Preparation and thermoelectric properties of BP films on SOI and sapphire substrates

The chemical vapor deposited (CVD) BP films on Si(100) (190 nm)/SiO _x (370 nm)/Si(100) (625 μm) (SOI) and sapphire (R-plane) (600 μm) substrates were prepared by the thermal decomposition of the B₂H₆–PH₃–H₂ system in the temperature range of 800–1050 °C for the deposition time of 1.5 h. The BP films were epitaxially grown on the SOI substrate, but a two-step growth method, i.e., a buffer layer at lower temperature and sequent CVD process at 1000 °C for 1.5 h was effective for obtaining a smooth film on the sapphire substrate. The electrical conduction types and electrical properties of these films depended on the growth temperature, gases flow rates and substrates. The thermal conductivity of the film could be replaced by the substrate, so that the calculated thermoelectric figure-of-merit (Z) for the BP films on the SOI substrate was 10⁻⁴–10⁻³/K at 700–1000 K. Those on the sapphire substrate were 10⁻⁶–10⁻⁵/K for the direct growth and 10⁻⁵–10⁻⁴/K for the two-step growth at 700–900 K, indicating that the film on a sapphire by two-step growth would reduce the defect concentrations and promote the electrical conductivity.     

Effect of thicknesses of copper catalyst and oxide sublayer on morphology of ZnO nanorods

The influence of thicknesses of a ZnO sublayer and a copper catalyst film on the morphology of ZnO nanorods grown by carbothermal synthesis on α-Al₂O₃(11–20) substrates has been studied. An increase in the Cu catalyst film thickness leads to a growth in the diameters, heights, and surface density of nanorods. As the ZnO sublayer thickness is increased, the average diameter of nanorods also increases, while their lengths and surface density decrease. The effect of elevated temperatures on the thermal decomposition of ultrathin Cu films deposited on α-Al₂O₃ substrates has been studied. The photoluminescence characteristics of nanorod arrays have been measured at high levels of optical pumping. An increase in the pumping level to 250–280 kW/cm² leads to superluminescence of the nanorods.     

Sol–gel derived Co<Subscript>3</Subscript>O<Subscript>4</Subscript> thin films: effect of annealing on structural,morphological and optoelectronic properties

Nanocrystalline Co₃O₄ thin films were prepared on glass substrates by using sol–gel spin coating technique. The effect of annealing temperature (400–700 °C) on structural, morphological, electrical and optical properties of Co₃O₄ thin films were studied by X-ray diffraction (XRD), Scanning Electron Microscopy, Electrical conductivity and UV–visible Spectroscopy. XRD measurements show that all the films are nanocrystallized in the cubic spinel structure and present a random orientation. The crystallite size increases with increasing annealing temperature (53–69 nm). These modifications influence the optical properties. The morphology of the sol–gel derived Co₃O₄ shows nanocrystalline grains with some overgrown clusters and it varies with annealing temperature. The optical band gap has been determined from the absorption coefficient. We found that the optical band gap energy decreases from 2.58 to 2.07 eV with increasing annealing temperature between 400 and 700 °C. These mean that the optical quality of Co₃O₄ films is improved by annealing. The dc electrical conductivity of Co₃O₄ thin films were increased from 10⁻⁴ to 10⁻² (Ω cm)⁻¹ with increase in annealing temperature. The electron carrier concentration (n) and mobility (μ) of Co₃O₄ films annealed at 400–700 °C were estimated to be of the order of 2.4–4.5 × 10¹⁹ cm⁻³ and 5.2–7.0 × 10⁻⁵ cm² V⁻¹ s⁻¹ respectively. It is observed that Co₃O₄ thin film annealing at 700 °C after deposition provide a smooth and flat texture suited for optoelectronic applications.     

Thickness dependent growth of needle-like and flower-like ZnO nanostructures

Needle-like nanorods and micron-scale flower-like structures of ZnO were synthesized by thermal evaporation of metallic zinc films with different thicknesses, followed by thermal annealing. Needle-like nanorods of ZnO were found through out the sample surface after annealing of the 1.3 μm thick Zn film. Three-dimensional crystalline nanorod-based flower-like structures of ZnO were also observed after annealing of the relatively thick (3.3 μm) Zn film. Thermal annealing of the Zn films was done at 800 °C in air for different time durations (30, 45, and 90 min). The flower size and number increase with increase in film thickness for the same annealing temperature and time. The X-ray diffraction results show that both the needle-like nanorods and flower-like structures are hexagonal wurtzite structure of ZnO. The room temperature PL spectrum shows a strong defect related violet emission peak centered at 441 nm for both the structures. The possible growth process based on root growth technique is proposed.     

Effect of Thickness on the Properties of Ga-doped Nano-ZnO Thin Films Prepared by RF Magnetron Sputtering

Nano transparent conductive oxide (TCO) Ga-doped ZnO (GZO) thin films with thickness from 260 nm to 620 nm were prepared on glass substrates by RF magnetron sputtering from a powder target with 3 at.% Ga₂O₃. The substrate temperature was kept at 300 °C. The effect of thickness on the structural, electrical, and optical properties of GZO thin films was investigated. It shows that the nano-GZO films are dense and flat, and have polycrystalline structure with preferentially in the (002) orientation. With the increase of thickness, the crystallinity and the grain sizes of the films are improved, meanwhile the carrier concentration increases and the lowest resistivity of 3.685×10⁻³ Ω cm occurs in the 620 nm thick GZO film. The average optical transmittance of all the films is over 80% in the visible range. Decreasing the thickness, the optical transmission of the films increase, and the absorption edge shifts to shorter wavelength, which means the optical band gap is broadened.     

Effects of annealing temperature and thickness on microstructure and properties of sol–gel derived multilayer Al-doped ZnO films

Transparent conductive multilayer Al-doped ZnO (AZO) films were prepared by the spin-on technique with rapid thermal annealing process at low temperature. The effects of annealing temperature and thickness on microstructure, growth behavior, electrical properties and optical properties of AZO films were investigated. It was found that AZO films exhibited stronger preferred c-axis-orientation, the electrical resistivity decreased as it would be expected with the increase of annealing temperature from 400 to 500 °C and the increase of the number of layers in the film from 1 to 6, but the electrical resistivity tended to keep at a certain lowest value of 2.7 × 10⁻⁴ Ω cm when the annealing temperature was above 500 °C and the number of layers did not exceed 6. The average optical transmittance of AZO films was over 90% when number of layers in the film did not exceed 4 and decreased as this number increases, but the annealing temperature had little effect on the average optical transmittance of AZO films.     

Effect of gamma radiation on electrical and optical properties of (TeO<Subscript>2</Subscript>)<Subscript>0·9</Subscript> (In<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0·1</Subscript> thin films

We have studied in detail the gamma radiation induced changes in the electrical properties of the (TeO₂)_0·9 (In₂O₃)_0·1 thin films of different thicknesses, prepared by thermal evaporation in vacuum. The current–voltage characteristics for the as-deposited and exposed thin films were analysed to obtain current versus dose plots at different applied voltages. These plots clearly show that the current increases quite linearly with the radiation dose over a wide range and that the range of doses is higher for the thicker films. Beyond certain dose (a quantity dependent on the film thickness), however, the current has been observed to decrease. In order to understand the dose dependence of the current, we analysed the optical absorption spectra for the as-deposited and exposed thin films to obtain the dose dependences of the optical bandgap and energy width of band tails of the localized states. The increase of the current with the gamma radiation dose may be attributed partly to the healing effect and partly to the lowering of the optical bandgap. Attempts are on to understand the decrease in the current at higher doses. Employing dose dependence of the current, some real-time gamma radiation dosimeters have been prepared, which have been found to possess sensitivity in the range 5–55 μGy/μA/cm². These values are far superior to any presently available real-time gamma radiation dosimeter.     

Impact of various irradiation photon energies and gamma doses on the optical properties of ZnSe nanostructure thin film

ZnSe thin films were prepared by thermal evaporation technique under high vacuum (10⁻⁶ Torr) at 300 K and different film thickness. The structure of thin films was measured using grazing incident in-plane X-ray diffraction (GIIXD) and shows single phase zinc blende structure. The particle sizes of the deposited films were estimated for low film thickness by TEM and high film thickness by GIIXD. The particle size of ZnSe films was decreased from ~8.53 to 3.93 nm as film thickness lowered from 200 to 20 nm which ensures the nanocrystalline structure. The optical transmission (T) and reflection (R) in the wavelength range 190–2,500 nm for irradiated and unirradiated ZnSe thin films under investigation were measured. The effect of irradiation of different energies in range (0.1–1.25 MeV) from X-ray, ¹³⁷Cs and ⁶⁰Co irradiation sources were studied for ZnSe thin films of 100 and 200 nm thicknesses. The dependence of the absorption spectra and refractive index were investigated for different energies irradiation sources. The ZnSe films show direct allowed interband transition. The effect of particle size of nanocrystalline ZnSe thin films for unirradiated and irradiated by gamma (γ) doses from ¹³⁷Cs on the optical properties was studied. Both the optical energy bandwidth and absorption coefficient (α) were found to be (γ) dose dependent.     

Physical properties of natively textured yttrium doped zinc oxide films by sol-gel

Sol-gel derived yttrium doped ZnO films of various thicknesses have been deposited by the dip coating technique. The investigations of microstructural, electrical and optical properties of post heat-treated films in air as a function of thickness have been made. It is found that high quality films are obtained at an annealing temperature of 550 ^∘C. The (002) preferential growth of both the doped and undoped ZnO films changes to (101) as the thickness of the films were increased. The full width at half maximum of (002) X-ray peak decreases with annealing temperature and the lattice constant is found to approach the value of bulk ZnO. Natively textured films have been obtained for film having thickness greater than 0.8 μm. The thinner films are found to be non-textured with high resistivity. The formation of the textured surface of the film is linked to the suppression of c-axis (002) orientation and the columnar growth in the thick film.     

Preparation and properties of a novel electrically conductive adhesive using a composite of silver nanorods, silver nanoparticles, and modified epoxy resin

A novel preparation method for a high-performance electrically conductive adhesive (ECA) which consisted of silver nanorods, silver nanoparticles and modified epoxy resin was developed. Silver nanorods (100 nm in diameter and 5 μm in length) were synthesized by reduction of silver nitrate with ethylene glycol in the presence of Pd seeds and poly (vinyl pyrrolidone) (PVP). Silver nanoparticles (50~60 nm) were synthesized using N, N′-Dimethylformanide as the reducing agent and PVP as the stabilizer. The nanorods and nanoparticles were dispersed well and no agglomerate in the matrix. The volume electrical resistivity tests showed the volume electrical resistivity of the ECA was closely related with the various sintering temperatures and time, and the ECA could achieve the volume electrical resistivity of (3–4) × 10⁻⁵ Ω cm after sintering at 160 °C for 20 min. Moreover, the results showed the as-prepared ECA was able to achieve low-temperature sintering and possessed excellent electrical, thermal, and mechanical properties.     

Investigations on the structural, optical and electrical properties of Nb-doped SnO2 thin films

Niobium-doped tin oxide thin films were deposited on glass substrates by the chemical spray pyrolysis method at a substrate temperature of 400 °C. Effects of Nb doping on the structural, electrical and optical properties have been investigated as a function of niobium concentration (0–2 at.%) in the spray solution. X-ray diffraction patterns showed that the films are polycrystalline in nature and the preferred growth direction of the undoped film shifts to (200) for Nb-doped films. Atomic force microscopy study shows that the surface morphology of these films vary when doping concentration varies. The negative sign of Hall coefficient confirmed the n-type conductivity. Resistivity of ~4.3 × 10⁻³ Ω cm, carrier concentration of ~5 × 10¹⁹ cm⁻³, mobility of ~25 cm² V⁻¹ s⁻¹ and an average optical transmittance of ~70% in the visible region (500–800 nm) were obtained for the film doped with 0.5 at.% niobium.     

Preparation and study of thickness dependent electrical characteristics of zinc sulfide thin films

Zinc sulfide thin films have been deposited onto glass substrates by chemical bath deposition. The various deposition parameters such as volume of sulfide ion source, pH of bath, deposition time, temperature etc are optimized. Thin films of ZnS with different thicknesses of 76–332 nm were prepared by changing the deposition time from 6–20 h at 30° C temperature. The effect of film thickness on structural and electrical properties was studied. The electrical resistivity was decreased from 1.83 × 10⁵ Ω-cm to 0.363 × 10⁵ Ω-cm as film thickness decreased from 332 nm to 76 nm. The structural and activation energy studies support this decrease in the resistivity due to improvement in crystallinity of the films which would increase the charge carrier mobility and decrease in defect levels with increase in the thickness.     

ZnO nanocrystalline thin films: a correlation of microstructural,optoelectronic properties

The compositional, structural, microstructural, dc electrical conductivity and optical properties of undoped zinc oxide films prepared by the sol–gel process using a spin-coating technique were investigated. The ZnO films were obtained by 5 cycle spin-coated and dried zinc oxide films followed by annealing in air at 600 °C. The films deposited on the platinum coated silicon substrate were crystallized in a hexagonal wurtzite form. The energy-dispersive X-ray (EDX) spectrometry shows Zn and O elements in the products with an approximate molar ratio. TEM image of ZnO thin film shows that a grain of about 60–80 nm in size is really an aggregate of many small crystallites of around 10–20 nm. Electron diffraction pattern shows that the ZnO films exhibited hexagonal structure. The SEM micrograph showed that the films consist in nanocrystalline grains randomly distributed with voids in different regions. The dc conductivity found in the range of 10⁻⁵–10⁻⁶ (Ω cm)⁻¹. The optical study showed that the spectra for all samples give the transparency in the visible range.     

Influence of the substrate temperature on the optical and electrical properties of Ga-doped ZnO thin films fabricated by pulsed laser deposition

Ga-doped (5 wt%) zinc oxide (GZO) thin films were fabricated on corning 1737 substrates at a fixed oxygen pressure of 200 mTorr at various substrate temperatures (100–300 °C) by using pulsed laser deposition (PLD) in order to investigate the microstructure, optical, and electrical properties of the GZO thin films. It was observed that all the thin films exhibit c-axis orientation and exhibit only a (002) diffraction peak. The GZO thin film, which was fabricated at 200 mTorr and 300 °C, showed the highest (002) orientation, and the full width at half maximum (FWHM) of the (002) diffraction peak was 0.38°. The position of the XRD peak shifted to a higher angle with increase in the substrate temperature. The optical transmittance in the visible region was greater than 85%. The Burstein-Moss effect, which causes a shift toward a high photon energy level, was observed. The electrical property indicated that the highest carrier concentration (2.33 × 10²¹ cm⁻³) and the lowest resistivity (3.72 × 10⁻⁴ Ωcm) were obtained in the GZO thin film fabricated at 200 mTorr and 300 °C.     

Ellipsometric studies of microscopic surface roughness of CdS thin films

Analysis of changes in surface roughness of CdS thin films with preparation temperature was carried out using variable angle spectroscopic ellipsometry (VASE). The films studied were prepared by spray pyrolysis technique, in the substrate temperature range 200–360°C. The VASE measurements were carried out in the visible region below the band gap (E _g=2·4eV) of CdS so as to reduce absorption by the film. The thickness of the films was in the range 500–600 nm. Bruggeman’s effective medium theory was used for analysis of the surface roughness of the film. The roughness of the film had a high value (∼ 65 nm) for films prepared at low temperature (200°C) and decreased with increase in substrate temperature. This reached minimum value (∼ 27 nm) in the temperature range 280–300°C. Thereafter roughness increased slowly with temperature. The growth rate of the films was calculated for different temperature ranges. It was found that the deposition rate decreases with the increase in substrate temperature and have an optimum value at 300°C. Above this temperature deposition rate decreased sharply. The scanning electron micrograph (SEM) of the film also showed that the film prepared at 280–300°C had very smooth surface texture.     

Sol–gel synthesis and electrical characterization of (Pb, Ca)TiO3 thin films

Calcium modified lead titanate films have been prepared on Pt/Ti/SiO₂/Si substrates using a sol–gel route. The sols were prepared from propanediol solutions of Pb(CH₃COO)₂·xH₂O, Ti(OC₃H₇)₂(CH₃COCHCOCH₃)₂ and Ca(NO₃)₂·xH₂O. Tetragonal phase (Pb, Ca)TiO3 films could be produced by firing the coatings at 650°C for 30 min. The limiting thickness of crack-free single layers was ∼0.4 μm, but 3 μm thick films could be made by a multiple deposition technique. Dielectric and ferroelectric parameters were determined for single layer 0.5 μm films for compositions up to 30 mol% Ca. The average values of remanent polarization, P_r and coercive field, E_c decreased with increasing Ca content from ∼11 μC cm⁻² and ∼125 Kv cm⁻¹ for a 10 mol% Ca composition to ∼8 μC cm⁻² and 80 kV cm⁻¹ for 30 mol% Ca films. It was noted that the statistical variation in electrical values across each film was greater than in PZT films made by a similar sol–gel route. Reasons for this are discussed in terms of the incidence of physical defects in the films. This revised version was published online in November 2006 with corrections to the Cover Date.     

The effect of post-annealing under CdCl<Subscript>2</Subscript> atmosphere on the properties of ITO thin films deposited by DC magnetron sputtering

Indium tin oxide (ITO) films deposited by DC magnetron sputtering were annealed under CdCl₂ atmosphere at different temperatures. The effects of CdCl₂ heat-treatment on the structural, electrical and optical properties of the films were investigated. The X-ray diffraction measurement proves the annealing results in a change of preferred orientation from (400) to (222). It is found the resistivity increases from 1.49 × 10⁻⁴ Ω cm of the as-deposited film to 6.82 × 10⁻⁴ Ω cm of the film annealed at 420 °C. The optical energy gap for the film varies from 3.97 to 3.89 eV. It is also found that the CdCl₂ heat-treatment results in narrowing the energy gap of ITO film.     

Thickness of ITO thin film influences on fabricating ZnO nanorods applying for dye-sensitized solar cell

Background and purposeIn order to compare the substrates influence on the properties of ZnO films and nanostructures, in this paper, the ITO substrates with different thicknesses were investigated.MethodITO thin films of different thickness (200 nme500 nm) were deposited on glass substrates by DC sputtering, on which ZnO nanorods were fabricated from as-deposited ZnO films by reducing annealing method.ResultsIt was found that the structural and electrical properties of ITO films were significantly influenced by the ITO film thickness. The roughness of ITO films was increased with increase in thickness. The Hall mobility of ITO films was also increased with the increase of film thickness; in contrast, the resistivity was decreased. The highest Hall mobility of 29.2 cm²/V s and the lowest resistivity of 1.303 × 10⁻⁴ Ω cm were obtained from 500 nm-thick ITO film. The structural properties of ZnO nanorods were significantly influenced by the ITO film thickness. The density of ZnO nanorods gradually decreased with the increase in thickness of ITO film.ConclusionThe overall conversion efficiency of demonstrated dye-sensitized solar cell was 2.11% with a fill factor 0.526, indicating high potential to be used as photoanodes in dye-sensitized solar cell applications.     

Photoelectrochemical behaviour of pulse plated CdS films

Cadmium sulphide (CdS) films were deposited by the pulse plating technique at room temperature and at different duty cycles in the range of 6–50% using AR grade 0.25 M cadmium sulphate and 0.30 M sodium thiosulphate at a deposition potential of −0.75 V (SCE). The total deposition time was kept constant at 1 h. The thickness of the films were around 2.0 μm. X-ray diffraction (XRD) studies indicate the formation of polycrystalline films with the cubic structure. The crystallite size increased from 23.0 to 27.5 nm as the duty cycle increased from 10 to 50%. Optical absorption studies indicated a direct band gap in the range of 2.40–2.80 eV as the duty cycle is decreased. XPS studies indicated the formation of CdS. Photoelectrochemical (PEC) cell measurements made with the photoelectrodes deposited at 50% duty cycle have exhibited higher conversion efficiency compared to earlier reports.     

Synthesis and variable temperature electrical conductivity studies of highly ordered TiO<Subscript>2</Subscript> nanotubes

Rafts of aligned, high aspect ratio TiO₂ nanotubes were fabricated by an electrochemical anodization method and their axial electrical conductivities were determined over the temperature range 225–400 °C. Length, outer diameter, and wall thickness of the nanotubes were approximately 60–80 μm, 160 nm, and 30 nm, respectively. Transmission electron microscopy studies confirmed that the TiO₂ nanotubes were initially amorphous, and became polycrystalline anatase after heat treatment at temperatures as low as 250 °C in air. The activation energy for conductivity over the temperature range 250–350 °C was found to be 0.87 eV. The conductivity values are comparable to those of nanocrystalline and nanoporous anatase thin films reported in literature.