Studies on micro-structural and electrical properties of spray-deposited fluorine-doped tin oxide thin films from low-cost precursor

Thin films of fluorine-doped tin oxide (SnO₂:F) on glass were prepared by spray pyrolysis technique using stannous chloride (SnCl₂) and ammonium fluoride (NH₄F) as precursors. The as-prepared films were characterized for their structural and electrical properties and are discussed in detail in this article. The surface morphology studies revealed that the films are grainy and the roughness of undoped films has been reduced on fluorine doping. X-ray diffraction (XRD) studies revealed that the films are polycrystalline. It further revealed that the undoped films grow along the preferred orientation of (211), whereas all the doped films grow along (200). The minimum sheet resistance 1.75 Ω/□ achieved in the present study for the films doped with 15 wt.% F is the lowest among the reported values for these materials prepared using SnCl₂ precursor. The electrical transport phenomenon has been analyzed in order to find out the possible scattering mechanism that limiting the mobility of charge carriers.     

Pyrosol deposition of fluorine-doped tin dioxide thin films

Fluorine-doped tin dioxide (SnO₂F) films were deposited from a tin tetrachloride solution in methanol utilizing a pyrosol deposition process. It is shown from thermodynamic calculations that the atmosphere during deposition is oxygen-rich and also suggested that chlorine and hydrogen chloride, which are produced during the deposition reaction, influence crystal growth. Detailed electrical, optical and structural properties of the material with respect to varying film thickness and substrate temperature are presented and discussed. Resistivity of the films deposited at 450 °C decreased from 6×10^–4 to 2×10^–4 cm, while the mobility increased from 14 to 45 cm²V^–1s^–1, respectively, when the film thickness was varied from 100 to 1650 nm. The carrier concentration was relatively unchanged for film thicknesses higher than 200 nm. Optimized SnO₂F films (600 nm) having a resistivity of 6×10^–4 cm, a carrier mobility of 20 cm²V^–1s^–1, a carrier concentration of 8×10²⁰ cm^–3 and a transmittance in excess of 80% are quite suitable as electrodes for amorphous silicon solar cells.     

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.     

Influence of the molar concentration and substrate temperature on fluorine-doped zinc oxide thin films chemically sprayed

The effect of both the molar concentration of the starting solution and the substrate temperature on the electrical, morphological, structural and optical properties of chemically sprayed fluorine-doped zinc oxide (ZnO:F) thin films deposited on glass substrates is analyzed in this work. All the starting solutions employed were aged for 10 days before the deposition. The results show that as the molar concentration increases, a decrease in the electrical resistivity values is obtained, reaching the minimum resistivity in films ZnO:F deposited from a 0.4 M solution at 500 °C. A further increase in the molar concentration leads to a very slight increase in the resistivity. On the other hand, as the substrate temperature is increased, the resistivity decreases and a tendency towards to minimum value is evidenced; taking the molar concentration as parameter, minimum values are reached at 500 °C. The obtaining of ZnO:F thin films, with a resistivity as low as 7.8 × 10^− 3 Ω cm (sheet resistance of 130 Ω/□ and film thickness of 600 nm) measured in as-deposited films is reported here for the first time. The concurrent effect of the high molar concentration of the starting solution, the substrate temperature values used, and the ageing of the starting solution, which might cause polymerization of the zinc ions with the fluorine species, enhance the electrical properties. The structure of the films is polycrystalline, with a (002) preferential growth. Molar concentration rules the surface morphology as at low concentration an hexagonal and porous structure is developed changing to a uniform compact and small grain size surface in the films deposited with the high molar concentrations.     

Thickness-dependent properties of sprayed iridium oxide thin films

Iridium oxide thin films with variable thickness were deposited by spray pyrolysis technique (SPT), onto the amorphous glass substrates kept at 350 °C. The volume of iridium chloride solution was varied to obtain iridium oxide thin films with thickness ranging from 700 to 2250 Å. The effect of film thickness on structural and electrical properties was studied. The X-ray diffraction (XRD) studies revealed that the as-deposited samples were amorphous and those annealed at 600 °C for 3 h in milieu of air were polycrystalline IrO₂. The crystallinity of Ir-oxide films ameliorate with film thickness thereby preferred orientation along (1 1 0) remains unchanged. The infrared spectroscopic results show Ir–O and Ir–O₂ bands. The room temperature electrical resistivity (ρ_RT) of these films decreases with increase in film thickness. The p-type semiconductor to metallic transition was observed at 600 °C.     

On the effect of acetic acid on physical properties of chemically sprayed fluorine-doped ZnO thin films

Fluorine-doped zinc oxide (ZnO:F) thin films onto sodocalcic glass substrates, starting from a highly concentrated starting solution (0.4 M) containing zinc acetate and hydrofluoric acid diluted in a mixture of deionized water, acetic acid , and methanol, using the chemical spray deposition technique, were deposited and characterized. The effect of the acetic acid content in the starting solution, and the substrate temperature on the electrical resistivity, structure, morphology and optical characteristics was studied. The samples were polycrystalline in nature, but as the acetic acid content in the starting solution increases, the preferential orientation shows a switching from (002) to (101). For a predetermined deposition temperature, as the acetic acid content increases, the film resistivity values show an increase. The minimum resistivity in the order of 6 × 10^− 3 Ω cm was found for the films deposited with the lowest acetic acid content used. The surface morphology varies from agglomerated grains to rod-like shaped grains as a function of the acetic acid content.     

Modeling the optical properties of tin oxide thin films

Searching the many papers reporting on the optical characteristics of tin oxide thin films, an obvious question arises: what is the origin of the very large differences in the reported optical and electrical properties of these films? The objective of the present work is to resolve this question by applying a modeling approach, simulating the refractive index of SnO, SnO₂, SnO + SnO₂, and porous tin oxide films in the visible range of the spectrum under various structure and composition conditions. Using the semi-empirical model of Wemple and DiDomenico for the dielectric function below the interband absorption edge of ionic and covalent solids, and the effective-medium theory of Bruggeman, the refractive indices of SnO, SnO₂, several mixtures of SnO and SnO₂ and various porous tin oxide films were calculated. The resulting data are compared with some published data to suggest the compositional and structural characteristics of the reported oxides. The correlation between the optical properties of the studied thin films and film composition is also indicated. It is proposed that the large spread in reported optical data is possibly a spread in the composition of the samples.     

溶剂体系对c轴择优取向ZnO薄膜结构和光学性质的影响

以无水乙醇、乙二醇甲醚、乙二醇甲醚/乙醇混合溶液(1∶1)为溶剂体系,采用溶胶-凝胶法制备了ZnO透明薄膜,并利用场发射扫描电镜、X射线衍射和反射光谱仪等研究了溶剂体系对薄膜组成、结构和光学性能的影响。结果表明,3种溶剂所制备的ZnO薄膜均为六方纤锌矿型结构,具有c轴择优取向;以乙二醇单甲醚/乙醇混合溶液(1∶1)为溶剂制备的ZnO薄膜平整、致密,在可见光区域透光率达到90%左右,禁带宽度为3.25eV,具备制作薄膜太阳能电池透明导电电极材料的应用价值。     

Dopant effects in sprayed tin oxide films

The effects of donor impurities such as antimony, fluorine and antimony plus fluorine on the structural, electrical and optical properties of tin oxide films prepared by spray pyrolysis have been studied. The dopant concentration in the films, as determined by Auger analysis, is less than that in the solution. The mobility of free carriers in doped tin oxide films is found to depend on the nature and the concentration of the dopant. Fluorine doping of tin oxide films results in the highest conductivity and the highest optical transmission in the visible region.     

UV absorption studies of undoped and fluorine-doped tin oxide films

Tin oxide films undoped and doped with fluorine are prepared by a chemical vapour deposition technique involving the oxidation of SnCl₂. For fluorine doping a mixture of trifluoroacetic acid and deionized water is used. The optical properties of these films are investigated in the UV region (225–350 nm). It is observed that for an undoped tin oxide the absorption edge lies at 3.71 eV. It is also observed that for fluorine-doped tin oxide films the absorption edge shifts towards higher energies, which is related to the Moss-Burstein shift. In the case of fluorine-doped tin oxide films, depending on the fluorine concentration, the absorption edge lies in the range 3.89–4.07 eV. Undoped and fluorine-doped tin oxide films show a direct transition at 4.03 eV and in the range 4.17–4.29 eV respectively, whereas the indirect transition for undoped and fluorine-doped tin oxide films occurs at 3.35 eV and in the range 3.55–3.67 eV respectively.     

Preparation and properties of sprayed undoped and fluorine doped tin oxide films

Thin films of undoped and fluorine doped tin oxide have been prepared on borosilicate glass plates by a spray pyrolysis technique. The effect of process parameters, such as tin chloride concentration in the precursor solution, substrate to nozzle distance, carrier gas (air) flow rate, substrate temperature, and doping level of fluorine in the spray solution, on the physical properties of the tin oxide thin films have been investigated. The grown films were polycrystalline in nature above 350°C, the [110] reflection having the maximum intensity in all cases. Films of about 10⁻³ Ω cm resistivity and high visible transparency of about 88% have been obtained at the optimum substrate temperature of 425°C and fluorine doping concentration of 57 at.%. The optical investigations show that the optimized films have a direct allowed bandgap of 4.25 eV and indirect allowed bandgap of 2.71 eV.     

Effect of platinum distribution on the hydrogen gas sensor properties in tin oxide thin films

Tin oxide and platinum layers were deposited on oxidized silicon wafers by ion-beam sputtering. The hydrogen gas sensing properties of undoped films and platinum-doped films were examined at 300°C for films annealed at 500°C. It was observed that the surface platinum when annealed together with the tin oxide film increased the sensitivity and reduced the response time compared with those of undoped films. Longer annealing tended to shift the optimum sensor thickness to a thicker side; the optimum thickness changed from 17 to 37 nm as the annealing time increased from 2 to 50 h. The interdiffusivity between the platinum and the tin atoms in the bulk was negligibly small at 300°C.     

Electron microscopy studies of sprayed thin tin dioxide films

Electron microscopy studies of thin tin dioxide films (about 80 nm thick) prepared by spraying from a mixture of SnCl₄ and ethyl acetate indicate that such films are amorphous at substrate temperatures of 340–350°C, partially crystallized at 360–410°C and polycrystalline at 420–500°C. Grains about 40 nm in size were found for all polycrystalline structures, independent of the substrate temperature. Only SnO₂, having values of the interplanar spacing d in agreement with the ASTM data, was clearly identified by electron diffraction. Scanning electron microscopy studies indicate smooth surfaces with pits of varying size and density depending on the preparation conditions.     

Effect of sputtering pressure and annealing temperature on the properties of indium tin oxide thin films

Indium tin oxide (ITO) thin films were deposited on glass substrates by RF sputtering system at different sputtering pressure (SP) (20–34 mTorr) and room temperature. The sputtering pressure effects on the deposition rate, electro-optical and structural properties of the as-deposited films were systematically investigated. The optimum sputtering pressure of 27 mTorr, giving a good compromise between electrical conductivity and optical transmittance was found to deposit films. The films were heat-treated in vacuum (200–450 °C) and their electro-optical and structural properties investigated with temperature. A criterion factor Q, which is the ratio between the normalized average transmission to normalized resistivity was defined. It has been observed that Q has its maximum value for heat treatment at 400 °C and the X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis proves the films have preferred crystal growth towards (2 2 2) direction and average size of grains are 35–40 nm.     

Effect of substrate temperature and precursor ratio on properties of thin ZnS films sprayed by improved method

Zinc sulphide (ZnS) thin films were prepared by improved spray pyrolysis (ISP) method. The ISP parameters, such as carrier gas flow rate, solution flow rate and substrate temperature, were controlled with an accuracy of ±0.25 lpm, ±1 ml/h and ±1 °C, respectively. The solution was sprayed in a pulsed mode. The substrate temperature was optimized by analyzing substrate temperature dependent properties of thin films. The thin film deposited at a temperature of 450 °C was dense and fairly smooth with satisfactory crystallinity and very small impurity content. The effect of precursor ratio in the solution on structural, compositional and optical properties of thin ZnS films, deposited at a temperature of 450 °C, was studied. A gradual increase in band gap energy from 3.524 eV to 3.634 eV, refractive index from 2.5 to 2.9 and dielectric constant from 6.6 to 8.7 were observed with the variation of solution precursor (Zn:S) ratio from (1:2) to (1:6). The structural and compositional studies support this kind of enhancement in optical properties. The results show that the thin ZnS film prepared by ISP at the substrate temperature of 450 °C from a solution with specific precursor ratio can be used for optoelectronic and photovoltaic applications.     

Lanthanum concentration dependence of electrical properties in tin oxide thin films

Lanthanum doped tin oxide thin films were prepared on boron-silicon glass substrates by spray pyrolysis. Lanthanum concentration was varied from 0 to 1.0 wt%. The microstructures, sheet resistance and thermal stabilities of the lanthanum doped tin oxide thin films have been investigated in order to determine the role of this dopant on electrical properties. X-ray diffraction (XRD) result shows the deposited thin film is mainly rutile SnO₂. And atomic force microscopy (AFM) reveals that the thin film has smooth surface with no cracks and defects. And it exhibits a typical bimodal grain size distribution with an average grain size of 95 nm. The sheet resistances of the thin films have a complex dependence on the lanthanum concentration. With increasing lanthanum concentration, the sheet resistances of tin oxide thin films were slightly increased and then abruptly decreased. Moreover, when the lanthanum concentration of 0.5 wt% was reached, the specimen exhibits excellent electrical properties. Because of its effectiveness in improving homogeneity of operating surface temperature and thermal stability, lanthanum appears to be an attractive additive for the tin oxide thin films.     

Optical properties of vapour chopped and nonchopped tin oxide thin films

The optical properties of vapour chopped and nonchopped undoped tin oxide thin films (1000 Å) have been studied. The films were prepared by thermal oxidation (in air) of vacuum evaporated vapour chopped and nonchopped tin thin films. Effect of varied oxidation temperature and duration was also studied. As revealed by X ray diffraction studies, polycrystalline (both tetragonal and orthorhombic) films were obtained. The refractive indices ranged from 1.575 to 1.667 and 1.600 to 1.810 for nonchopped and vapour chopped tin oxide films respectively. Refractive index was found to increase after ageing. The effect was found lower in vapour chopped films. The direct band gap values ranged from 2.18 to 2.51 and 2.95 to 3.08 for nonchopped and vapour chopped tin oxide films respectively. The films showed high optical transmittance in the visible range of the spectrum. It was observed that the heat treatment process strongly affects the properties of the films. The vapour chopped tin oxide thin films showed higher packing density, optical band gap and refractive index with lower ageing effect.     

Preparation of sprayed tin oxide transparent conducting films and their structural and electrical properties

The aim of the present work was to prepare fairly uniform transparent and conductive tin oxide films using an inexpensive and easily controllable method. The technique adopted was spray deposition. The design of the spray deposition apparatus takes into consideration its reliability in controlling the different parameters affecting the formation of deposited layers, as well as its adaptability for large-area applications. The growth rate was independent of substrate type but increased with increasing substrate temperature. The crystalline structure of the films was found to be a function of substrate temperature and film thickness. The X-ray diffraction patterns showed that the preferred orientation was (110) and the grain size was in the range of a few tens of nanometres. The variation of sheet resistance with deposition parameters was studied and an empirical formula relating the sheet resistance to the film thickness was obtained. The prepared tin oxide layers of thicknesses up to 200 nm were found to have a transparency of about 80% to 85%, which makes them suitable for application as a single antireflecting coating of silicon solar cells.