AFM, XPS and RBS studies of the growth process of CdS thin films on ITO/glass substrates deposited using an ammonia-free chemical process

This paper deals with a detailed study of the growth stages of CdS thin films on ITO/glass substrates by chemical bath deposition (CBD). The chemical and morphological characterization was done through X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS), and atomic force microscopy (AFM) techniques. On the other hand, optical transmission and X-ray diffraction (XRD) measurements were performed in order to study the optical and structural properties of the films. The time, the chemistry, and morphology of the different stages that form the growth process by CBD were identified through these results. Furthermore, clear evidence was obtained of the formation of Cd(OH)₂ as the first chemical species adhered to the substrate surface which forms the first nucleation centers for a good CdS formation and growth. On the other hand, the ITO coating caused growth stages to occur earlier than in just glass substrates, with which we can obtain a determined thickness in a shorter deposition time. We were able to prove that CBD is a good technique for the manufacture of thin films of semiconductor materials, since the CdS film does not have any impurities. Completely formed films were transparent, uniform, with good adherence to the substrate, of a polycrystalline nature with a hexagonal structure. These results indicate that films obtained by CBD are good candidates to be applied in different optoelectronic devices.     

Controlled synthesis, characterization and optical properties of CdS nanocrystalline thin films via chemical bath deposition (CBD) route

In this study, the CdS nanocrystalline thin films obtained from an ammonia-free chemical bath deposition process. The crystallites with a size range of 10–20 nm in diameter with zinc blend (cubic) and wurtzite (hexagonal) crystal structure and strong photoluminescence were prepared from the mixture solutions of: cadmium chloride dihydrate as a cadmium source, thiourea as a sulfur source and sodium citrate dihydrate as a complexing agent for cadmium ions. The well-cleaned glass used as a substrate for thin films deposition. The obtained samples were characterized by the techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), atomic force microscopy (AFM) and fluorescence spectroscopy. Also, the effect of two parameters such as pH and temperature of reaction on the synthesis of CdS nanocrystals was studied. Finally, it was found the CdS nanocrystals showed sharp excitation features and strong “band-edge” emission.     

Ammonia-free method for synthesis of CdS nanocrystalline thin films through chemical bath deposition technique

The preparation of thin films of CdS by chemical bath deposition is mostly based on the utilisation of ammonia as a complexing agent for cadmium ions. Here we report on a technique based on sodium citrate dihydrate that eliminates the problems of ammonia volatility and toxicity. The crystallites with a size range of 10–20 nm in diameter with zinc blend (cubic) and wurtzite (hexagonal) crystal structures and strong photoluminescence were prepared from the mixture solutions of: cadmium chloride dihydrate as a cadmium source, thiourea as a sulfur source and sodium citrate dihydrate as a complexing agent for cadmium ions. The well-cleaned glass used as a substrate for thin film deposition. The obtained samples were characterized by the techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscope (AFM) and fluorescence spectroscopy. Also, the effect of two operating conditions, (i) pH, and (ii) the temperature of reaction on the synthesizing of CdS nanocrystals was examined. Finally, it was found that the CdS nanocrystals showed sharp excitation features and strong ”band-edge” emission.     

Spectroscopic analysis of CIGS2/CdS thin film solar cell heterojunctions on stainless steel foil

This paper presents a spectroscopic analysis of the interface between a CuIn_1−xGa_xS₂ (CIGS2) absorber and a CdS buffer layer on stainless steel foil by Auger electron spectroscopy (AES), inverse photoemission spectroscopy (IPES) and photoelectron spectroscopy (PES) such as X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). By combining these spectroscopic techniques, detailed information about the electronic and chemical properties of the CIGS2 surface and the CdS/CIGS2 interface can be obtained. The gallium concentration in CIGS2 films was found to increase continuously towards the Mo back contact. XPS analysis showed the presence of KCO₃ on the surface of CdS, deposited on etched and un-oxidized samples indicating diffusion of potassium. No potassium was observed on oxidized as well as samples having thicker CdS (50 nm) indicating the effectiveness of oxidation and chemical bath deposition (CBD) process in cleaning the sample surface effectively. In addition, investigation of the electronic level alignment at the interface has been carried out by combining PES and IPES. Conduction band offset of −0.45 (±0.15) eV and a valence band offset of −1.06 (±0.15) eV were measured. These unfavorable conditions limit efficiency of CIGS2 thin film solar cells.     

Optimisation of the CBD CdS deposition parameters for ZnO/CdS/CuGaSe2/Mo solar cells

We present an optimisation of our recipe for the CdS chemical bath deposition process as applied to solar cells based on polycrystalline CuGaSe₂ (CGSe) absorber layers prepared in two stages by physical vapour deposition. We investigate the influence of the ammonia (NH₃) and the thiourea (H₂NCSNH₂) concentration, both being constituents of the chemical bath deposition (CBD) solution, at a deposition temperature of 80 °C on the microstructural and optical properties of CdS layers and on ZnO/CdS/CuGaSe₂/Mo device parameters. The composition of the CdS layers and their thickness were determined using X-ray Fluorescence Analysis. Transmission and reflection measurements performed at 300 K were used for the calculation of absorption and optical band gap energy (E_g). The E_g values of the films varied from 2.41 to 2.46 eV depending on deposition conditions. Cubic phase of the as-grown layers was identified by X-ray diffraction analysis. An improvement in the investigated solar cells efficiency was achieved when the ammonia concentration was increased and the thiourea concentration was reduced, compared to the previously used standard HMI recipe. The influence of the CBD CdS preparation recipe on the ZnO/CdS/CuGaSe₂/Mo electrical and photoelectrical properties is discussed.     

Surface chemical states of barium zirconate titanate thin films prepared by chemical solution deposition

Ba(Zr_0.05Ti_0.95)O₃ (BZT) thin films grown on Pt/Ti/SiO₂/Si(1 0 0) substrates were prepared by chemical solution deposition. The structural and surface morphology of BZT thin films has been studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results showed that the random oriented BZT thin film grown on Pt/Ti/SiO₂/Si(1 0 0) substrate with a perovskite phase. The SEM surface image showed that the BZT thin film was crack-free. And the average grain size and thickness of the BZT film are 35 and 400 nm, respectively. Furthermore, the chemical states and chemical composition of the films were determined by X-ray photoelectron spectroscopy (XPS) near the surface. The XPS results show that Ba, Ti, and Zr exist mainly in the forms of BZT perovskite structure.     

Tribological behavior of in situ Ag nanoparticles/polyelectrolyte composite molecular deposition films

Multilayer polyelectrolyte films containing silver ions were obtained by molecular deposition method on a glass plate or a quartz substrate. The in situ Ag nanoparticles were synthesized in the multilayer polyelectrolyte films which were put into fresh NaBH₄ aqueous solution. The structure and surface morphology of composite molecular deposition films were observed by UV-vis spectrophotometer, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Tribological characteristic was investigated by AFM and micro-tribometer. It was found that the in situ Ag nanoparticles/polyelectrolyte composite molecular deposition films have lower coefficient of friction and higher anti-wear life than pure polyelectrolyte molecular deposition films.     

Influence of substrate temperature and ion-beam energy on the syntheses of aluminium nitride thin films by nitrogen-ion-assisted pulsed-laser deposition

Aluminium nitride (AlN) thin films have been grown on Si(100), Si(111) and Sapphire Al₂O₃(001) substrates by pulsed KrF excimer laser (wavelength 248 nm, duration 30 ns) ablation of an AlN target with the assistance of nitrogen-ion-beam bombardment. The influence of process parameters such as substrate temperature and ion-beam energy has been investigated in order to obtain high-quality AlN films. The AlN films deposited by pulsed-laser deposition (PLD) have been characterized by X-ray diffraction (XRD) to determine the crystalline quality, grain size and growth orientation with respect to the substrate. The XRD spectra of AlN films on Si(100), Si(111) and Sapphire substrates yield full-width-half-maximum (FWHM) values of approximately 1.6. The bonding characteristics in the films have been evaluated by Raman spectroscopy. The chemical composition of the films has been characterized by X-ray photoelectron spectroscopy (XPS). The surface morphology of the films has been measured by atomic force microscopy (AFM). At a substrate temperature of at least 600 °C, polycrystalline AlN films with orientations of AlN(100) and AlN(101) have been synthesized. PACS 68.55.-a; 81.15.Fg; 77.84.Bw     

Photoemission studies of initial oxidation for ultra-thin zinc film on 6H-SiC(0 0 0 1) surface with synchrotron radiation

The thermal oxidation process of metallic zinc on 6H-SiC(0 0 0 1) surface has been investigated by using atomic force microscopy (AFM), synchrotron radiation photoelectron spectroscopy (SRPES) and XPS methods. The AFM images characterize the surface morphology of ZnO film formed during the thermal oxidation and SRPES record the valence band, Si 2p and Zn 3d spectra at different stages. The O 1s peak is recorded by XPS because of the energy limit of the synchrotron radiation. Our results reveal that the silicon oxides layer of SiC substrate can be reduce by hot metallic zinc atom deposition. The oxygen atoms in the silicon oxides are captured by the zinc atoms to form ZnO_x at the initial stage and as a result, the oxidized SiC surface are deoxidized. After the zinc deposition with the final thickness of 2.5 nm, the sample is exposed in oxygen atmosphere and annealed at different temperatures. According to the evolution of peaks integrated intensities, it is considered that the Zn/SiC system will lose zinc atoms during the annealing in oxygen flux at high temperature due to the low evaporation temperature of pure zinc. After further annealing in oxygen flux at higher temperature, the substrate is also oxidized and finally the interface becomes a stable SiC-SiO_x-ZnO sandwich structure.     

Structural, optical and photoelectrochemical characterization of CdS nanowire synthesized by chemical bath deposition and wet chemical etching

Nanocrystalline thin films of CdS have been grown onto flexible plastic and titanium substrates by a simple and environmentally benign chemical bath deposition (CBD) method at room temperature. The films consist of clusters of CdS nanoparticles. The clusters of CdS nanoparticles in the films were successfully converted into nanowire (NW) networks using chemical etching process. The possible mechanism of the etching phenomenon is discussed. These films were examined for their structural, surface morphological and optical properties by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV-vis spectrophotometry techniques, respectively. Photoelectrochemical (PEC) investigations were carried out using cell configuration as n-CdS/(1 M NaOH + 1 M Na₂S + 1 M S)/C. The film of nanowires was found to be hexagonal in structure with the preferential orientation along the (0 0 2) plane. The nanowires have widths in the range of 50-150 nm and have lengths of the order of a few micrometers. Optical studies reveal that the CdS nanowires have value of band gap 2.48 eV, whereas it is 2.58 eV for nanoparticles of CdS. Finally, we report on the ideality of junction improvement of PEC cells when CdS nanoparticles photoelectrode converted into nanowires photoelectrode.     

Optical properties of cadmium sulfide nanocrystal film prepared by electrochemical synthesis at liquid-liquid interface

Dendritic nanocrystalline CdS film was deposited at liquid-liquid interface of surfactants and an electrolyte containing 4 mmol L⁻¹ cadmium chloride (CdCl₂) and 16 mmol L⁻¹ thioacetamide (CH₃CSNH₂) with an initial pH value of 5 at 15 °C by electrochemical synthesis. The nanofilm was characterized by transmission electron microscopy (TEM), field emission scanning electron microscope (FE-SEM), atomic force microscopy (AFM), ultraviolet visible (UV-vis) absorption spectroscopy and fluorescence spectroscopy. The surface morphology and particle size of the nanofilm were investigated by AFM, SEM and TEM, and the crystalline size was 30-50 nm. The thickness of the nanofilm calculated by optical absorption spectrum was 80 nm. The microstructure and composition of the nanofilm was investigated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), showing its polycrystalline structure consisting of CdS and Cd. Optical properties of the nanofilm were investigated systematically by UV-vis absorption and fluorescence spectroscopy. A λ_onset blue shift compared with bulk CdS was observed in the absorption spectra. Fluorescence spectra of the nanofilm indicated that the CdS nanofilm emitted blue and green light. The nanocomposites film electrode will bring about anodic photocurrent during illumination, showing that the transfer of cavities produces photocurrent.     

Revisiting the physical processes of vapodeposited thin gold films on chemically modified glass by atomic force and surface plasmon microscopies

The preparation of very thin (at the scale of a few tens of nanometers) gold films by thermal evaporation and deposition on a solid substrate (glass) remains a key step for the elaboration of transparent and sensitive optical biosensors. We study the influence of the glass surface treatment and its thermal conductivity on the structure and composition of evaporated gold films. Using a combination of atomic force microscopy (AFM), high resolution surface plasmon resonance (SPR) imaging, and X-ray photoelectron spectroscopy (XPS), we demonstrate that the grafting of a layer of long chain mercaptant, using 11-mercaptoundecyltrimethoxysilane (S_ξSi), prior to gold deposition produces a drastic modification of gold inner and surface textures. A thorough investigation of AFM image topography by 2D wavelet-based segmentation method reveals the flat conical shape of the gold surface grains and their shape invariance with the glass surface chemical treatment. However, this treatment leads to a drastic decrease of the mean size and polydispersity of these grains by a factor of 2, thereby lowering the gold surface roughness. The rationale is that the combination of surface forces and thermal transfer drives the formation of homogeneous and flatter gold films.     

Synthesis of CdS nanostructures using template-assisted ammonia-free chemical bath deposition

CdS micro- and nano-structures (micro/nanotubes and nanostructured films) were obtained by ammonia-free chemical bath deposition using polymer templates (ion track-etched polycarbonate membranes and poly(styrene-hydroxyethyl methacrylate) nanosphere arrays). The semiconductor structures were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), optical absorption, photoluminescence and electrical measurements. The diameters of CdS tubes are between 300 nm and few microns and the lengths are up to tens of micrometers. The SEM images prove that the CdS films are nanostructured due to the deposition on the polymer nanosphere arrays. For both CdS structures (tubes and films) the XRD patterns show a hexagonal phase. The optical studies reveal a band gap value of about 2.5?2.6 eV and a red luminescence at ～1.77 eV. A higher increase of conductivity is observed for illuminating the CdS nanostructured film when compared to the simple semiconductor film. This is a consequence of the periodic patterning induced by the polymer nanosphere array.     

TEM and XPS studies on CdS/CIGS interfaces

Copper indium gallium selenide (CIGS) was deposited by metallic precursors sputtering and subsequently submitted to a selenization process. The upper CdS layers were deposited by chemical bath deposition (CBD) technique. The CdS/CIGS interfaces were investigated by Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). As checked by XPS analysis, the CIGS surface exhibited a hydroxide-terminated CdSe layer when treated with Cd Partial Electrolyte solution (Cd PE). Its thickness was roughly estimated to several nanometers. A 100 nm thick CdS layer was deposited onto CIGS surface. The TEM images revealed a clear and sharp interface between CdS and CIGS. XPS analysis showed a CIGS surface covered by a pinhole free and homogeneous CdS layer. XPS depth profile measurement of the CdS/CIGS interface did not evidence elemental inter-diffusion between the CIGS and CdS layers, in very good agreement with TEM observations.     

Investigation of ageing characteristics and identification of surface chemical changes on SrGa2S4:Ce display phosphor under electron beam bombardment

Cathodoluminescent ageing characteristics of SrGa₂S₄:Ce³⁺ under prolonged electron beam bombardment was studied and the data are presented. The cathodoluminescent intensity with an increasing Coulomb loading was observed to degrade under different primary electron beam voltages. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to monitor the surface chemical changes during electron beam bombardment and after the degradation process. Auger peak to peak heights monitored during the ageing process suggest a loss in S and C and an initial increase in oxygen concentration on the surface. XPS results indicate the formation of a SrO overlayer due to electron stimulated surface chemical reactions (ESSCRs).     

Influence of surface temperature and surface modifications on the initial layer growth of para-hexaphenyl on mica (0 0 1)

Ultra-thin films of para-hexaphenyl (6P) were prepared on muscovite mica (0 0 1) utilizing organic molecular beam deposition (OMBD) under well defined ultra high vacuum (UHV) conditions. The 6P growth characteristics were studied as a function of substrate temperature and substrate surface conditions. For the initial state of layer growth, thermal desorption spectroscopy (TDS) was used to verify the existence of a wetting layer. In this monomolecular continuous wetting layer, the molecules lie flat on the surface and are rather strongly bonded. For thicker films, in-situ X-ray photoelectron spectroscopy (XPS) in combination with (TDS) was applied to reveal the kinetics of the layer growth. Ex-situ atomic-force microscopy (AFM) was used to determine the film morphology. In particular, the influence of surface modifications (carbon contamination, sputtering) on 6P layer growth was investigated. XPS and low energy electron diffraction (LEED) were used to characterize the mica surface before the film deposition. TDS and AFM revealed a considerable change in film growth, from a needle-like island growth of flat laying molecules on top of the wetting layer (for the air cleaved mica) to terrace-like film growth of standing molecules, without a wetting layer (after surface modifications).     

Surface modification of silica-coated zirconia by chemical treatments

Zirconia surface modification by various chemical treatments after silica coating by sandblasting was investigated in this study. The surface of silica-coated dental zirconia was hydroxylated by treatment with different acids at room temperature for 4 h, rinsed with deionized water and air-dried. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Shifts in binding energies for Zr 3d_5/2 and Si 2p peaks were observed after treatment with acids, thereby showing a change in the chemical states of zirconium and silicon on the surface layer of silica-coated zirconia. The XPS analysis revealed that the silica-coated zirconia (SiO₂-ZrO₂) surfaces had changed to hydrous silica-coated zirconia (SiO₂-ZrO₂·nH₂O). One-way ANOVA analysis revealed there was significant difference in both surface roughness parameters of silica-coated zirconia after chemical treatments and the surface topography varied depending on the acid treatment.     

Structure and composition of the segregated Cu in V2O5/Cu system

We have investigated segregation of copper at the surface of V₂O₅ films deposited onto Cu substrate by employing surface analysis techniques. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) confirmed that the Cu is segregated at the surface and its chemical state is Cu₂O. According to secondary ion mass spectroscopy (SIMS) and glow discharge spectroscopy (GDS), the Cu concentration inside the deposited V₂O₅ layer is low. Ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS) revealed the segregation alters the surface local density of states. Surface analysis of deposited samples in ultra high vacuum (UHV) condition verified that the segregation occurs during the deposition. We have extended kinetic tight binding Ising model (KTBIM) to explain the surface segregation during the deposition. Simulation data approve the possibility of surface segregation during room temperature deposition. These results point out that on pure Cu substrate, oxidation occurs during the segregation and low surface energy of Cu₂O is the original cause of the segregation.     

Surface functionalisation of polymer nanofibres by sputter coating of titanium dioxide

The surface properties of nanofibres are of importance in various applications. In this work, electrospun polyamide nanofibres were used as substrates for creating functional nanostructures on the nanofibre surfaces. A RF magnetron sputter coating was used to deposit the functional layer of titanium dioxide (TiO₂) onto the nanofibres. Atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and environmental scanning electron microscopy (ESEM) were employed to study the topography, grain structure and wetting of the nanofibre surfaces, respectively. The AFM results indicated a significant difference in the morphology of the nanofibres before and after the TiO₂ sputter coating. The XRD analysis showed the amorphous structures of the TiO₂ deposition layer. XPS spectra reflected the chemical features of the deposited nanostructures. The ESEM observation revealed that the surface wettability of TiO₂ sputter coated nanofibres was significantly improved after UV irradiation.     

Direct production of carbon nanotubes decorated with Cu2O by thermal chemical vapor deposition on Ni catalyst electroplated on a copper substrate

Carbon nanotubes (CNTs) decorated with Cu₂O particles were grown on a Ni catalyst layer deposited on a Cu substrate by thermal chemical vapor deposition from liquid petroleum gas. Ni catalyst nanoparticles with different sizes were produced in an electroplating system at 45 °C using the corrosive effect of H₂SO₄ which was added to solution. These nanoparticles provide the nucleation sites for CNT growth avoiding the need for a buffer layer. The surface morphology of the Ni catalyst films and CNT growth over this catalyst was studied by scanning electron microscopy (SEM). High temperature surface segregation of the Cu substrate into the Ni catalyst layer and its exposition to O₂ at atmospheric environment, during the CNTs growth, lead to the production of CNTs decorated with about 6 nm Cu₂O nanoparticles. We used SEM to study the surface characteristics of Ni catalyst films and characteristic of grown CNTs. Raman spectroscopy, transmission electron microscopy (TEM), electron diffraction (EDX), X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) revealed the formation of CNTs. The selected area electron diffraction pattern, EDX, and XPS studies show that these CNTs were decorated with Cu₂O nanoparticles. This way of fabrication is the easiest and lowest cost method.