Deposited ZnS Thin Films Research Articles

Zinc and cadmium thioamidate complexes: rational design of single-source precursors for the AACVD of ZnS.

A series of zinc(II) thioamidate complexes [Zn{SC(iPr)NR}2]n for R = iPr (n = 2) (2), tBu (3) (n = 1), Ph (4) (n = 2) and Cy (5) (n = 2) and one cadmium(II) thioamidate complex [Cd{SC(iPr)NtBu}2]3, (6), were designed and synthesised as single-source precursors for AACVD ZnS and CdS. Solid-state structures of all four zinc(II) compounds revealed distorted tetrahedral or trigonal bipyramidal geometries, with varying tendencies for dimeric association, mediated by {Zn-S} bridging bonds. The thermogravimetric analysis identified the {tBu} derivertive, 3, as the most promising precursor based on its low decomposition onset (118 °C) and clean conversion to ZnS. This was attributed to the greater availability of β-hydrogen atoms promoting the pyrolysis mechanism. The corresponding cadmium thioamide 6 was found to crystallise as a trimetallic molecule which lacked the thermal stability to be considered viable for AACVD. Hence, 3 was used to deposit ZnS thin films by AACVD at 200-300 °C. Powder X-ray diffraction confirmed phase-pure growth of hexagonal wurtzite ZnS, with approximate crystallite sizes of 15-20 nm. Scanning electron microscopy revealed densely packed spherical nanoclusters. The morphology and crystallinity were most consistent for depositions between 250-300 °C. Energy dispersive X-ray spectroscopy indicated slightly sulfur-deficient stoichiometries.

Investigation of the Effect of Deposition Time and Annealing on the Structural and Optical Properties of Chemically Deposited ZnS Thin Films

Abstract: The Chemical bath deposition (CBD) technique was successfully used to deposit ZnS thin films. The effects of deposition time and annealing on the structural and optical properties of the obtained thin films were investigated by X-ray diffraction (XRD), and UV-Vis spectrophotometer, respectively. The XRD pattern for the as-deposited ZnS thin films deposited for 30 min showed three peaks at 2θ values of 28.75o, 48.05o, and 56.47o corresponding to (111) (main peak), (220), and (311) reflection planes, indicating cubic structure. Comparing these peaks with that of the films prepared for 60 min, the latter have narrower widths, are more intense, sharper, and produce another weak peak at 2θ angle of 33.51o. Post-annealing treatment results in even more intense and sharper peaks, with their width narrowing further, causing an improvement in the crystallinity. The grains detected from the SEM micrograph are well-defined with spherical shapes of varying sizes. The grain shape changes after annealing due to coalescence of the grains. According to the EDX result, Zn and S were present in the prepared film. The as-deposited films showed a maximum transmittance of 82.1% at the visible region, which increased to 89.2% after annealing. The optical energy band gap was found to be in the ranges of 3.58 - 3.75 eV and 3.41 - 3.73 eV for the as-deposited and annealed films, respectively, signifying that the energy band gap decreases with annealing. Keywords: Chemical bath deposition, Zinc sulphide, Optical properties, Annealing, Thin films, Deposition time.

Role of deposition parameters on the properties of the fabricated heterojunction ZnS/p-Si Schottky diode

Heterojunction diodes of ZnS/p-Si have been fabricated using the chemical bath deposition (CBD) technique at two different deposition durations under both stirring and non-stirring conditions. The x-ray diffraction (XRD) patterns indicate the deposited ZnS films exhibit good crystallinity with the growth direction along the (111) planes of a cubic zinc blend structure. The crystallite size of all the deposited ZnS thin films have been calculated using the Scherer formula and found to be in the range of 2.2–2.7 nm which is very close (∼4 nm) to the size estimated using transmission electron microscopy (TEM). The surface morphology of the deposited ZnS thin films were studied by scanning electron microscopy (SEM) and it was observed that spherical nanoparticles agglomerated with the increase in deposition time. Furthermore, the optical properties of the deposited ZnS thin films were studied using UV-visible (UV-VIS) and photoluminescence (PL) spectroscopy. The effective calculated band gap was found in the range from 3.7–3.82 eV for all the samples, however PL spectra shows multiple emissions in the as-deposited ZnS films, indicating the presence of intrinsic defects, The characteristics of the fabricated ZnS/p-Si heterojunction diode was studied by measuring the dark current-voltage (I–V) relation using thermionic emission model. Electrical parameters such as barrier height, saturation current, ideality factor and series resistance were extracted from the I–V characteristics of the fabricated Schottky diodes. The barrier potential for all the ZnS/p-Si heterojunction diodes range between 0.829–0.857. Moreover, the calculated ideality factor was found very close to the ideal value of the diode (1.34 and 1.43) in the devices fabricated under stirring conditions.

Investigation of the effect of zinc precursors onto structural, optical and electrical properties of CBD deposited ZnS thin films

Investigation of the effect of zinc precursors onto structural, optical and electrical properties of CBD deposited ZnS thin films

Physical properties of chemically deposited ZnS thin films: Role of the solubility curves and species distribution diagrams

A physicochemical analysis was carried out on the chemically deposited ZnS/glass thin films. Solutions were prepared by considering the better chemical conditions obtained from the solubility curves (SCs) and the species distribution diagrams (SDDs) for the ZnCl2–NH4NO3-KOH-SC(NH2)2 system. The range of pH from 11.73 to 12.47 is proposed as a chemical condition obtained from the SCs and SDDs. The effects of these conditions on the structural, optical, morphological and stoichiometric properties of the films deposited at 70 °C were analyzed by x-ray diffraction (XRD), UV–Vis spectroscopy, scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS). XRD results show a main cubic crystalline structure (sphalerite) with (111) orientation, while the bandgap energy of the samples increases value from 3.44 eV to 3.72 eV with the increased pH. XPS technique evidenced a reduction of the Zn(OH)2 compound while the [Zn]/[S] ratio improved when the pH increased. SEM images allowed observing homogeneous surfaces and coalescence of grains on the surface of films.

The effect of 8 MeV electron beam irradiation on the structural, optical and photoluminescence properties of ZnS thin films

The thermal deposited ZnS thin films with thickness ranging from 400 nm to 1000 nm were irradiated with various doses of 8 MeV electrons. The structural, optical and photoluminescence properties of the irradiated films were studied in detail. The films with thickness ranging from 600 nm to 650 nm were found to have highest tolerance to irradiation. The crystallite size of these films increased after irradiation. The irradiated films were also found to have high absorption in the UV region and their bandgap showed only marginal variation of 2.57%. The photoluminescence spectra of the films were unaffected by irradiation, confirming the stability of their energy band structure. Thus, the thermal deposited ZnS films were found to be highly suitable for UV photo-detection in the radiation environment.

Kinetic of growth of chemically deposited ZnS films near room temperature conditions

The growth mechanism of ZnS films deposited by chemical bath at temperatures ranging between 25 °C and 55 °C was investigated by a kinetic analysis. For the ZnS thin films deposition, zinc chloride, potassium hydroxide, ammonium nitrate, and thiourea were used as chemical reagents. The effects of the bath temperature, thiourea concentration and deposition time on the ZnS formation were investigated. The species distribution diagrams suggested that the growth mechanism involves bisulfide ions (HS−) for the sulfur species as well as zinc hydroxide complexes ( and ). Higher thickness and growth rates were observed for increased bath temperature. The activation energy of the system was estimated as 127 ± 5 kJ/mole and the order of reaction with a value of 1.03 ± 0.05. These results suggest that the growing mechanism of the ZnS films is a chemical process with formation and absorption of Zn(OH)2 in the substrate followed by an exchange reaction between OH− and HS− ions. Chemical, morphological and optical characterizations of the deposited ZnS thin films are reported. The formation of the ZnS and Zn(OH)2 compounds was identified by x-ray photoelectron spectroscopy technique. An increase in the [Zn]/[S] ratio from 0.367 at 25 °C to 0.906 at 55 °C, was obtained. The estimated bandgap energy of the deposited films ranged from 3.5 to 3.8 eV. Optical transmittance higher than 80% above 500 nm on films makes proper as optical window for thin film solar cells.

Properties of sputtered ZnS thin films for photovoltaic application

ZnS thin films were deposited on soda lime glass substrates by RF magnetron sputtering technique with the variation in radio-frequency (RF) power. A study on the structural, surface topography, surface morphology, optical and electrical properties of the deposited ZnS thin films was carried out through XRD, AFM, FESEM, UV–vis spectrometry and Hall Effect measurement. Structural determination by XRD method indicated that the films are of polycrystalline nature with a strong preferential orientation along the (111) reflection plane exhibiting a cubic or zinc-blende structure for the films deposited at 60 W and above. The optical properties of the films were ascertained for the photon wavelength ranging from 350 to 900 nm. The bandgap was found in the range of 3.22–3.44 eV. The surface roughness and the morphology of the films were also affected by the RF powers as observed from the AFM and FESEM images.

Study of the early growth stages of chemically deposited ZnS thin films from a non-toxic solution

ZnS thin films grown by the chemical bath deposition method have been under intense investigation due to their applications in solar cells. In this work, the early growth stages of ZnS thin films deposited by means of a non-toxic solution are studied by measuring the morphological, chemical and optical properties of the films obtained at different deposition time (5, 10, 20, 30, 60, 90 and 120 min). From the atomic force microscopy (AFM) studies, it was seen that the substrate surface is not fully covered before 20 min, and the growth exponential value changed from 0.5 to 1.8. The x-ray photoelectron spectroscopy (XPS) measurements revealed the presence of the Zn–O bond and the absence of the S–Zn bond at the earliest deposition time. Additionally, from XPS analysis, a shift in the signals for O1s and Zn2p3/2 for the sample grown at 30 min was observed due to the formation of Zn(OH)2. Finally, the UV-Vis spectrophotometry measurements showed that all samples have a high transmittance (>80%) and that the band gap value decreased as the deposition time increased.

Photoluminescence and Raman spectroscopic analysis of PV deposited ZnS thin films

Structural, optical and emission characteristics of the physical vapor deposited (PVD) ZnS films were investigated. Refined patterns of x-ray diffractogram (XRD) revealed that the grown films have zinc blende (ZB) structure with preferential orientation along (111) plane. Optical spectrograph was analyzed to derive various optical parameters, including optical band gap, which assess the device suitability of grown films. Raman spectra, fitted with Lorentzian peak, provided insight on frequency shift as an effect of elastic and thermal strains. Intensified peaks in photoluminescence spectra reflected observable recombination process due to larger band gap and assisted in study of defect-states in the grown films. In brief, the paper reports on influence of thickness and post deposition annealing on film characteristics analyzed through spectroscopic techniques.

A low cost and single source atmospheric pressure vapor phase epitaxy of ZnS for thin film photovoltaic applications

A novel and low cost vapor phase epitaxy (VPE) method using a single non-volatile source has been used to deposit ZnS thin films on soda lime glass substrates. Utilization of the non-volatile source eliminates the need for expensive and sophisticated reactors commonly used in conventional VPE. Instead, this experiment was carried out using inexpensive and easily attainable apparatus. The vapor phase reaction process described is also more compatible to the industry standard dry deposition processes of the other layers in the thin film solar cell stack for Cu(In,Ga)Se2 (CIGS), Cu2ZnSnS4 (CZTS) and CdTe photovoltaic (PV) devices. In this experiment, the substrate temperature was varied from 400 to 480 °C and the ZnS thin films produced were analyzed using X-ray diffractometry (XRD), optical spectroscopy, field emission scanning electron microscope (FESEM) and Hall effect measurement system. The films were found to be hexagonal structured except for the film deposited at 480 °C, where the film was found to be cubic. The thickness, bandgap and resistivity of the deposited films ranged from 54 to 351 nm, 3.18 to 3.83 eV and 2 × 103 to 1.6 × 104 Ω·cm respectively.

Influence of thiourea on the synthesis and characterization of chemically deposited nano structured zinc sulphide thin films

Nanocrystalline ZnS thin films incorporated onto glass substrates by chemical bath deposition method were investigated by X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM), UV–Visible Spectroscopy (UV–Vis), Photoluminescence (PL), and Raman Spectroscopy (FT-R) studies. The influence of thiourea on the structural and optical properties of ZnS films was discussed in detail. The X-ray diffraction patterns confirmed the rhombohedral structure of ZnS thin films. AFM measurements indicated the nano carpet-like surface morphology variation in film with influence of thiourea. The optical absorption studies in the wavelength range of 200–800 nm showed that the band gap energy of ZnS has decreased from 3.69–3.36 eV, as thiourea varied from 0.2–0.8 M. The PL spectra divulged a shift near band edge emission around 360 nm, when thiourea concentration was increased. The Stokes shift calculations from PL emission spectra supported the relative intensities estimated from PL energy band spectra and showed a dependable repeatability for different molar concentrations. The Zn–S complexes were identified in the Raman spectra and the Raman spectral shift for 0.6 M composition of thiourea, which was found to be appreciable for ZnS thin films. The characterization studies clearly exposed the effect of thiourea onto the chemically deposited ZnS thin films.

Effect of Pulse Energy on the Optical Properties of ZnS Thin Films Prepared Using Pulse Laser Technique

In these work four samples of ZnS thin films deposited on glass substrate using pulse laser deposition method with different pulse energies, the effect of the laser pulse energy on the optical properties of the four ZnS thin films fabricated was studied. Q-Switched Nd: YAG laser with the fundamental wavelength 1064 nm, laser Pulse energies of (125, 150, 175, and 200) mj with fixed number of pulses of 20, and pulse repetition rate of 5 Hz were used. The target to the substrate distance and angle were kept fixed. The film thicknesses were measured using FESEM measurement tool. The thickness of the deposited ZnS thin films was found to be linearly dependent on the pulse energy used. The transmission spectra in the tested region (532 to 915) nm were found to be in the range from 0.41 to 0.59% depending on the ZnS thin film thickness, and for each ZnS thin film the transmission spectrum is unique. The refractive indices of all samples were determined; and for each sample and it were found to change with wavelength, the highest refractive index of 5.6 at 915 nm was obtained for the sample of the smallest thickness 0.49 microns. Transmission spectra, absorption coefficients and the refractive indices they were in good agreement with the literature.

Effect of Dye Extracts on the Optical Properties of Chemically Deposited ZnS Thin Films

Effect of Dye Extracts on the Optical Properties of Chemically Deposited ZnS Thin Films

The properties of spray-deposited zinc sulfide thin films using trisodium citrate complexant

Zinc sulfide thin films were deposited on soda-lime glass substrates by chemical spray pyrolysis technique using trisodium citrate complexant. The films were deposited at substrate temperatures of 300 to 400 °C using 0.1 M zinc chloride, 0.1 M thiourea, and 0.01 M trisodium citrate. The thin films’ transmittance is between 40.83 and 50.86% at 550 nm, and energy band gap varied between 3.52 and 3.72 eV. The films have very high refractive indices in the range of 3.07 to 4.02 at 550 nm. The X-ray diffraction (XRD) analyses of the films revealed hexagonal structure. The crystallite size changed between 7.030 and 45.486 nm. The surface morphology is flower-like, dense, and contains rectangular nanorod-like particles and cracks. Raman spectroscopy revealed the presence of first-order (transverse optic (TO) and longitudinal acoustic), second-order (2TO and 2LO), and third-order (3LO) Raman phonons. The Fourier transform infrared spectra indicate the presence of chemical bonds in the thin films, while photoluminescence indicates near band edge emissions. The contact angle revealed hydrophilic nature of the deposited ZnS thin films.

Chemical spray pyrolysis deposition of zinc sulphide thin films using ethylenediaminetetraacetic acid disodium salt complexant

Zinc sulphide thin films were deposited by chemical spray pyrolysis technique. The films were deposited at substrate temperatures of 300, 350 and 400 °C using 0.1 M zinc chloride, 0.1 M thiourea and 0.01 M ethylenediaminetetraacetic acid disodium salt (EDTA). The films have moderate transmittance between 56.00 and 77.73% over visible range, and the band gap energy varies between 3.87 and 4.12 eV. The films have high values of refractive index in the range 1.67 to 2.71 at 550 nm. The X-ray diffraction analysis of the films shows the presence of hexagonal and cubic structures. The crystallite size ranges between 6.122 and 43.023 nm. The surface morphology is flower-like, sheet-like and island-shaped nanoclusters. Raman spectroscopy reveals the presence of first-order, second-order and third-order Raman phonons. The Fourier transform infrared spectra show the presence of chemical bonds in the thin films while photoluminescence shows near band edge emission. The contact angle indicated hydrophilic nature of the deposited ZnS thin films.

Electrochemical deposition of ZnS thin films and their structural, morphological and optical properties

The most interesting feature of electrodeposition is the ability to control the driving force of the deposition process by simply controlling the applied potential. In this paper, we report a study about the effect of applied potential on the properties of ZnS thin films grown by electrochemical deposition. The electrochemical, morphological, structural and optical properties of the ZnS thin films were investigated in terms of different applied potential ranging from −1 to −1.3V vs. SCE. Surface roughness was estimated from atomic force microscopy (AFM) and surface morphology was investigated by scanning electron microscopy (SEM), respectively. According to the results, the surface morphology of the deposits was influenced by the applied potential, showing different rates of growth. The frequencies of nucleation and growth of ZnS crystallites during the ZnS deposition tends to be higher when the applied potentials become more negative. X-ray diffraction analysis show that the films prove crystalline behavior with the Blend structure and have preferably growth along (200) direction having c-axis perpendicular to the substrate surface. The variations of crystallite grain size, lattice constant, microstrain and dislocation densities are observed for the ZnS thin films, the dislocation density and microstrain were found to vary inversely with the crystallite size. As the deposition potential increased the optical band gap Eg was found to decrease from 3.74 to 3.50eV and the carrier densities increase from 4.33×1020 to 3.21×1022cm−3. This variation in Eg is related to the Urbach tail. All experimental observations are discussed in detail.

Properties of sputtered ZnS and ZnS:A (A = Er, Yb) films grown at low substrate temperatures

Radio-frequency magnetron sputtering was used to deposit ZnS thin films doped with rare earth elements (Er and Yb) on glass substrates at two different temperatures (room temperature and 180 °C). The crystal structure, surface morphology, and optical properties of the deposited films were studied as a function of the deposition variables as a first step in the possible application of lanthanide-doped ZnS as buffer layer and/or window layer in thin film photovoltaic devices. From the x-ray diffraction patterns, it was found that the samples have preferred orientation depending on the substrate temperature and on the doping level. Moreover, optical constants were determined from spectroscopic ellipsometry measurements and found that samples of undoped ZnS, in particular, exhibit optical constants that are in good agreement with those reported in the literature for the two ZnS crystalline phases. The band gap (Eg) was calculated using the extinction coefficient (k), which was reduced according to the degree of doping. Also, based on ultraviolet-visible spectroscopy measurements, it was determined that the films have a transmittance of 80% for wavelengths above 350 nm, and a shift of the absorption edge to longer wavelengths when the degree of doping was increased. Finally, photoluminescence measurements were performed at room and low temperature using below bandgap excitation to probe defect states within the gap, as well as the luminescent characteristics of the Er3+ and Yb3+ ions in the ZnS host.

Physicochemical Analysis and Characterization of Chemical Bath Deposited ZnS Films at Near Ambient Temperature

A physicochemical analysis, based on the species distribution diagrams and the solubility curves, is proposed for CDB-ZnS films deposition. The effect of the temperature and the HS−/Zn(OH)42− ion concentrations on ZnS films formation are discussed. ZnS thin films were deposited by chemical bath deposition (CBD) technique at near ambient temperatures (25, 40 and 55°C) from ZnCl2/KOH/NH4NO3/SC(NH2)2 solutions. From the physicochemical analysis it was found that by increasing the bath temperature and the [HS−]/[Zn(OH)42−] ratio, better conditions for obtaining ZnS films of good quality are obtained. Chemical, stoichiometric, and structural characterizations of the deposited ZnS thin films are reported. The formation of ZnS and/or Zn(OH)2 compounds, was identified by X-ray photoelectron spectroscopy (XPS) analysis. The stoichiometric [S]/[Zn] ratio of films was measured in a range between 0.22 to 0.77. X-ray diffraction (XRD) results show amorphous structures for almost all the films, excepting for ZnS samples prepared with the high temperature and [HS−]/[Zn(OH)42−] ratio when a cubic sphalerite phase (111) preferential structure was identified. The bandgap energy of the ZnS samples changes from 3.4 to 3.7 eV when the [HS−]/[Zn(OH)42−] ratio is modified.

Morphology and growth behavior of O2-free chemical bath deposited ZnS thin films

We investigate the role of reagent concentrations and ambient O2 on the morphology and growth behavior of ZnS thin films grown with the chemical bath deposition method. We investigate the role of substrate on film morphology, and find significant differences between films deposited on SiO2 versus Si. The films are also sensitive to dissolved O2 in the bath, as it causes a layer of SiO2 to form at the ZnS/Si interface during deposition. Degassing of solutions and an N2 atmosphere are effective to minimize this oxidation, allowing deposition of ZnS films directly onto Si. Under these conditions, we examine film properties as they relate to reagent bath concentrations. As the reagent concentrations are decreased, both the film roughness and growth rate decrease linearly. We also observe deformation and shifting of X-ray diffraction peaks that increases with decreasing reagent concentrations. The shifts are characteristic of lattice compression (caused by the substitution of oxygen for sulfur), and the deformation is characteristic of distortion of the lattice near crystal grain interfaces (caused by tensile stress from interatomic forces between neighboring crystal grains). At the weakest concentrations, the low roughness suggests a mixed growth mode in which both clusters and individual ZnS nanocrystallites contribute to film growth. With increasing reagent concentrations, the growth mode shifts and becomes dominated by deposition of clusters.