From silver nanoparticles to thin films: Evolution of microstructure and electrical conduction on glass substrates

Silver nanoparticles embedded in a dielectric matrix are investigated for their potential as broadband-absorbing optical sensor materials. This contribution focuses on the electrical properties of silver nanoparticles on glass substrates at various morphological stages. The electrical current through thin films, consisting of silver nanoparticles, was characterized as a function of film thickness. Three distinct conductivity zones were observed. Two relatively flat zones (“dielectric” for very thin films and “metallic” for films thicker than 300-400 Å) are separated by a sharp transition zone where percolation dominates. The dielectric zone is characterized by isolated particle islands with the electrical conduction dominated by a thermally activated tunneling process. The transition zone is dominated by interconnected silver nanoclusters—a small increase of the film thickness results in a large increase of the electrical conductivity. The metallic conductivity zone dominates for thicknesses above 300-400 Å.     

Photoelectrochemical properties of electrochemically deposited CdIn2S4 thin films

Thin films of CdIn₂S₄ have been deposited on to stainless steel and fluorine-doped tin oxide (FTO)-coated glass substrates from aqueous acidic bath using an electrodeposition technique. Ethylene diamine tetra-acetic acid (EDTA) disodium salt is used as complexing agent to obtain good-quality deposits by controlling the rate of the reaction. The different preparative parameters like concentration of bath, deposition time, bath temperature, pH of the bath have been optimized by the photoelectrochemical (PEC) technique in order to get good-quality photosensitive material. Different techniques have been used to characterize CdIn₂S₄ thin films. Optical absorption shows the presence of direct transition with band gap energy 2.17 eV. The X-ray diffraction (XRD) analysis of the as-deposited and annealed films showed the presence of polycrystalline nature. Energy-dispersive analysis by X-ray (EDAX) study for the sample deposited at optimized preparative parameters shows that the In-to-Cd ratio is almost 2 and S-to-Cd ratio is almost 4. Scanning electron microscopy (SEM) for samples deposited at optimized preparative parameters reveals that spherical grains are uniformly distributed over the surface of the substrate indicates the well-defined growth of polycrystalline CdIn₂S₄ thin film. PEC characterization of the films is carried out by studying photoresponse, spectral response and photovoltaic output characteristics. The fill factor (ff) and power conversion efficiency (η) of the cell are 69 and 2.94%, respectively.     

Characterization of Cu(In,Ga)Se2 thin films prepared by evaporationfrom ternary compounds

Thin films of Cu(In,Ga)Se₂ were fabricated by evaporation from ternary CuGaSe₂ and CuInSe₂ compounds for photovoltaic device applications and their properties were investigated. From XRF analysis, the Cu:(In+Ga):Se atomic ratio in all thin films was approximately 1:1:2. The Ga/(In+Ga) atomic ratio in the thin films changed linearly from 0 to 1.0 with increasing the [CGS]/([CGS]+[CIS]) mole ratio in the evaporating materials. However, for thin films prepared at the [CGS]/([CGS]+[CIS]) mole ratio above 0.4, the composition by EPMA analysis was not consistent with that by XRF analysis. The result of EPMA analysis showed that the surface of a thin film was Cu-rich. XRD studies demonstrated that the thin films prepared at the [CGS]/([CGS]+[CIS]) mole ratio under 0.2 had a chalcopyrite Cu(In,Ga)Se₂ structure and the preferred orientation to the 112 plane. On the other hand, XRD patterns of the thin films produced at the [CGS]/([CGS]+[CIS]) mole ratio above 0.6 showed the diffraction lines from a chalcopyrite Cu(In,Ga)Se₂ and a foreign phase. The separation of a peak was observed near 2θ=27°, indicative the graded Ga concentration in Cu(In,Ga)Se₂ thin film.     

Effect of indium doping in CdO thin films prepared by spray pyrolysis technique

Preparation of transparent and conducting indium doped CdO thin films by spray pyrolysis on glass substrate is reported for various concentration of indium (2-8 wt%) in the spray solution. The electrical, optical and structural properties of indium doped CdO films were investigated using different techniques such as Hall measurement, optical transmission, X-ray diffraction and scanning electron microscope. X-ray analysis shows that the undoped CdO films are preferentially orientated along (2 0 0) crystallographic direction. Increase of indium doping concentration increases the films packing density and reorient the crystallites along (1 1 1) plane. A minimum resistivity of 4.843×10⁻⁴ Ω cm and carrier concentration of 3.73×10²⁰ cm⁻³ with high transmittance in the range 300-1100 nm were achieved for 6 wt% indium doping. The band gap value increases with doping concentration and reaches a maximum of 2.72 eV for 6 wt% indium doping from 2.36 eV of that of undoped film. The minimum resistivity achieved in the present study is found to be the lowest among the reported values for In-doped CdO films prepared by spray pyrolysis method.     

Properties of highly oriented spray-deposited fluorine-doped tin oxide thin films on glass substrates of different thickness

Transparent conducting thin films of fluorine-doped tin oxide (FTO) have been deposited onto the preheated glass substrates of different thickness by spray pyrolysis process using SnCl₄·5H₂O and NH₄F precursors. Substrate thickness is varied from 1 to 6 mm. The films are grown using mixed solvent with propane-2-ol as organic solvent and distilled water at optimized substrate temperature of 475 °C. Films of thickness up to 1525 nm are grown by a fine spray of the source solution using compressed air as a carrier gas. The films have been characterized by the techniques such as X-ray diffraction, optical absorption, van der Pauw technique, and Hall effect. The as-deposited films are preferentially oriented along the (2 0 0) plane and are of polycrystalline SnO₂ with a tetragonal crystal structure having the texture coefficient of 6.19 for the films deposited on 4 mm thick substrate. The lattice parameter values remain unchanged with the substrate thickness. The grain size varies between 38 and 48 nm. The films exhibit moderate optical transmission up to 70% at 550 nm. The figure of merit (φ) varies from 1.36×10⁻⁴ to 1.93×10⁻³ Ω⁻¹. The films are heavily doped, therefore degenerate and exhibit n-type electrical conductivity. The lowest sheet resistance (R_s) of 7.5 Ω is obtained for a typical sample deposited on 4 mm thick substrate. The resistivity (ρ) and carrier concentration (n_D) vary over 8.38×10⁻⁴ to 2.95×10⁻³ Ω cm and 4.03×10²⁰ to 2.69×10²¹ cm⁻³, respectively.     

Effect of AgNO3 additive/doping on microstructure and superconductivity of the Pb doped Hg: 1223 thin film prepared through spray pyrolysis

We report successful fabrication of Ag doped Hg:1223 films by reacting Ba₂Ca_2.3Cu_3.3O_z(Ag_y): y=0, 0.02, 0.025, 0.05, 0.1 and 0.2 precursors deposited by spray pyrolysis on SrTiO₃ (100) substrates, in controlled Hg+Pb ambient, in an evacuated sealed quartz tube at 820 °C for 4 h. The effects of AgNO₃ addition on the superconducting properties of Hg/(Pb):1223 films are studied. The addition of low concentration of silver e.g. y≈0.025 results in a slight increase in T_c (R=0) from 125 to 126 K and the dc critical current density (J_c) decreases with the increasing Ag in Hg(Pb):1223 (Ag_y) films. The microstructural details exhibit the curious characteristics of spiral like features for lower concentrations of silver i.e. up to y=0.05. These improvements are believed to be due to the liberation of oxygen through the dissociation of AgNO₃ at higher temperature and passivation of weak link effects through the segregation of silver at these grain boundaries. The addition of silver content y≥0.05 resulted in the decrease in transition temperature. The J_c is observed to decrease steadily with increasing Ag content. The microstructural features, e.g. spiral are also found to deteriorate with increasing silver content. The deterioration in superconducting properties at high Ag content is believed to be mainly due to the formation of Ag-Hg amalgam.     

Orientation-dependent dielectric properties of Ba(Sn0.15Ti0.85)O3 thin films prepared by sol-gel method

The orientation-dependent dielectric properties of barium stannate titanate (Ba(Sn_0.15Ti_0.85)O₃, BTS) thin films grown on (1 0 0) LaAlO₃ single-crystal substrates through sol-gel process were investigated. The nonlinear dielectric properties of the BTS films were measured using an inter-digital capacitor (IDC). The results show that the in-plane dielectric properties of BTS films exhibited a strong sensitivity to orientation. The upward shift of Curie temperature (T_c) of the highly (1 0 0)-oriented BTS thin films is believed to be attributing to a tensile stress along the in-plane direction inside the film. A high tunability of 47.03% was obtained for the highly (1 0 0)-oriented BTS films, which is about three times larger than that of the BTS films with random orientation, measured at a frequency of 1 MHz and an applied electric field of 80 kV/cm. This work clearly reveals the highly promising potential of BTS films for application in tunable microwave devices.     

Effect of annealing on structural and electrical properties of the Li–Mn–O thin films, prepared by high frequency RF magnetron sputtering

Li–Mn–O thin film cathode materials are prepared by high frequency (27.12 MHz) RF magnetron sputtering. The high RF frequency gives higher deposition rates without compromising on the quality of the films. This investigation focuses on the effects of post-annealing on the micro-structural, morphological and electrical properties of Li–Mn–O films. It is observed that with the increase of annealing temperature the crystallinity as well as the electrical conductivity of the films increases. The films annealed at 600–700 °C are found to have high structural perfection and good electrical properties.     

A new molecular silver precursor for the preparation of thin conductive silver films

The synthesis and characterization of a new molecular silver precursor is reported. The presented complex [Ag(DioxoNic)₂]NO₃ (DioxoNic=(2,2-Dimethyl-1,3-dioxolan-4-yl)methyl nicotinate) can be obtained by the reaction of silver(I) nitrate and (2,2-Dimethyl-1,3-dioxolan-4-yl)methyl nicotinate in ethanol. The product crystallizes in the monoclinic space group P21/c (No. 14). Concentrated ethanolic solutions allow the fabrication of thin films via dip coating. Using UV-irradiation and subsequent moderate temperature treatment compact films of elemental silver can be obtained. The resulting silver films show excellent electrical properties with sheet resistances down to 0.7 Ω/sq at a film thickness of 25 nm corresponding to a specific electrical resistance of 1.75×10⁻⁸ Ωm very close to the value of bulk silver. For the potential application in optoelectronic devices, the complex was tested as an ink in a soft printing process for the preparation of patterned silver films.     

Preparation and characterization of electrodeposited Sb2Se3 thin films from non-aqueous media

Semiconducting Sb₂Se₃ thin films have been prepared onto the stainless steel and fluorine doped tin oxide coated glass substrates from non-aqueous media using an electrodeposition technique. The electrodeposition potentials for different bath compositions and concentrations of solution have been estimated from the polarization curves. SbCl₃ and SeO₂ in the volumetric proportion as 1:1 with their equimolar solution concentration of 0.05 M form good quality films. The films are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and optical absorption techniques. The SEM studies show that the film covers the total substrate surface with uneven surface morphology. The XRD patterns of the films obtained by varying compositions and concentrations show that the as-deposited films are polycrystalline with relatively higher grain size for 1:1 composition and 0.05 M concentration. The optical band gap energy for indirect transition in Sb₂Se₃ thin films is found to be 1.195 eV.     

Electrosynthesis and characterization of CdSe thin films: Optimization of preparative parameters by photoelectrochemical technique

CdSe thin films have been electrodeposited potentiostatically onto stainless-steel and fluorine-doped tin oxide-coated glass substrates from an aqueous acidic bath using cadmium acetate ((CH₃COO)₂Cd·2H₂O) as a Cd ion source. Preparative parameters such as deposition potential, solution concentration, bath temperature, pH of the electrolytic bath and deposition time have been optimized by using photoelectrochemical (PEC) technique to obtain well adherent and uniform thin films. The electrodeposits were dark brown in colour. The films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical absorption techniques. XRD studies reveal that films are polycrystalline, with hexagonal crystal structure. SEM shows that the films are compact, with spherical grains. Optical absorption studies reveal that the material exhibits a direct optical transition having band gap energy ∼1.72 eV. PEC study shows that the films are photoactive.     

Characterization of SnO2 obtained from the thermal oxidation of vacuum evaporated Sn thin films

Tin oxide has been prepared by thermal oxidation of evaporated tin thin films onto pyrex glass substrates. Films oxidation was achieved in air at a temperature of 600 °C with varied duration from 20min to 3 h. Structural, optical and electrical properties of the films were characterized by means of X-ray diffraction, UV–vis spectroscopy and electrical resistivity measurements respectively. The X-ray analysis revealed the transformation of Sn into SnO₂ with preferential orientation along (101) plans. No intermediate phases such as SnO and Sn₃O₄ were evidenced. It was also found that the SnO₂ crystallites orientation changed with the annealing time due to the strain energy effect. Both band gap energy and electrical resistivity decrease with annealing time due to the crystalline quality improvement and films densification. We have noticed that oxidation at 600 °C for 3 h leads to transparent and conductive films with suitable properties for photovoltaic applications.     

Synthesis and materials properties of transparent conducting In2O3 films prepared by sol-gel-spin coating technique

Transparent conducting indium oxide (In₂O₃) thin films have been prepared on glass substrates by the simple sol-gel-spin coating technique. These films have been characterized by X-ray diffraction, resistivity and Hall effect measurements, optical transmission, scanning electron microscopy and atomic force microscopy for their structural, electrical, optical and morphological properties. The influence of spin parameters, number of coating, process temperature on the quality of In₂O₃ films are studied. In the operating range of deposition, 400-475 °C, all the films showed predominant (2 2 2) orientation. Films deposited at optimum process conditions exhibited a resistivity of 2×10⁻² Ω cm along with the average transmittance of about 80% in the visible spectral range (400-700 nm).     

Deposition and characterization of transparent and conductive sprayed ZnO:B thin films

Highly transparent and conductive Boron doped zinc oxide (ZnO:B) thin films were deposited using chemical spray pyrolysis (CSP) technique on glass substrate. The effect of variation of boron doping concentration in reducing solution on film properties was investigated. Low angle X-ray analysis showed that the films were polycrystalline fitting well with a hexagonal wurtzite structure and have preferred orientation in [002] direction. The films with resistivity 2.54×10⁻³ Ω-cm and optical transmittance >90% were obtained at optimized boron doping concentration. The optical band gap of ZnO:B films was found ∼3.27 eV from the optical transmittance spectra for the as-deposited films. Due to their excellent optical and electrical properties, ZnO:B films are promising contender for their potential use as transparent window layer and electrodes in solar cells.     

Synthesis of nanocrystalline ZnTiO3 perovskite thin films by sol-gel process assisted by microwave irradiation

Nanocrystalline ZnTiO₃ thin films have been grown on Si (1 0 0) at room temperature by using simple, cost effective sol-gel process assisted by microwave irradiation for thermal treatment. For comparison purpose the deposited films have subjected to two kinds of annealing treatments: first set by using conventional annealing and second set by irradiating the deposited films at different microwave powers for 10 min. In both treated films, formation of cubic phase ZnTiO₃ structure has been observed. It is evident that there is a dramatic structural modification when the deposited films are exposed to microwave. It is evident that there is a dramatic change in the morphological properties of the films irradiated in microwave compared to the conventional annealing temperature. Microwave exposed films have shown 19% of Zn, 19% of Ti and 62% of O in the films close to the stiochiometry of the ZnTiO₃, where as annealed films have shown 18% of Zn, 17.5% of Ti, and 64.5% of O in the films of ZnTiO₃. Plausible mechanism for the formation of cubic phase of ZnTiO₃ at low microwave powers has also been discussed. This new innovative microwave heating could open a door for the advanced technologies to cut down the process cost in post treatment of the materials.     

Annealing of thin Zr films on Si1−xGex(0≤x≤1): X-ray diffraction and Raman studies

X-ray diffraction experiments have been combined with Raman scattering and transmission electron microscopy data to analyze the result of rapid thermal annealing (RTA) applied to Zr films, 16 or 80 nm thick, sputtered on Si_1−xGe_x epilayers (0≤x≤1). The C49 Zr(Si_1−xGe_x)₂ is the unique phase obtained after complete reaction. ZrSi_1−xGe_x is formed as an intermediate phase. The C49 formation temperature T_f is lowered by the addition of Ge in the structure. Above a critical Ge composition close to x=0.33, a film microstructure change was observed. Films annealed at temperatures close to T_f are continuous and relaxed. Annealing at T>T_f leads to discontinuous films: surface roughening resulting from SiGe diffusion at film grain boundaries occurred. Grains are ultimately partially embedded in a SiGe matrix. A reduction in the lattice parameters as well as a shift of Raman lines are observed as T exceeds T_f. Both Ge non-stoichiometry and residual stress have been considered as possible origins for these changes. However, as Ge segregation has never been detected, even by using very efficient techniques, it is thought that the changes originate merely from residual stress. The C49 grains are expected to be strained under the SiGe matrix effect and shift of the Raman lines would indicate the stress is compressive. Some simple evaluations of the stress values indicate that it varies between −0.3 and −3.5 GPa for 0≤x≤1 which corresponds to a strain in the range (−0.11, −1.15%). X-ray and Raman determinations are in good agreement.     

Close space vapor transport method for Bi2Te3 thin films deposition: Influence of the type of substrate

Bismuth telluride thin films have been grown by close space vapor transport (CSVT) technique as a function of substrate temperature (T_sub). Both N- and P-type samples can be obtained by this method which is a relatively simple procedure, which makes the method interesting for technological applications. The samples were deposited onto amorphous glass and polycrystalline CdTe film substrates in the substrate temperature range 300-425 °C, with a fixed gradient between source and substrate of 300 °C. The influence of the type of substrate and substrate temperature in the CSVT chamber on the physical properties of the films is presented and discussed.     

Property enhancement of transparent conducting zinc oxide thin films—Effect of simultaneous (Sn+F) doping

Undoped and simultaneously (Sn+F) doped ZnO thin films were fabricated using a simplified spray pyrolysis technique and the effects of Sn doping level on their electrical, structural, optical and surface morphological properties were studied. The XRD patterns confirmed the hexagonal wurtzite structure of ZnO. The minimum electrical resistivity of 0.45×10⁻² Ω cm was obtained for ZnO films having Sn+F doping levels of 8+20 at%. All the films exhibited average optical transmittance of 85% in the visible region, suitable for transparent electrode applications. The overall quality of the fabricated films was confirmed from photoluminescence (PL) studies. The PL and surface morphological studies along with the elemental analysis showed the increase of Sn diffusion into the ZnO lattice which was consistent with the concentration of Sn in the starting solution. The results of the analysis of physical properties of simultaneously doped ZnO films proved that these films might be considered as promising candidates for solar cells and other opto-electronic applications.     

Reaction kinetics of α-CuInSe2 formation from an In2Se3/CuSe bilayer precursor film

The reaction pathway and kinetics of α-CuInSe₂ formation from a glass/In₂Se₃/CuSe polycrystalline bilayer precursor film were investigated using time-resolved, in situ high-temperature X-ray diffraction. Bilayer glass/In₂Se₃/CuSe precursor films were deposited on thin glass substrates in a migration enhanced molecular beam epitaxial deposition system. These films were then temperature ramp annealed or isothermally soaked while monitoring the phase evolution. The initial In₂Se₃ and CuSe reactant phases were directly transformed to α-CuInSe₂ without any detectable intermediate phase. Kinetic parameters were estimated using the Avrami and parabolic diffusion controlled reaction models. The parabolic reaction model fitted the experimental data better than the Avrami model over the entire temperature range (230-290 °C) of the set of isothermal experiments, with an estimated activation energy of 162 (±5) kJ/mol.