉ and one unit cell of Bi₄Ti₃O₁₂. As the annealing temperature increases from 600 to 750 °C, uniform and crack-free films, better crystallinity and ferroelectric properties can be obtained, but the pyrochlore phase in BTN–BIT films annealed over 800 °C would impair the ferroelectric properties. With the increase of O₂ flow rate from 0.5 to 1.5 L/min, both remanent polarization P_r and coercive electric field E_C increase, which are mainly attributed to reduction of the vacanvies of Bi and oxide ions in the films. Natural-superlattice-structured BTN–BIT thin films having 2–1 superlattice annealed at 750 °C in O₂ ambient with a flow rate of 1.5 L/min exhibit superior ferroelectric properties of P_r = 23.5 μC/cm² and E_C = 135 kV/cm.     

Influence of Al2O3 on low-field spin-polarized tunneling magnetoresistance of (1 − x) La0.7Ca0.3MnO3 + x Al2O3 composites

In this study we report the effect of Al₂O₃ on the low field magnetoresistance (LFMR) of (1 ? x) La_0.7Ca_0.3MnO₃ + x Al₂O₃ composite synthesized through a solid-state reaction method combined with an energy milling method. Based upon a spin-polarized tunneling of conduction electrons at the grain boundaries, we have proposed a phenomenological model to explain the observed electrical transport behavior over the whole temperature range (5  300 K), especially the gradual drop of metal-insulator transition temperature (T_p = T_max) as a function of increasing Al₂O₃ content, while the ferromagnetic–paramagnetic transition temperature (T_C) remains almost constant (T_C = 250 K).