Deposition and optical properties of nanocrystalline ZnS thin films by a chemical method

Highly transparent and adherent nanocrystalline large-area ZnS thin films were grown on the slide glass and the SnO₂-coated glass substrate by chemical deposition using an aqueous solution containing zinc sulfate and thioacetamide. The films were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, UV–Vis transmission and photoluminescence. The SEM morphologies of the films revealed a string-like structure formed by the smaller particle on the ZnS film surface. The average particle size was confirmed, using XRD analysis and TEM observation, to be about 3–4 nm.     

Laser ablation and deposition of wide bandgap semiconductors: plasma and nanostructure of deposits diagnosis

Nanostructured CdS and ZnS films on Si (100) substrates were obtained by nanosecond pulsed laser deposition at the wavelengths of 266 and 532 nm. The effect of laser irradiation wavelength on the surface structure and crystallinity of deposits was characterized, together with the composition, expansion dynamics and thermodynamic parameters of the ablation plume. Deposits were analyzed by environmental scanning electron microscopy, atomic force microscopy and X-ray diffraction, while in situ monitoring of the plume was carried out with spectral, temporal and spatial resolution by optical emission spectroscopy. The deposits consist of 25–50 nm nanoparticle assembled films but ablation in the visible results in larger aggregates (150 nm) over imposed on the film surface. The aggregate free films grown at 266 nm on heated substrates are thicker than those grown at room temperature and in the former case they reveal a crystalline structure congruent with that of the initial target material. The observed trends are discussed in reference to the light absorption step, the plasma composition and the nucleation processes occurring on the substrate.     

倾斜式生长ZnS纳米柱状薄膜及其透射性能的研究

采用倾斜式生长的方法,在本底真空为3×10-4 Pa,生长率为0.2 nm·s-1的条件下,通过改变衬底的法线方向与入射粒子流的夹角α,在ITO导电玻璃衬底上制备了ZnS纳米薄膜。在α=80°和85°时,样品的Ｘ射线衍射谱证实了不同倾斜角时所制备薄膜中均有纳米ZnS晶体形成,扫描电子显微镜(SEM)图像显示,所形成的薄膜均呈现出了柱状结构,并且倾斜角为85°时所得到的纳米柱直径大于80°时所得结果;在α=0°时,相应测量结果表明,虽然在不同衬底上也形成了纳米ZnS晶体薄膜,但并未见柱状结构,而是形成了一层均匀且致密的薄膜。对两种薄膜结构的生长动力学过程作了分析。ITO衬底上薄膜的透射光谱表明ZnS柱状薄膜能够提高可见光的透过率,因此对柱状ZnS纳米薄膜的研究将有利于提高电致发光器件的发光效率。     

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.     

Al-doped ZnS layers synthesized by solution growth method

ZnS is one of the potential candidates as a window/buffer layer for solar photovoltaic applications. Al-doped ZnS nanocrystalline films were grown by a simple and economic process, chemical solution growth method. The layers were prepared for different Al-dopant concentrations that vary in the range, 0-10 at. %. The effect of Al-doping on the composition, structure, optical, electrical and photoluminescence properties of the synthesized layers was determined using appropriate techniques. The elemental composition of a typical sample with 6 at. % ‘Al’ in ZnS was Zn = 44.9 at. %, S = 49.8 at. % and Al = 5.3 at. %. The films were nanocrystalline in nature and showed (111) plane of ZnS as the preferred orientation for all the doping concentrations. The layers with 6 at. % of Al showed a crystallite size of ∼9 nm. The FTIR studies confirmed the presence of ZnS in the layers. The layers showed an average transmittance of ∼75% in the visible region. The change of photoluminescence behaviour with dopant concentration was also studied. The electrical resistivity was considerably decreased from 10⁷ Ωcm to 10³ Ωcm with Al-doping. The detailed analysis of results will be presented and discussed.     

Green route fast synthesis and characterization of chemical bath deposited nanocrystalline ZnS buffer layers

Zinc sulphide (ZnS) thin films are deposited using chemical bath deposition method on the glass substrates in an aqueous alkaline reaction bath of zinc acetate and thiourea along with non-toxic complexing agent tri-sodium citrate at 95 °C. The results show noteworthy improvement in the growth rate of the deposited ZnS thin films and thickness of the film increases with the deposition time. From X-ray diffraction patterns, it is found that the ZnS thin films exhibit hexagonal polycrystalline phase reflecting from (101) and (0016) planes. The high resolution transmission electron microscopy studies confirmed the formation of hexagonal phase from the d-value calculation which was 0.3108 nm. X-ray photoelectron spectroscopy reveals that the Zn–S bonding energy is at 1022.5 and 162.1 eV for Zn 2p_3/2 and S 2p_1/2 states, respectively. Field emission scanning electron microscopy study shows that deposited thin films are highly uniform, with thin thickness and completely free from large ZnS clusters which usually form in aqueous solutions. Atomic force microscopy investigates that root mean square values of the ZnS thin films are from 3 to 4.5 nm and all the films are morphologically smooth. Energy dispersive spectroscopy shows that the ZnS thin films are relatively stoichiometric having Zn:S atomic ratio of 55:45. It is shown by ultraviolet–visible spectroscopy that ～90% transmittance and ～10% absorbance for the ZnS films in the visible region, which is significantly higher than that reported elsewhere and the band gap energy of the ZnS films is found to be 3.76, 3.74, and 3.71 eV, respectively.     

Application of optical 3D measurement on thin film buckling to estimate interfacial toughness

The shape-from-focus (SFF) method has been widely studied as a passive depth recovery and 3D reconstruction method for digital images. An important step in SFF is the calculation of the focus level for different points in an image by using a focus measure. In this work, an image entropy-based focus measure is introduced into the SFF method to measure the 3D buckling morphology of an aluminum film on a polymethylmethacrylate (PMMA) substrate at a micro scale. Spontaneous film wrinkles and telephone-cord wrinkles are investigated after the deposition of a 300 nm thick aluminum film onto the PMMA substrate. Spontaneous buckling is driven by the highly compressive stress generated in the Al film during the deposition process. The interfacial toughness between metal films and substrates is an important parameter for the reliability of the film/substrate system. The height profiles of different sections across the telephone-cord wrinkle can be considered a straight-sided model with uniform width and height or a pinned circular model that has a delamination region characterized by a sequence of connected sectors. Furthermore, the telephone-cord geometry of the thin film can be used to calculate interfacial toughness. The instability of the finite element model is introduced to fit the buckling morphology obtained by SFF. The interfacial toughness is determined to be 0.203 J/m² at a 70.4° phase angle from the straight-sided model and 0.105 J/m² at 76.9° from the pinned circular model.     

Chemical bath-deposited ZnS thin films: Preparation and characterization

Chemical bath deposition of ZnS thin films from NH₃/SC(NH₂)₂/ZnSO₄ solutions has been studied. The effect of various process parameters on the growth and the film quality are presented. The influence on the growth rate of solution composition and the structural, optical properties of the ZnS thin films deposited by this method have been studied. The XRF analysis confirmed that volume of oxygen of the as-deposited film is very high. The XRD analysis of as-deposited films shows that the films are cubic ZnS structure. The XRD analysis of annealed films shows the annealed films are cubic ZnS and ZnO mixture structure. Those results confirmed that the as-deposited films have amorphous Zn(OH)₂. SEM studies of the ZnS thin films grown on various growth phases show that ZnS film formed in the none-film phase is discontinuous. ZnS film formed in quasi-linear phase shows a compact and a granular structure with the grain size about 100 nm. There are adsorbed particles on films formed in the saturation phase. Transmission measurement shows that an optical transmittance is about 90% when the wavelength over 500 nm. The band gap (E_g) value of the deposited film is about 3.51 eV.     

Structure and photoluminescence properties of ZnS films grown on porous Si substrates

ZnS films were deposited on porous silicon (PS) substrates with different porosities. With the increase of PS substrate porosity, the XRD diffraction peak intensity decreases and the surface morphology of the ZnS films becomes rougher. Voids appear in the films, due to the increased roughness of PS structure. The photoluminescence (PL) spectra of the samples before and after deposition of ZnS were measured to study the effect of substrate porosity on the luminescence properties of ZnS/PS composites. As-prepared PS substrates emit strong red light. The red PL peak of PS after deposition of ZnS shows an obvious blueshift. As PS substrate porosity increases, the trend of blueshift increases. A green emission at about 550 nm was also observed when the porosity of PS increased, which is ascribed to the defect-center luminescence of ZnS. The effect of annealing time on the structural and luminescence properties of ZnS/PS composites were also studied. With the increase of annealing time, the XRD diffraction peak intensity and the self-activated luminescence intensity of ZnS increase, and, the surface morphology of the ZnS films becomes smooth and compact. However, the red emission intensity of PS decreases, which was associated with a redshift. White light emission was obtained by combining the luminescence of ZnS with the luminescence of PS.     

Synthesis and characterization of nanostructural CuS–ZnS binary compound thin films prepared by spray pyrolysis

We have investigated the effect of zinc concentration ([Zn]/[Cu]=0–100 at%) on nanostructural, optical and electrical properties of CuS–ZnS binary thin films grown on glass substrate by the spray pyrolysis technique. X-ray diffraction analysis showed that the films were crystallized with mixed structures of CuS hexagonal and ZnS cubic structure. UV–vis optical measurements analysis showed that these binary films have a relatively high absorption coefficient (～10⁵ cm^?1) in the visible spectrum with a direct band gap in the range of 2.57–2.45 eV in agreement with the corresponding room temperature PL spectra. The electrical studies showed that all these samples have a p-type conductivity and the free hole density decreases with increasing [Zn]/[Cu] molar ratio, in agreement with the reflectance spectra of the layers, originating from plasma oscillations.     

The growth of Y2SiO5:Ce thin films with pulsed laser deposition

Y₂SiO₅:Ce phosphor thin films were grown onto Si(100) substrates with pulsed laser deposition (PLD) using a 248-nm KrF excimer laser. Process parameters were varied during the growth process and the effect on the surface morphology and cathodoluminescence (CL) was analysed. The process parameters that were changed included the following: gas pressure (vacuum (5×10⁻⁶ Torr), 1×1⁻² Torr and 1 Torr O₂), different gas species (O₂, Ar and N₂ at a pressure of 455 mTorr), laser fluence (1.6±0.1 J cm⁻² and 3.0±0.3 J cm⁻²) and substrate temperature (400 and 600°C). The surface morphology was investigated with atomic force microscopy (AFM). The morphology of the thin films ablated in vacuum and 10 mTorr ambient O₂ showed more or less the same trend. An increase in the pressure to 1 Torr O₂, however, showed a definite increase in deposited particle sizes. Ablation in N₂ gas resulted in small particles of 20 nm in diameter and ablation in O₂ gas produced bigger particles of 20, 30 and 40 nm as well as an agglomeration of these particles into bigger size clusters of 80 to 100 nm. Ablation in Ar gas led to particle sizes of 30 nm and the particles were much more spherically defined and evenly distributed on the surface. The higher fluence deposition led to bigger particle and grain sizes as well as thicker layers with respect to the lower fluence. The particle sizes of the higher fluence vary mainly between 130 and 140 nm and the lower fluence sizes vary between 50 and 60 nm. The higher fluence particles consist of smaller particles ranging from 5 to 30 nm as measured with AFM. The surface structure of the thin film ablated at 400°C substrate temperature is less compact (lesser agglomeration of particles than at 600°C). The increase in substrate temperature definitely resulted in a rougher surface layer. CL was measured to investigate the effect of the surface morphology on the luminescent intensities. The increased O₂ ambient (1 Torr) resulted in a higher CL intensity compared to the thin films ablated in vacuum. The thin film ablated in Ar gas showed a much higher CL intensity than the other thin films. Ablation at a high fluence resulted in a higher CL intensity. The higher substrate temperature resulted in better CL intensities. The more spherically shaped particles and rougher surface led to increase CL intensities.     

Synthesis of thin diamond films from faceted nanosized crystallites

Diamond films consist of crystallites having nanometer grains were deposited using low methane concentration by hot filament chemical vapor deposition (HFCVD). The results show that films consist of nanodiamond grains with grain sizes ranging from 20 nm to 200 nm having thickness dependent size. Increasing the deposition time, the grain size increases and hence the thickness of the film increases. The diamond nucleation (nucleation density 10¹⁰ cm⁻²) is comparable to that obtained by biasing the substrate. The use of low methane concentration for the formation of nano crystallites improves the quality of the film as indicated by Raman spectroscopy. The distance between the filament and substrate is increased while maintaining the substrate temperature. The effects of this large separation on the gas phase chemistry are discussed which helps to understand the reduced size of the crystallites under input gas ratios when microcrystallines are obtained.     

Preserving long-term emission stability of carbon nanotube field emitter using aluminum layer

Carbon nanotube (CNT) field emitter was fabricated, and then its emission stability was evaluated with three different anode structures; indium tin oxide (ITO)/glass, ZnS:Cu,Al(green phosphor)/ITO/glass, and Al/ZnS:Cu,Al/ITO/glass. It was found that the electron emission from CNTs to the phosphor layer degrades much faster than the emission to ITO layer does. The current decay time from 100 μA/cm² to 50 μA/cm² for ITO/glass and ZnS:Cu,Al/ITO/glass were 250 h and 20 h, respectively. Such rapid decay in emission current with the phosphor-coated anode was found to be attributed to the formation of Zn particles on CNTs during the field emission. However, the deposition of aluminum layer on the phosphor, in other words, using the anode structure of Al/ZnS:Cu,Al/ITO/glass recovered the stability that is comparable to that with an ITO/glass. The aluminum layer was found to efficiently prevent phosphor elements from being degassed, preserving the long-term emission stability of carbon nanotubes.     

Nanostructured ZnO thin films by chemical bath deposition in basic aqueous ammonia solutions for photovoltaic applications

This paper presents further insights and observations of the chemical bath deposition (CBD) of ZnS thin films using an aqueous medium involving Zn-salt, ammonium sulfate, aqueous ammonia, and thioure. Results on physical and chemical properties of the grown layers as a function of ammonia concentration are reported. Physical and chemical properties were analyzed using scanning electron microscopy (SEM), X-ray energy dispersive (EDX), and X-ray diffraction (XRD). Rapid growth of nanostructured ZnO films on fluorine-doped SnO₂ (FTO) glass substrates was developed. ZnO films crystallized in a wurtzite hexagonal structure and with a very small quantity of Zn(OH)₂ and ZnS phases were obtained for the ammonia concentration ranging from 0.75 to 2.0 M. Flower-like and columnar nanostrucured ZnO films were deposited in two ammonia concentration ranges, respectively: one between 0.75 and 1.0 M and the other between 1.4 and 2.0 M. ZnS films were formed with a high ammonia concentration of 3.0 M. The formation mechanisms of ZnO, Zn(OH)₂, and ZnS phases were discussed in the CBD process. The developed technique can be used to directly and rapidly grow nanostructured ZnO film photoanodes. Annealed ZnO nanoflower and columnar nanoparticle films on FTO substrates were used as electrodes to fabricate the dye sensitized solar cells (DSSCs). The DSSC based on ZnO-nanoflower film showed an energy conversion efficiency of 0.84%, which is higher compared to that (0.45%) of the cell being constructed using a photoanode of columnar nanoparticle ZnO film. The results have demonstrated the potential applications of CBD nanostructured ZnO films for photovoltaic cells.     

Role of deposition temperature and Sn content on structural,optical & electrical properties of In2O3 thin films

High quality indium tin oxide (ITO) thin films (In_2−xSn_xO₃: x = 0, 0.1 and 0.2) have been grown by using pulsed laser deposition technique on quartz substrates. The structural, morphological, optical and electrical investigations of deposited films have been studied as a function of substrate deposition temperatures and the Sn compositions. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) patterns affirm that each film is polycrystalline in nature with cubic bixbyite single phase structure which preferentially oriented along (222) Miller plane. The existence of chemical bonding and functional groups was investigated by FTIR spectroscopy. The TEM micrograph of films (@450°C) for x = 0.1 and x = 0.2 reveal spherical morphology with average particle size 63 nm and 51 nm, respectively. The SEM and AFM images show uniform flower like surface morphology and well-demonstrated nanosized spherical particles, respectively. The widening of the band gap of all the films were exclusively defined by Burstein-Moss shift. The Hall measurement reveals that each film is degenerate with n-type semiconducting nature along with high mobility. Low resistivity (2.024 × 10⁻⁴ Ω-cm) and high transparency (92.58%) along with high carrier concentration (8.915 × 10²⁰ cm⁻³) were optimized for x = 0.1 film at 450°C deposition temperature.     

Influence of argon ambience on the structural,morphological and optical properties of pulsed laser ablated zinc sulfide thin films

Nanostructured zinc suplhide thin films are successfully deposited on quartz substrates using pulsed laser deposition (PLD) under different argon pressures (0, 5, 10, 15 and 20 Pa). The influence of argon ambience on the microstructural, optical and luminescence properties of zinc sulfide (ZnS) thin films is systematically investigated. The GIXRD data suggests rhombohedral structure for ZnS films prepared under different argon ambience. Self-assembly of grains into well-defined patterns along the y direction is observed in the AFM image of the film deposited under argon pressure 20 Pa. All the films show a blue shift in optical band gap. This can be due to the quantum confinement effect and less widening of conduction and valence band for the films with less thickness and smaller grain size. The PL spectra of the different films are recorded at excitation wavelengths 250 nm and 325 nm and the spectra are interpreted. The PL spectra of the films recorded at excitation wavelength 325 nm show intense yellow emission. The film deposited under an argon pressure of 15 Pa shows the highest PL intensity for excitation wavelength 325 nm. For the PL spectra (excitation at 250 nm), the highest PL intensity is observed for the film prepared under argon free ambience. In our study, 15 Pa is the optimum argon pressure for better crystallinity and intense yellow emission when excited at 325 nm.     

Field emission and photoluminescence characteristics of ZnS nanowires via vapor phase growth

Large-area ZnS nanowires were synthesized through a vapor phase deposition method. X-ray diffraction and electron microscopy results show that the products are composed of single crystalline ZnS nanowires with a cubic structure. The nanowires have sharp tips and are distributed uniformly on silicon substrates. The diameter of the bases is in the range of 320-530 nm and that of the tips is around 20-30 nm. The strong ultraviolet emission in the photoluminescence spectra also demonstrates that the ZnS nanowires are of high crystalline perfection. Field emission measurements reveal that the ZnS nanowires have a fairly low threshold field, which may be ascribed to their very sharp tips, rough surfaces and high crystal quality. The perfect field emission ability of the ZnS nanowires makes them a promising candidate for the fabrication of flexible cold cathodes.     

Field emission from ZnS nanorods synthesized by radio frequency magnetron sputtering technique

The field emission property of zinc sulphides nanorods synthesized in the thin film form on Si substrates has been studied. It is seen that ZnS nanorod thin films showed good field emission properties with a low-macroscopic turn-on field (2.9-6.3 V/μm). ZnS nanorods were synthesized by using radio frequency magnetron sputtering of a polycrystalline prefabricated ZnS target at a relatively higher pressure (10⁻¹ mbar) and at a lower substrate temperature (233-273 K) without using any catalyst. Transmission electron microscopic image showed the formation of ZnS nanorods with high aspect ratio (>60). The field emission data were analysed using Fowler-Nordhiem theory and the nearly straight-line nature of the F-N plots confirmed cold field emission of electrons. It was also found that the turn-on field decreased with the decrease of nanorod's diameters. The optical properties of the ZnS nanorods were also studied. From the measurements of transmittance of the films deposited on glass substrates, the direct allowed bandgap values have been calculated and they were in the range 3.83-4.03 eV. The thickness of the films was ∼600 nm.     

Growth, structural and optical properties of CdxZn1−xS thin films deposited using spray pyrolysis technique

Cd_xZn_(1−x)S (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) thin films were deposited by the chemical spray pyrolysis technique using a less used combination of chemicals. Depositions were done at 573 K on cleaned glass substrates. The composition, surface morphology and structural properties of deposited films were studied using EDAX, SEM and X-ray diffraction technique. XRD studies reveal that all the films are crystalline with hexagonal (wurtzite) structure and inclusion of Cd into the structure of ZnS improved the crystallinity of the films. The value of lattice constant ‘a’ and ‘c’ have been observed to vary with composition from 0.382 to 0.415 nm and 0.625 to 0.675 nm, respectively. The band gap of the thin films varied from 3.32 to 2.41 eV as composition varied from x = 0.0–1.0. It was observed that presence of small amount of cadmium results in marked changes in the optical band gap of ZnS.     

The influence of doping element on structural and luminescent characteristics of ZnS thin films

For the fabrication of green and blue emitting ZnS structures the elements of I, III, and VII groups (Cu, Al, Ga, Cl) are used as dopants. The influence of type of impurity, doping technique, and type of substrate on crystalline structure and surface morphology together with luminescent properties was investigated. The doping of thin films was realized during the growth process and/or post-deposition thermal treatment. ZnS thin films were deposited by physical (EBE) and chemical (MOCVD) methods onto glass or ceramic (BaTiO₃) substrates. Closed spaced evaporation and thermodiffusion methods were used for the post-deposition doping of ZnS films. X-ray diffraction (XRD) techniques, atomic force microscopy (AFM), and measurements of photoluminescent (PL) spectra were used for the investigations. It was shown that the doping by the elements of I (Cu) and III (Al, Ga) groups does not change the crystal structure during the thermal treatment up to 1000 C, whereas simultaneous use of the elements of I (Cu) and VII (Cl) groups leads to decrease of the phase transition temperature to 800 C. The presence of impurities in the growth process leads to a grain size increase. At post-deposition treatment Ga and Cl act as activators of recrystallization process. The transition of ZnS sphalerite lattice to wurtzite one leads to the displacement of the blue emission band position towards the short-wavelength range by 10 nm.