Growth, structure and optical characterization of high quality ZnO thin films obtained by spray pyrolysis

Zinc oxide thin films were prepared by spray pyrolytic decomposition of zinc acetate onto a glass substrate. Auger spectroscopy showed that the film stoichiometry is close to the ZnO phase with a little excess of oxygen. X-ray diffraction spectra show that the structure belongs to the hexagonal wurtzite crystal type, with a mean crystallite size in the range 20–33 nm. Under optimized deposition conditions films are c-axis oriented, having a full width at half-maximum (FWHM) value of the (002) X-ray diffraction line of 0.23°. Microstructure was analysed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high resolution electron microscopy (HREM). In regard to the crystal growth, a critical temperature was found to be around 600 K. Above this critical temperature the film is c-axis oriented and almost all grains became round shaped. Optical constants, n and k, were determined using only transmittance data and a direct band gap of 3.28 eV was deduced.     

Synthesis and characterization of electrolyte-grade 10%Gd-doped ceria thin film/ceramic substrate structures for solid oxide fuel cells

In the present research, spray pyrolysis technique is employed to synthesize 10%Gd-doped ceria (GDC) thin films on ceramic substrates with an intention to use the "film/substrate" structure in solid oxide fuel cells. GDC films deposited on GDC substrate showed enhanced crystallite formation. In case of NiO-GDC composite substrate, the thickness of film was higher (~ 13 μm) as compared to the film thickness on GDC substrate (~ 2 μm). The relative density of the films deposited on both the substrates was of the order of 95%. The impedance measurements revealed that ionic conductivity of GDC/NiO-GDC structure was of the order of 0.10 S/cm at 500 °C, which is a desirable property for its prospective application.     

Synthesis and characterization of nanostructured nickel oxide thin films prepared with chemical spray pyrolysis

Nickel oxide thin films have been deposited in an open atmosphere onto glass substrates by chemical spray pyrolysis using aqueous nickel acetate solutions and air as driving gas. The films show a strong variation in the surface morphology depending on the substrate temperature and the precursor solution flux. At 350 °C substrate temperature, a reticular tissue-like film morphology is obtained, becoming the reticular nickel oxide fibres of the film thicker with increasing precursor solution flux. At 450 °C substrate temperature, the film growth rate is 4 times slower and a highly symmetric self-ordering of the material at nanometer length scale occurs. These films consist of interconnected grains separated by pores, both of about 100 nm in size. XRD and TEM revealed that the films are cubic NiO, being the crystallite size around 10 nm. The optical band gap of the films decreases strongly for increasing film thickness from 4.3 eV to 3.65 eV.     

Spray pyrolysis deposition of zinc oxide nanostructured layers

Highly structured ZnO layers comprising well-shaped hexagonal rods were prepared by spray pyrolysis deposition of zinc chloride aqueous solutions in the temperature range of 490-560 °C. The layers were characterised by SEM, XRD and SAED. A flat ZnO film evolves into the structured layer consisting of single crystalline hexagonal elongated prisms at growth temperatures close to 500 °C and above. The rise of both the growth temperature and solution concentration increases rod dimensions. The deposition of the 0.1 mol/l solution at ∼500 °C results in crystals with a diameter of 200-300 nm and length of 800 nm. However, the rods grown at 560 °C indicate a width in the range of 400-600 nm and a length of up to 2500 nm. The deposition of the 0.05 mol/l solution at 560 °C results in the rods with a diameter of 100-300 nm and a length of 1500 nm. The increase of the concentration up to 0.2 mol/l results in branched crystals, mainly tripods with a similar leg size of 600-700 nm in width and 3000 nm in length. According to XRD, the ZnO layers grown from the 0.1 mol/l solution in the temperature range of 450-560 °C are c-axis-oriented, independent of morphology. The XRD peaks intensities ratio (I₀₀₂/I₁₀₁) of the samples deposited at 560 °C changes from 9 to 1.3 by an increase in the solution concentration from 0.05 to 0.2 mol/l and indicates that c-axis orientation vanishes at higher concentrations. We showed that ZnO nanorods with the length to diameter ratio of 30 can be prepared by spray technique using indium tin oxide-covered glass substrates instead of bare glass.     

Initial stages of deposition and film formation during spray pyrolysis — Nickel oxide, cerium gadolinium oxide and mixtures thereof

The formation of nickel oxide (NiO), cerium gadolinium oxide (CGO) and NiO-CGO thin films by air blast spray pyrolysis was studied at two scales. First, single droplets of precursor were deposited on sapphire substrates and the morphology of the formed residue was studied as a function of the substrate surface temperature, type of metal salt, salt saturation, and organic solvent in the precursor. Second, the synthesis of continuous films from repetitively deposited droplets and crack formation in the films were studied as a function of substrate temperature and salt decomposition kinetics. Nitrates, acetates, perchlorates and chlorides of nickel, cerium and gadolinium were the metal salts used, and mixtures of ethanol or water with di-, tri- and tetraethylene glycol were used as solvents.Regular ring- or disc-shaped deposits were formed from single droplets that evaporated without boiling and were mainly observed with metal acetate- and chloride-based spray solutions or at low substrate temperatures. Disc-shaped residues were obtained for saturated salt solutions and changed to rings with diminishing rim thickness with decreasing salt saturation. The formation of bubbles in the droplet from boiling or salt decomposition during evaporation resulted in the distortion of the circular shape and was predominantly observed for metal nitrate-based precursors and at high substrate surface temperatures.Continuous, dense and crack-free films of CGO and NiO-CGO with thicknesses up to 500 and 800 nm, respectively, were prepared from metal nitrate/chloride mixtures in a tetraethylene glycol-based solvent. The maximum crack-free thickness decreased with decreasing deposition temperature and was correlated to the metal salt decomposition kinetics.     

Doping induced structural and compositional changes in ZnO spray pyrolysed films and the effects on optical and electrical properties

Highly transparent, conductive ZnO films have been deposited by spray pyrolysis of a zinc acetate based solution. Quality films are yielded as our process is analogous to an aerosol assisted chemical vapour deposition rather than a droplet deposition spray pyrolysis technique. The properties of the films are governed by the additives to the base precursor solution. When aluminium acetylacetonate is added to the precursor solution, ZnO:Al films are grown with free charge carrier concentrations of more than 10²⁰ cm⁻³. The carrier density and mobility are measured by both Hall probe and near infrared spectroscopy. Film growth and grain size, morphology and orientation are altered using an increased percentage of ZnCl₂ in the precursor, which results in a 10 fold increase in charge carrier mobility. An investigation is presented correlating the composition of the precursor solution with the chemical, structural, electrical and optical properties of the grown films.     

Temperature and pH dependence of the electroless Ni–P deposition on silicon

The sensitization, activation, nucleation and growth of electroless Ni–P deposition on silicon in an acid plating bath with sodium hydrophosphite as reducing agent and sodium succinate as complexing agent were studied by transmission electron microscopy, field emission scanning electron microscopy and atomic force microscopy. The results show that a continuous polycrystalline SnCl₂ film was formed on the silicon surface in the sensitization process, and small crystalline Pd particles were dispersedly produced on SnCl₂ film in the activation process. In the initial deposition stage, the small Ni–P particles had already emerged on the silicon surface in a deposition time of less than 2 s. When Ni–P particles grew, their size increased but their number decreased, and they later developed into a columnar structure. The deposition rate of the electroless Ni–P deposit increased as the pH value and the temperature of the plating bath increased (from 1.36 to 29.66 μm/h). The activation energy of the electroless Ni–P deposition on silicon increased as the pH value of the plating bath decreased (from 68.8 to 79.4 kJ/mol).     

Anodic oxide growth on aluminium surfaces modified by cathodic deposition of Ni and Co

Surface conditions similar to those found in aluminium alloys of practical use were assessed by cathodic deposition of transitions metals (Ni and Co) from different electrolytes. Fundamental aspects concerning with the growth of anodic oxide films at potentials lower than 10 V in neutral acetate buffer solution on these modified surfaces were analysed by common electrochemical techniques complemented with scanning electron microscopy and transmission electron microscopy. In both potentiodynamic and galvanostatic modes, the growth of aluminium oxide competes with the dissolution of deposited metal particles. The formation of a thin aluminium barrier oxide film beneath them shifts the dissolution potential over to 1.5 V towards more positive values. Some particles get progressively embedded in the matrix of the growing alumina and act as cation sources, increasing the film conductivity and diminishing the established electric field in the oxide. This effect is more pronounced with Co deposits due to its high active dissolution rate, before passivation occurs. Then, the generation of a two-layer film is explained in terms of the precipitation of metal hydroxide at the solution side on the oxide barrier film.     

Structure and magnetic characterizations of cobalt ferrite films prepared by spray pyrolysis

Cobalt ferrite films were prepared by spray pyrolysis with post-annealing. For the as-deposited film, the differential scanning calorimetry measurement shows a crystallization peak at around 375 °C during the isochronal heating at 20 °C/min, and the X-ray diffraction pattern shows its amorphous-like characteristic. The isothermal post-annealings were performed for 2 h at various temperatures from 400 to 700 °C, leading to the crystallization of films, forming the spinel structure. The cross-sectional analysis with scanning electron microscopy shows that the film's thickness keeps almost constant after annealing, and the layered granular structure appears when the annealing temperature is high. The magnetic hysteresis loops of as-deposited and annealed films show that both the saturation magnetization and coercivity increase with the annealing temperature, due to the crystallization of CoFe₂O₄ phase.     

钙钛矿型中低温固体氧化物燃料电池阴极材料研究进展

中低温固体氧化物燃料电池的研制是固体氧化物燃料电池商业化的必然趋势,阴极材料的研制是影响其发展的关键问题之一.本文综述了近年来固体氧化物燃料电池ABO3型钙钛矿阴极材料的研究情况,并提出了其发展方向.     

Electron microscopy and physical studies of a new tungsten doped indium oxide transparent conductor

Advanced electron microscopy techniques - HREM and EFTEM - were used to study a new transparent conductive material: Indium Tungsten Oxide (IWO, in this work), which was synthesized from an aerosol. Indium oxide thin films doped with tungsten were deposited by spray pyrolysis for several %W concentration using (NH₄)10W₁₂O₄₁ and InCl₃ dissolved in ethanol as starting compounds. The W⁶⁺ ions fulfill the electronic and structural requirements to work like a donor impurity in In₂O₃ materials. The solution was sprayed onto silicon and glass substrates kept at 525 °C. Structural analysis was carried out from X-ray diffraction and selected area electron diffraction results reveal the structure of In₂O₃ with very small variations in crystallographic parameters. From electron microscopy observations, the nanostructured nature of the IWO compound was derived and from EFTEM micrographs the distribution of W reveals an increasing concentration in grain boundaries. Also scanning electron and atomic force microscopy techniques were used in order to determine the morphological and topological characteristics of the films. Also the number of carrier increases when W grows up to 5% weight and remains constant close to η_e = 28 × 10¹⁹ cm^− 3. The mobility grows up 2% W weight and the decreases monotonically. The optical transmittance of the IWO was close to 78% for all the cases. From XPS studies, low intensity signals form W orbitals were detected. The identified signals correspond to W-4d_5/2 and to W-4f_7/2 which were found very close to 250 and 37 eV respectively. These signals correspond to W atoms surrounded by O atoms in a similar configuration to the one reported for WO₃.     

喷雾热解法制备掺氟氧化锡导电玻璃及其性能

采用喷雾热解法,以四氯化锡和氟化铵为原料、喷瓶为雾化装置,在载玻片上制得氟掺杂二氧化锡(FTO)透明导电薄膜。运用XRD、SEM、紫外-可见分光光度计和四探针测试仪分别对薄膜进行了表征。研究了喷涂次数、衬底温度、前体浓度、掺杂浓度和醇水比对FTO薄膜光电性能的影响。结果表明,当衬底温度为500℃,SnCl4.5H2O浓度为0.81mol/L,NH4F浓度为0.1mol/L,醇水比为8:2,喷涂100次时,薄膜的光电性能较好,其方块电阻为13Ω/□,平均透光率为79%。     

Effect of precursor concentration on structural and optical properties of ZnO microrods by spray pyrolysis

ZnO films have been prepared by spray pyrolysis technique on glass substrate at 500 °C. Zinc Chloride has been used as a precursor. Effect of precursor concentration on structural and optical properties has been investigated. Homogenous films are obtained with precursor concentration rating between 0.1 M and 0.4 M. X-ray diffraction patterns show that ZnO films are polycrystalline with (002) plane as preferential orientation. Field emission scanning electron microscopy images show that ZnO films consist of microrods that their length increases with increasing precursor concentration and tallest microrods obtain by spraying precursor with 0.3 M concentration. The optical transmittance spectrum shows that transmittance increases with decreasing of the concentration and transmittance reaches to a maximum value of about 80% for the visible region ZnO films prepared with 0.1 M. Photoluminescence spectra at room temperature show an ultraviolet emission at 3.21 eV that can be related to band gap and two visible emissions at 2.88 eV and 2.38 eV.     

Optimization of indium tin oxide by pulsed DC power on single junction amorphous silicon solar cells

We investigated the optimal deposition conditions of a thin indium tin oxide (ITO) film on an amorphous silicon (a-Si) single-junction solar cell using pulsed DC magnetron sputtering. Thin ITO films were deposited while power, deposition time, pressure, gas flow and temperature were varied to find such conditions. The efficiency of a-Si solar cells with ITO films was 6.65% at the optimal conditions — a pulsed DC power of 40 W, a deposition time of 460 s, a pressure of 0.53 Pa, gas flow of 16 sccm and 151 °C. On the other hand, an a-SiGe tandem solar cell with the ITO films made at the optimal conditions yields an efficiency of 7.20%. We have also examined the surface morphology of ITO coated a-Si solar cells, using atomic force microscopy. Interestingly, a change in power does not alter the surface morphology at small length scales, whereas at large scales, the lower power sample had a lower surface roughness than the samples made with higher powers. We also find that for the range of deposition conditions examined, the value of the roughness exponent does not change with α ? 2/3 and a thin layer of ITO does not modify the surface morphology significantly.     

TiO2 pore-filling and its effect on the efficiency of solid-state dye-sensitized solar cells

Solid-state dye-sensitized solar cells have been the focus of much attention over the past few years. We have recently been able to demonstrate efficiencies in excess of 4% and further improvements are expected. Here we compare a range of different hole conductor materials and investigate a number of key parameters which affect their performance. Wetting and pore-filling of the nanoporous TiO₂ layer by the hole transporter appears to play a critical role in determining the final efficiency of the cell. A comparison of our results shows the importance of complete filling in contrast to just wetting of the nanoporous TiO₂ layer, which is generally underestimated.     

Texture control of fluorine-doped tin oxide thin film

The texture control of transparent oxide thin film, the crystalline orientation, is very important, because it is related to the electrical resistivity and the optical transparency. It is known that the crystal orientation could be controlled by varying precursor source, gas flow rate, and deposition temperature. We deposited fluorine-doped tin oxide (FTO) thin film on aluminoborosilicate glass by spraying water-based solution and ethyl alcohol-added solution. We showed in this research that (200) and (301) preferential orientation of FTO thin film can be controlled by the addition of ethyl alcohol to FTO coating solution. (200) oriented FTO thin film deposited from ethyl alcohol-added solution comprises of pyramidal crystallites with {111} polar faces, which contain {101} contact twin planes. {101} contact twin planes forms salient reentrant angle which provides nucleating sites and makes crystallites grow abnormally. (301) orientation is thought through Periodic Bond Chains of tin hydroxide which forms prismatic long crystallites. Prismatic crystallites are comprised of {110} crystal faces which contain {101} repeated contact twin. It is very helpful to control (200) or (301) oriented crystal formation in transparent conducting oxide film, because the texture affects the electrical and optical properties of transparent conducting oxide film. We suggest that ethyl alcohol addition plays a role to form crystallites with {111} polar faces corresponding to (200) preferential orientation. The crystal morphologies are changed by doping elements, precursor sources, deposition conditions like flow rate and temperatures, and solvents.     

Study on physical properties of indium-doped zinc oxide deposited by spray pyrolysis technique

Transparent conducting indium doped zinc oxide (IZO) thin films have been deposited on soda-lime glass substrates by the spray pyrolysis technique. The structural, electrical, and optical properties of these films were investigated as a function of substrate temperature. In this work the substrate temperature was varied between 350 °C and 500 °C. X-ray diffraction pattern reveals that at 350 °C dominant peak is (100) orientation. By increasing substrate temperature from 350 °C to 450 °C, sheet resistance decreases, from 302 Ω/□ to 26 Ω/□, then at 500 °C increases to 34 Ω/□. In the useful range for deposition (i.e. 450 °C to 500 °C), the orientation of the films was predominantly (002). The lowest sheet resistance (26Ω/□) is obtained at substrate temperature of about 450 °C with the transmittance of about 75%. Study of scanning electron microscopy images shows that films deposited at 400 °C, have grain size as large as 574 nm, while with increasing substrate temperature to 450 °C, grain size becomes smaller and reaches to a value of about 100 nm with spherical shape. At 500 °C grain size value would be around 70 nm with the same spherical shape.     

Imposed layer-by-layer growth of ZnO on GaN/sapphire substrates using pulsed laser interval deposition

The oxide semiconductor ZnO is of high interest for electrooptical applications due to its direct and wide band gap in the UV region. We present our results on pulsed laser deposition growth of ZnO on GaN-buffered Al₂O₃ substrates. Using in-situ reflection high energy electron diffraction, intensity oscillations were recorded and used to apply the technique of interval deposition. A significant improvement of structural thin film quality was achieved due to the expansion of the high quality from the first layers to the whole film thickness.     

Enhancement of light output power in GaN-based light-emitting diodes using indium tin oxide films with nanoporous structures

We fabricated GaN-based light-emitting diodes (LEDs) with a transparent ohmic contact made from nanoporous indium tin oxide (ITO). The nanoporous structures are easily made and controlled using a simple wet etching technique. The transmittance, sheet resistance, and root-mean-square surface roughness of the nanoporous ITO films are correlated strongly with the etch times. On the basis of the experimental values of these parameters, we choose an optimum etch time of 50 s for the fabrication of LEDs. The wall-plug efficiency of the LEDs with nanoporous ITO is increased by 35% compared to conventional LEDs at an injection current of 20 mA. This improvement is attributed to the increase in light scattering at the nanoporous ITO film-to-air interface.