Effect of ambient hydrogen sulfide on the physical properties of vacuum evaporated thin films of zinc sulfide

Evaporated thin films of zinc sulfide (ZnS) have been deposited in a low ambient atmosphere of hydrogen sulfide (H₂S ∼10⁻⁴ Torr). The H₂S atmosphere was obtained by a controlled thermal decomposition of thiourea [CS(NH₂)₂] inside the vacuum chamber. It has been observed that at elevated substrates temperature of about 200 °C helps eject any sulfur atoms deposited due to thermal decomposition of ZnS during evaporation. The zinc ions promptly recombine with H₂S to give better stoichiometry of the deposited films. Optical spectroscopy, X-ray diffraction patterns and scanning electron micrographs depict the better crystallites and uniformity of films deposited by this technique. These deposited films were found to be more adherent to the substrates and are pinhole free, which is a very vital factor in device fabrication.     

Effect of sintering aid (CdCl2) on the optical and structural properties of CdZnS screen-printed film

Sulphides of zinc and cadmium have been utilized effectively in various opto-electronic devices. In the present work cadmium zinc sulphide (Cd_0.4Zn_0.6S) thin film has been deposited on ultra clean glass substrate by a simple inexpensive screen-printing method using cadmium sulphide, zinc sulphide, anhydrous cadmium chloride and ethylene glycol. Cadmium chloride has been used as sintering aid and ethylene glycol as a binder. Effect of sintering aid on the optical and structural properties of prepared cadmium zinc sulphide film has been investigated. The optical band gap (E_g) of the film has been studied by using reflection spectra in wavelength range 325–600 nm. It is found that reflection spectra suffer a drastic fall at two places, which is indicative of two band gaps of film viz. 2.38 eV and 2.9 eV corresponding to CdS and Cd_0.6Zn_0.4S, respectively. This is suggestive of the fact that cadmium zinc sulphide is a wide band gap semiconducting material. X-ray diffraction also confirms the formation of Cd_0.6Zn_0.4S composition.     

Ion-induced secondary electron emission from ZnS thin films deposited by closed-spaced sublimation

ZnS thin films were deposited on soda lime glass and aluminum substrates by close-spaced sublimation technique. The change in composition, structural and optical properties of the films was investigated as a function of the substrate temperature. The deposited films were stoichiometric and crystalline in nature having cubic structure oriented only along (1 1 1) plane. The energy band gap of the films deposited at the substrate temperature of 150, 250 and 350 °C was 3.52, 3.58 and 3.63 eV respectively. These films were then bombarded with 2-10 keV energy pulsed Ar⁺ beam and their electron yield was determined from impinging ion and emitted electron currents. The electron yield of ZnS films was much high as compared to the metals. The electron yield of ZnS films increased with energy of the incident ion and got saturated at about 8 keV. The most important result of this study was that the electron yield of ZnS films having same composition was different. Monte Carlo simulations performed to interpret these experimental findings showed that the dissimilar electron yields of ZnS films is due to the combined effect of energy band gap, surface barrier potential and density of the films.     

Effect of deposition parameters on structural, optical and electrical properties of nanocrystalline ZnSe thin films

This paper presents the chemical bath deposition of zinc selenide (n-ZnSe) nanocrystalline thin films on non-conducting glass substrates, in an aqueous alkaline medium using sodium selenosulphate as Se²⁻ ion source. The X-ray diffraction studies show that the deposited ZnSe material is nanocrystalline with a mixture of hexagonal and cubic phase. The direct optical band gap ‘E_g’ for the as-deposited n-ZnSe films is found to be 3.5 eV. TEM studies show that the ZnSe nanocrystals (NCs) are spherical in shape. Formation of ZnSe has been confirmed with the help of infrared (IR) spectroscopy by observing bands corresponding to the multiphonon absorption. We demonstrate the effect of the deposition temperature and reactant concentration on the structural, optical and electrical properties of ZnSe films.     

Maxwell stress distributions in chiral sculptured thin films due to obliquely incident plane waves

When a circularly polarized plane wave is obliquely incident on a chiral sculptured thin film (STF), the Maxwell stress dyadic exhibits a decreasing periodic variation across the thickness of the film. Normal and tangential surface tractions exist on the two faces of the chiral STF, as well as a net normal pressure across the film. These stresses are affected by the incidence angle of light, and are maximized when (i) the incident plane wave and the chiral STF are co-handed, (ii) the wavelength falls within a regime called the Bragg regime, (iii) the ratio of film thickness to the structural period of the chiral STF reaches a saturation value, and (iv) the deviation from normal incidence is small.     

Highly efficient transparent Zn2SiO4:Mn phosphor film on quartz glass

Highly efficient transparent Zn₂SiO₄:Mn²⁺ film phosphors on quartz substrates were deposited by the thermal diffusion of sputtered ZnO:Mn film. They show a textured structure with some preferred orientations. Our film phosphor shows, for the best photoluminescence (PL) brightness, a green PL brightness of about 20% of a commercial Zn₂SiO₄:Mn²⁺ powder phosphor screen. The film shows a high transmittance of more than 10% at the red-color region. The excellence in PL brightness and transmittance can be explained in terms of the textured crystal growth with a continuous gradient of Zn₂SiO₄: Mn²⁺ crystals.     

Optical properties of ZnO and ZnO:Ce layers grown by spray pyrolysis

In this paper, zinc oxide (ZnO) and cerium-doped zinc oxide (ZnO:Ce) films were deposited by reactive chemical pulverization spray pyrolysis technique using zinc and cerium chlorides as precursors. The effects of Ce concentration on the structural and optical properties of ZnO thin films were investigated in detail. These films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) measurements. All deposited ZnO layers at the temperature 450 °C are polycrystalline and indicate highly c-axis oriented structure. The dimension of crystallites depends on incorporation of Ce atoms into the ZnO films. The photoluminescence spectra of the films have been studied as a function of the deposition parameters such as doping concentrations and post grows annealing. Photoluminescence spectra were measured at the temperature range from 13 K to 320 K.     

Crystalline and photoluminescence characteristics of YVO4:Sm thin films grown by pulsed laser deposition under oxygen pressure

YVO₄:Sm³⁺ films were deposited on Al₂O₃ (0 0 0 1) substrates at various oxygen pressures changing from 13.3 to 46.6 Pa by using the pulsed laser deposition method. The crystallinity and surface morphology of these films were investigated by means of X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. The XRD pattern confirmed that YVO₄:Sm³⁺ film has zircon structure and the AFM study revealed that the films consist of homogeneous grains ranging from 100 to 400 nm. The room temperature photoluminescence (PL) spectra showed that the emitted radiation was dominated by a reddish-orange emission peak at 602 nm radiating from the transition of (⁴G_5/2→⁶H_7/2). The crystallinity, surface morphology, and photoluminescence spectra of thin-film phosphors were highly dependent on the deposition conditions, in particular, the substrate temperature. The surface roughness and photoluminescence intensity of these films showed similar behavior as a function of oxygen pressure.     

Switching phenomenon and optical properties of Se85Te10Bi5 films

Se₈₅Te₁₀Bi₅ films of different thicknesses ranging from 126 to 512 nm have been prepared. Energy-dispersive X-ray (EDX) spectroscopy technique showed that films are nearly stoichiometric. X-ray diffraction (XRD) measurements have showed that the Se₈₅Te₁₀Bi₅ films were amorphous. Electrical conduction activation energy (ΔE_σ) for the obtained films is found to be 0.662 eV independent of thickness in the investigated range. Investigation of the current voltage (I-V) characteristics in amorphous Se₈₅Te₁₀Bi₅ films reveals that it is typical for a memory switch. The switching voltage V_th increases with the increase of the thickness and decreases exponentially with temperature in the range from 298 to 383 K. The switching voltage activation energy (ε) calculated from the temperature dependence of V_th is found to be 0.325 eV. The switching phenomenon in amorphous Se₈₅Te₁₀Bi₅ films is explained according to an electrothermal model for the switching process. The optical constants, the refractive index (n) and the absorption index (k) have been determined from transmittance (T) and reflectance (R) of Se₈₅Te₁₀Bi₅ films. Allowed non-direct transitions with an optical energy gap (E_g^opt) of 1.33 eV have been obtained. ΔE_σ is almost half the obtained value of E_g^opt, which suggested band to band conduction as indicated by Davis and Mott.     

Luminescent characteristics of CaTiO3:Pr thin films prepared by pulsed laser deposition method with various substrates

CaTiO₃:Pr³⁺ films were deposited on different substrates such as Al₂O₃ (0 0 0 1), Si (1 0 0), MgO (1 0 0), and fused silica using pulsed laser deposition method. The crystallinity and surface morphology of these films were investigated by XRD and SEM measurements. The films grown on the different substrates have different crystallinity and morphology. The FWHM of (2 0 0) peak are 0.18, 0.25, 0.28, and 0.30 for Al₂O₃ (0 0 0 1), Si (1 0 0), MgO (1 0 0), and fused silica, respectively. The grain sizes of phosphors grown on different substrates were estimated by using Scherrer's formula and the maximum crystallite size observed for the thin film grown on Al₂O₃ (0 0 0 1). The room temperature PL spectra exhibit only the red emission peak at 613 nm radiated from the transition of (¹D₂ → ³H₄) and the maximum PL intensity for the films grown on the Al₂O₃ (0 0 0 1) is 1.1, 1.4, and 3.7 times higher than that of the CaTiO₃:Pr³⁺ films grown on MgO (1 0 0), Si (1 0 0), and fused Sillica substrates, respectively. The crystallinity, surface morphology and luminescence spectra of thin-film phosphors were highly dependent on substrates.     

The preparation and refractive index of BST thin films

Radio-frequency magnetron sputtering technique is used to deposit Ba_0.65Sr_0.35TiO₃ (BST) thin films on fused quartz substrates. In order to prepare the high-quality BST thin films, the crystallization and microstructure of the films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). More intense characteristic diffraction peaks and better crystallization can be observed in BST thin films deposited at 600 °C and subsequently annealed at 700 °C. The refractive index of the films is determined from the measured transmission spectra. The dependences of the refractive index on the deposition parameters of BST thin films are different. The refractive index of the films increases with the substrate temperature. At lower sputtering pressure, the refractive index increases from 1.797 to 2.197 with pressure increase. However, when the pressure increases up to 3.9 Pa, the refractive index reduces to 1.86. The oxygen to argon ratio also plays an important effect on the refractive index of the films. It has been found that the refractive index increases with increase in the ratio of oxygen to argon. The refractive index of BST thin films is strongly dependent on the annealing temperature, which also increases as the annealing temperature ascends. In a word, the refractive index of BST thin films is finally affected by the films’ microstructure and texture.     

Ellipsometric characterization of PbI2 thin film on glass

The optical properties of polycrystalline lead iodide thin film grown on Corning glass substrate have been investigated by spectroscopic ellipsometry. A structural model is proposed to account for the optical constants of the film and its thickness. The optical properties of the PbI₂ layer were modeled using a modified Cauchy dispersion formula. The optical band gap E_g has been calculated based on the absorption coefficient (α) data above the band edge and from the incident photon energy at the maximum index of refraction. The band gap was also measured directly from the plot of the first derivative of the experimental transmission data with respect to the light wavelength around the transition band edge. The band gap was found to be in the range of 2.385±0.010 eV which agrees with the reported experimental values. Urbach's energy tail was observed in the absorption trend below the band edge and was found to be related to Urbach's energy of 0.08 eV.     

Enhanced luminescent characteristics of laser-ablated GdVO4:Eu thin films by Li-doping

Li-doping has been used to improve luminescent characteristics of thin films. Influence of Li-doping on the crystallization, surface morphology and luminescent properties of GdVO₄:Eu³⁺ films have been investigated. Crystallinity and surface morphology of thin films have been very important factors to determine luminescent characteristics and depended on the deposition conditions. The GdVO₄:Eu³⁺ and Li-doped GdVO₄:Eu³⁺ thin films have been grown using pulsed laser deposition method on Al₂O₃ (0 0 0 1) substrates at a substrate temperature of 600 °C under an oxygen pressure of 13.33-53.33 Pa. The crystallinity and surface morphology of the films were investigated using X-ray diffraction (XRD) and atomic force microscope (AFM), respectively. A broadband incoherent ultraviolet light source with a dominant excitation wavelength of 310 nm and a luminescence spectrometer have been used to measure photoluminescence spectra at room temperature. The emitted radiation was dominated by the red emission peak at 619 nm radiated from the transition of ⁵D₀-⁷F₂ of Eu³⁺ ions. Particularly, the peak intensity of Li-doped GdVO₄ films was increased by a factor of 1.7 in comparison with that of GdVO₄:Eu³⁺ films. The enhanced luminescence results not only from the improved crystallinity but also from the reduced internal reflections caused by rougher surfaces. The luminescent intensity and surface roughness exhibited similar behavior as a function of oxygen pressure.     

Mg substitutions in ZnO-Al2O3 thin films and its effect on the optical absorption spectra of the nanocomposite

ZnO-Al₂O₃ nanocomposite thin films were prepared by sol-gel technique. The room temperature synthesis was mainly based on the successful peptization of boehmite (AlO(OH)) and Al(OH)₃ compounds, so as to use it as matrix to confine ZnO nanoparticles. The relative molar concentrations of xZnO to (1 − x) Al₂O₃ were varied as x = 0.1, 0.2 and 0.5. The optical absorption spectra of the thin films showed intense UV absorption peaks with long tails of variable absorption in the visible region of the spectra. The ZnO-Al₂O₃ nanocomposites thin films were doped with MgO by varying its molar concentrations as y = 0.05, 0.75, 0.1, 0.125, 0.15 and 0.2 with respect to the ZnO present in the composite. The MgO doped thin films showed suppression of the intense absorption peaks that was previously attained for undoped samples. The disappearance of the absorption peaks was analyzed in terms of the crystalline features and lattice defects in the nanocomposite system. The bulk absorption edge, which is reportedly found at 3.37 eV, was shifted to 5.44 eV (for y = 0.05), 5.63 eV (for y = 0.075) and maximum to 5.77 eV (for y = 0.1). In contrast, beyond the concentration, y = 0.1 the absorption edges were moved to 5.67 eV (for y = 0.125), 5.61 eV (for y = 0.15) and to 5.49 eV (for y = 0.2). This trend was explained in terms of the Burstein-Moss shift of the absorption edges.     

Highly enhanced photoluminescence and X-ray excited luminescence of Li doped Gd2O3:Eu thin films

Transparent Li-doped Gd₂O₃:Eu³⁺ thin-film phosphors were prepared by a modified sol-gel method. The effect of the Li⁺ ions on luminescent properties of the thin film was investigated. The results indicated that incorporation of Li⁺ ions into Gd₂O₃ lattice could result in a remarkable increase on photoluminescence or X-ray excited luminescence, and the strongest emission was observed from Gd_1.84Li_0.08Eu_0.08O_3−δ film, in which the intensity was increased by a factor of 1.9 or 2.3 in comparison with that of Gd_1.92Eu_0.08O₃ film. And it could be achieved the highest intensity for sintering the Gd_1.84Li_0.08Eu_0.08O_3−δ film at 700 °C. Such a temperature is much lower than the typical solid-state reaction temperature for its powder phosphors. This kind of transparent thin-film phosphors may promise for application to micro X-ray imaging system.     

The Al-doping and post-annealing treatment effects on the structural and optical properties of ZnO:Al thin films deposited on Si substrate

Al-doped ZnO (ZnO:Al) thin films with different Al contents were deposited on Si substrates using the radio frequency reactive magnetron sputtering technique. X-ray diffraction (XRD) measurements showed that the crystallinity of the films was promoted by appropriate Al content (0.75 wt.%). Then the ZnO:Al film with Al content of 0.75 wt.% was annealed in vacuum at different temperatures. XRD patterns revealed that the residual compressive stress decreased at higher annealing temperatures. While the surface roughness of the ZnO:Al film annealed at 300 °C became smoother, those of the ZnO:Al films annealed at 600 and 750 °C became rougher. The photoluminescence (PL) measurements at room temperature revealed a violet, two blue and a green emission. The origin of these emissions was discussed and the mechanism of violet and blue emission of ZnO:Al thin films were suggested. We concluded that the defect centers are mainly ascribed to antisite oxygen and interstitial Zn in annealed (in vacuum) ZnO:Al films.     

Microstructures and optical properties of Cu-doped ZnO films prepared by radio frequency reactive magnetron sputtering

Pure and Cu-doped ZnO (ZnO:Cu) thin films were deposited on glass substrates using radio frequency (RF) reactive magnetron sputtering. The effect of substrate temperature on the crystallization behavior and optical properties of the ZnO:Cu films have been studied. The crystal structures, surface morphology and optical properties of the films were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and a fluorescence spectrophotometer, respectively. The results indicated that ZnO films showed a stronger preferred orientation toward the c-axis and a more uniform grain size after Cu-doping. As for ZnO:Cu films, the full width at half maxima (FWHM) of (0 0 2) diffraction peaks decreased first and then increased, reaching a minimum of about 0.42° at 350 °C and the compressive stress of ZnO:Cu decreased gradually with the increase of substrate temperature. The photoluminescence (PL) spectra measured at room temperature revealed two blue and two green emissions. Intense blue-green luminescence was obtained from the sample deposited at higher substrate temperature. Finally, we discussed the influence of annealing temperature on the structural and optical properties of ZnO:Cu films. The quality of ZnO:Cu film was markedly improved and the intensity of blue peak (∼485 nm) and green peak (∼527 nm) increased noticeably after annealing. The origin of these emissions was discussed.     

Optical characterization of thermally evaporated thin films of As40S40Se20 chalcogenide glass by reflectance measurements

40 S₄₀Se₂₀, deposited by thermal evaporation, were obtained in the 400 nm to 2200 nm spectral region. The optical constants of this amorphous material were computed using an optical characterization method based mainly on the ideas of Minkov and Swanepoel of utilising the upper and lower envelopes of the spectrum, which allows us to obtain both the real and imaginary parts of the complex refractive index, and the film thickness. Thickness measurements made by a surface-profiling stylus have been carried out to cross-check the results obtained by the optical method. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model. The optical band gap has been determined from absorption coefficient data by Tauc’s procedure. Finally, the photo-induced and thermally induced changes in the optical properties of a-As₄₀S₄₀Se₂₀ thin films were also studied, using both transmission and reflection spectra. Received: 11 February 1998/Accepted: 16 February 1998     

Structural characterization and optical properties of ZnSe thin films

Zinc selenide (ZnSe) thin films (d = 0.11-0.93 μm) were deposited onto glass substrates by the quasi-closed volume technique under vacuum. Their structural characteristics were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The experiments showed that the films are polycrystalline and have a zinc blende (cubic) structure. The film crystallites are preferentially oriented with the (1 1 1) planes parallel to the substrate surface. AFM images showed that the films have a grain like surface morphology. The average roughness, R_a = 3.3-6.4 nm, and the root mean square roughness, R_rms = 5.4-11.9 nm, were calculated and found to depend on the film thickness and post-deposition heat treatment.The spectral dependence of the absorption coefficient was determined from transmission spectra, in the range 300-1400 nm.The values of optical bandgap were calculated from the absorption spectra, E_g = 2.6-2.7 eV.The effect of the deposition conditions and post-deposition heat treatment on the structural and optical characteristics was investigated.     

Write-once blue laser recording using silicon doped SbOx thin films prepared by reactive dc-magnetron sputtering

Si:SbO_x films have been deposited by reactive dc-magnetron sputtering from a Sb target with Si chips attached in Ar + O₂ with the relative O₂ content 7%. The as-deposited films contained Sb metal, Sb₂O₃, SiO, Si₂O₃ and SiO₂. The crystallization of Sb was responsible for the changes of optical properties of the films. The results of the blue laser recording test showed that the films had good writing sensitivity for blue laser beam (406.7 nm), and the recording marks were still clear even if the films were deposited in air 60 days, which demonstrated that doping silicon in SbO_x films can improve the stability of SbO_x films. High reflectivity contrast of about 36% was obtained at a writing power 6 mW and writing pulse width 300 ns.