Characterization of antimony-doped tin oxide films for solar cell applications

Transparent conducting undoped tin oxide (SnO₂) and antimony-doped tin oxide (ATO) films were deposited onto Pyrex glass and single-crystal silicon substrates using an inexpensive chemical vapour deposition system. SnCl₂ and SbCl₃ were used as the source reagents with oxygen and nitrogen respectively as the carrier gases. The deposition conditions were as follows: temperature, 350–500 °C; oxygen flow rate, 0.8–3.25 1 min^-1; nitrogen flow rate, 0–0.1 1 min^-1; deposition time, 5–20 min. The antimony concentration in the film and its physical properties were the same on both substrates. A figure of merit (Tr¹⁰/R_sh where Tr is the transmission at a particular wavelength and R_sh is the sheet resistance) was used to compare the performance of these films. The maximum figure of merit for SnO₂ films $\text{(1.43 × 10}{}^{-3}\text{Ω}{}^{-1}\text{(}\text{Tr}\text{= 95\% and R}{}_{\mathrm{<mtext>sh</mtext>}}\text{= 420 Ω/□))}$ was obtained when they were deposited at 500 °C with oxygen at a flow rate of 1 1 min^-1. The sheet resistance of antimony-doped films is a minimum at 3 mol.% Sb and the transmission decreases as the antimony concentration increases. The maximum figure of merit obtained for ATO films was $\text{6.78 × 10}{}^{-3}\text{Ω}{}^{-1}\text{(}\text{Tr}\text{= 90.6\% and R}{}_{\mathrm{<mtext>sh</mtext>}}\text{= 55 Ω/□)}$ for an antimony content of 3 mol.% and a nitrogen flow rate of 0.07 1 min^-1. These results are explained theoretically and are compared with those reported by other workers.     

Optical and electrical properties of SnO2 films prepared by chemical vapour deposition

Transparent and conducting SnO₂ films are prepared at 500°C on quartz substrates by chemical vapour deposition technique, involving oxidation of SnCl₂. The effect of oxygen gas flow rate on the properties of SnO₂ films is reported. Oxygen with a flow rate from 0·8–1·35 lmin⁻¹ was used as both carrier and oxidizing gas. Electrical and optical properties are studied for 150 nm thick films. The films obtained have a resistivity between 1·72 × 10⁻³ and 4·95 × 10⁻³ ohm cm and the average transmission in the visible region ranges 86–90%. The performance of these films was checked and the maximum figure of merit value of 2·03 × 10⁻³ ohm⁻¹ was obtained with the films deposited at the flow rate of 1·16 lmin⁻¹.     

Structural, electrical and moisture resistance properties of Ga-doped ZnO films

The structural, electrical and moisture resistance properties of Ga-doped ZnO (GZO) films with 200 nm thickness in terms of their dependence on oxygen gas flow rate (fO₂) during deposition were studied. GZO films are deposited on glass substrates by ion plating with DC arc discharge. After a reliability test at a temperature of 60 °C and a relative humidity of 95% for 500 h, the percentage of resistivity change of GZO films decreased from 16–20% to 3–11% with increasing fO₂ from 6–12 to 14–25 sccm. The minimum percentage of the resistivity change was observed in the GZO films deposited at fO₂ of 21 sccm and the resistivity after the reliability test was 3.5 × 10^{− 4} Ω cm. The effects of the intrinsic defects on the percentage of resistivity change are discussed on the basis of electrical and optical characteristics of GZO films.     

Properties of stannic oxide thin films produced from the SnCl4-H2O and SnCl4-H2O2 reaction systems

Transparent and conductive stannic oxide films were produced at the relatively low temperature of 250°C from the SnCl₄-H₂O and SnCl₄-H₂O₂ reaction systems by a chemical vapour deposition method. The films were not doped with impurities. Films formed from the first system are superior to those formed from the second with respect to electrical properties although they have a lower deposition rate at the same deposition temperature. The former system gives rise to films with resistivities in the range 10–10^-3 Ω cm between 250 and 400°C. The latter system produces films with resistivities in the range 10²–10^-2 Ω cm between 250 and 450°C. The electrical properties depend on the absorption of hydrogen peroxide as well as on the grain size, which depends on the deposition temperature and the reaction system. The spectral transmissivity for films 0.36–1.1 μm thick varies over the range 80–95% in the regions between 400 and 650 nm for both systems. Different reaction mechanisms take place in different temperature regions for both systems since there are two activation energies in the plot of deposition rate as a function of temperature.     

SnO2:F films synthesized by chemical vapour deposition technique using hydrofluoric acid as doping material

Highly transparent and highly conducting films of SnO₂:F were prepared by chemical vapour deposition technique. The films prepared at 350°C substrate temperature and 2·5 lit. min⁻¹ flow rate of oxygen showed maximum figure of merit. The optimum doping concentration of fluorine was 1·02 wt%. The Hall experiment showed that the films prepared at optimum conditions had high carrier concentration and high mobility.     

Deposition of improved optically selective conductive tin oxide films by spray pyrolysis

Antimony-doped SnO₂ films with a resistivity as low as 9×10^–4 cm were prepared by spray pyrolysis. Structural, electrical and optical properties were studied by varying the antimony concentration, film thickness and deposition temperature. About 94% average transmission in the visible region and about 87% infrared reflectance were obtained for antimony-doped SnO₂ films by a systematic optimization of the preparation parameters. As the best combination, an average transmission of 88% in the visible region and an infrared reflectance of 76% was possible for the doped SnO₂ films.     

Preparation and properties of reactively co-sputtered transparent conducting films

The method of reactive co-sputtering was used to determine the optimum dopant concentration for low resistivity In₂O₃/SnO₂ and SnO₂/Sb₂O₅ films. The optimum concentration of SnO₂ in In₂O₃ was approximately 10 mol. % and of Sb₂O₅ in SnO₂ about 7 mol. %. The resistivity increased sharply at lower dopant concentrations but changed only slightly at higher dopant concentrations. The lowest resistivity for reactively sputtered highly transparent In₂O₃/SnO₂ films was 1.5 × 10^-3 Ω cm and for SnO₂/Sb₂O₅ films 3 × 10^-3 Ω cm. Reactively sputtered In₂O₃/SnO₂ films show a strong (111) texture and have an extremely smooth surface.     

Electrical and optical properties of indium tin oxide films prepared on plastic substrates by radio frequency magnetron sputtering

The influence of deposition power, thickness and oxygen gas flow rate on electrical and optical properties of indium tin oxide (ITO) films deposited on flexible, transparent substrates, such as polycarbonate (PC) and metallocene cyclo-olefin copolymers (mCOC), at room temperature was studied. The ITO films were prepared by radio frequency magnetron sputtering with the target made by sintering a mixture of 90 wt.% of indium oxide (In₂O₃) and 10 wt.% of tin oxide (SnO₂). The results show that (1) average transmission in the visible range (400-700 nm) was about 85%-90%, and (2) ITO films deposited on glass, PC and mCOC at 100 W without supplying additional oxygen gas had optimum resistivity of 6.35 × 10⁻⁴ Ω-cm, 5.86 × 10⁻⁴ Ω-cm and 6.72 × 10⁻⁴ Ω-cm, respectively. In terms of both electrical and optical properties of indium tin oxide films, the optimum thickness was observed to be 150-300 nm.     

Process study of chemically vapour-deposited SnOx (x≈2) films

The physical characteristics of SnO_x(x≈2) films deposited onto Pyrex glass substrates by chemical vapour deposition are studied. The temperature dependence indicates that films deposited at 600 °C have good polycrystallinity. The electrical conductivity of the 600 °C films is mainly controlled by the variation in the SnCl₄ vapour flow rate. A subsequent thermal annealing process can even reduce the sheet resistance to 400ω/□. In addition, the visible absorption shows that the 600 °C films tend to lose their transparency in the short wavelength range of the visible spectrum.     

Transport properties of thin SnO2〈Sb〉 films grown by pulsed laser deposition

Thin SnO₂<Sb> films grown by pulsed laser deposition have been characterized by X-ray diffraction, optical spectroscopy, and scanning electron microscopy. The carrier mobility and concentration in the films have been determined as functions of target composition (0–8 at % Sb) using Hall effect measurements, and the resistivity of the films has been measured by a four-probe technique. The lowest resistivity (ρ = 2 × 10^?3 Ω cm) and the highest transmission (? 85%) of the films in the spectral range 400-800 nm have been obtained at a target composition Sb/(Sn + Sb) = 2 at %. The observed variation in the resistivity of the films is determined by changes in carrier concentration to a greater extent than by changes in carrier mobility. X-ray photoelectron spectroscopy results demonstrate that the predominant charge state of the antimony in the films is Sb⁵⁺.     

Studies on tin oxide films prepared by electron beam evaporation and spray pyrolysis methods

Transparent conducting tin oxide thin films have been prepared by electron beam evaporation and spray pyrolysis methods. Structural, optical and electrical properties were studied under different preparation conditions like substrate temperature, solution flow rate and rate of deposition. Resistivity of undoped evaporated films varied from 2.65 × 10⁻² ω-cm to 3.57 × 10⁻³ ω-cm in the temperature range 150–200°C. For undoped spray pyrolyzed films, the resistivity was observed to be in the range 1.2 × 10⁻¹ to 1.69 × 10⁻² ω-cm in the temperature range 250–370° C. Hall effect measurements indicated that the mobility as well as carrier concentration of evaporated films were greater than that of spray deposited films. The lowest resistivity for antimony doped tin oxide film was found to be 7.74 × 10⁻⁴ ω-cm, which was deposited at 350°C with 0.26 g of SbCl₃ and 4 g of SnCl₄ (SbCl₃/SnCl₄ = 0.065). Evaporated films were found to be amorphous in the temperature range up to 200°C, whereas spray pyrolyzed films prepared at substrate temperature of 300– 370°C were poly crystalline. The morphology of tin oxide films was studied using SEM.     

Thin tin oxide films of low conductivity prepared by chemical vapour deposition

Homogeneous SnO₂ films were produced from dibutylin diacetate by chemical vapour deposition at rates of 10–30 nm min^?1. The films obtained had conductiveties between 3 and 25Ω^?1 cm^?1 and they showed a high quantum efficiency for dye sensitization. Under various conditions a linear dependence of the thickness and resistivity of the films on deposition time was observed. The transmission of most of the films was greater than 90%. SnO₂ films with a thickness of more than 500 nm exhibit absorption. The refractive index of these films, as determined by UV-visible interference measurements in reflection, is between 1.8 and 2.2 in the wavelength range 300–860 nm and agrees well with the refractive index determined from interference microscope measurements. Heat treatment of the films exposed to air lowers their conductivity and refractive indices.     

Structural, optical and electrical properties of reactively sputtered Ag2Cu2O3 films

Ag₂Cu₂O₃ thin films were deposited on glass substrates by RF magnetron sputtering of an equiatomic silver-copper target (Ag_0.5Cu_0.5) in reactive Ar-O₂ mixtures. The reactive sputtering was done at varying power, oxygen flow rate and deposition temperature to study the influence of these parameters on the deposition of Ag₂Cu₂O₃ films. The film structure was determined by X-ray diffraction, while the optical properties were examined by spectrophotometry (UV-vis-NIR) and photoluminescence. Furthermore, the film thickness and resistivity were measured by tactile profilometry and 4-point probe, respectively. Additional mobility, resistivity and charge carrier density Hall effect measurements were done on a few selected samples. The best films in terms of stoichiometry and crystallography were achieved with a sputtering power of 100 W, oxygen and argon flow rates of 20 sccm (giving a deposition pressure of 1.21 Pa) and a deposition temperature of 250 °C. The optical transmittance and photoluminescence spectra of films deposited with these parameters indicate several band gaps, most prominently, a direct one of around 2.2 eV. Electrical characterization reveals charge carrier concentrations and mobilities in the range of 10²¹-10²² cm^− 3 and 0.01-0.1 cm²/Vs, respectively.     

Epitaxial growth of transparent tin oxide films on (0 0 0 1) sapphire by pulsed laser deposition

The growth of epitaxial SnO₂ on (0 0 0 1) sapphire using pulsed laser deposition is examined. X-ray diffraction analysis shows that the films are highly a-axis oriented SnO₂ with the rutile structure. Three distinct symmetry-equivalent in-plane epitaxial orientations were observed between the film and substrate. With increasing growth temperature, both the growth rate and surface roughness increase with columnar grain formation. Carrier concentration ranged from 10¹⁷ to 10¹⁹ cm⁻³, with mobility of 0.5-3 cm²/V s. The resistivity of the films increases with increasing growth temperature, suggesting a lower density of oxygen vacancy-related defects formed during high temperature deposition.     

F-doped SnO2 thin films grown on flexible substrates at low temperatures by pulsed laser deposition

Fluorine-doped tin oxide (SnO₂:F) films were deposited on polyethersulfone plastic substrates by pulsed laser deposition. The electrical and optical properties of the SnO₂:F films were investigated as a function of deposition conditions such as substrate temperature and oxygen partial pressure during deposition. High quality SnO₂:F films were achieved under an optimum oxygen pressure range (7.4-8 Pa) at relatively low growth temperatures (25-150 °C). As-deposited films exhibited low electrical resistivities of 1-7 mΩ-cm, high optical transmittance of 80-90% in the visible range, and optical band-gap energies of 3.87-3.96 eV. Atomic force microscopy measurements revealed a reduced root mean square surface roughness of the SnO₂:F films compared to that of the bare substrates indicating planarization of the underlying substrate.     

High-rate deposition of Ta-doped SnO2 films by reactive magnetron sputtering using a Sn-Ta metal-sintered target

Ta-doped SnO₂ films were deposited on glass substrate (either unheated or heated at 200 °C) by reactive magnetron sputtering with a Sn-Ta metal-sintered target using a plasma control unit (PCU) and mid-frequency (mf, 50 kHz) unipolar pulsing. The PCU feedback system precisely controlled the flow of the reactive and sputtering gases (O₂ and Ar, respectively) by monitoring either discharge impedance or the plasma emission of the atomic O* line at 777 nm. The planar target was connected to the switching unit, which was operated in unipolar pulse mode. Power density on the target was maintained at 4.4 W cm^− 2 during deposition. The lowest obtained resistivity for the films deposited on heated substrate was 6.4 × 10^− 3 Ωcm, where the deposition rate was 250 nm min^− 1.     

Characterizations of SnO2 and SnO2:Sb thin films prepared by PECVD

Structural characterizations of tin oxide (SnO₂) thin films, deposited by plasma-enhanced chemical vapor deposition (PECVD), were investigated with scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results show that the films are porous, the crystalline structure transforms from crystalline to amorphous phase as deposition temperature changes from 500°C to 200°C, and the chemical component is non-stoichiometric (Sn:O is 1.0716 prepared at 450°C with a value of O₂ flow 3.5 l/min). Sheet resistance of the thin films decreases with increasing of deposition temperature. Whereas, sheet resistance increases with increasing of oxygen flow. Tin oxide doped with antimony (SnO₂:Sb) thin films prepared by same method have a better selectivity to alcohol than to carbon monoxide; the maximum sensitivity is about 220%. The gas-sensing mechanism of SnO₂ thin films is commentated.     

Structural,electrical and optical properties of r.f.-magnetron-sputtered SnO2:Sb film

The effects of gas composition, pressure and substrate temperature on the properties of relatively thick (0.2–0.8 μm) SnO₂ films deposited onto fused quartz substrates by r.f. magnetron sputtering are reported. The lowest resistivity of about 2 × 10^?3ωcm was attained for high rate deposition conditions of about $\text{1000}\text{A}\text{?}\text{min}{}^{\mathrm{?1}}$ on substrates at a temperature of 400°C in an atmosphere of 10% O₂. This value corresponds to a carrier density of 3 × 10²⁰cm^?3 and a mobility of 10 cm²V^?1s^?1. The crystal structure was found to be sensitive to all the above parameters. Low resistivity films showed a highly preferred orientation of (101) parallel to the substrate.     

Undoped tin oxide thin films obtained by the sol gel technique,starting from a simple precursor solution

In this work, we report the synthesis and the structural, optical and electrical properties of undoped tin oxide thin films obtained by the sol–gel technique. The films have been prepared from a simpler precursor solution than other ones reported; it is based on stannous chloride (SnCl₂·2H₂O), ethanol, glycerol and triethylamine. The films are deposited on glass slide substrates and sintered at temperatures in the 300–550 °C range, in an open atmosphere. A second thermal treatment in vacuum is made in order to decrease the resistivity of the films. The X-ray diffraction patterns show the tetragonal phase of SnO₂ with a small preferential orientation in the (110) plane. All films show high optical transmission (~85%) and a direct band gap value around of 3.8 eV. The minimum resistivity value, 2 × 10⁻¹ Ohm-cm, is obtained for the films sintered at 300 and 350 °C and thermal treated in vacuum at 500 °C for 1 h. The decrease of the resistivity with the thermal treatment in vacuum is associated with an increase in the oxygen vacancies concentration.     

Subject index

Transparent conducting films of cadmium-tin oxide were prepared by d.c. reactive sputtering from a CdSn alloy target in an ArO₂ atmosphere. The electrical and optical properties of the films were found to depend on the oxygen concentration in the gas mixture. The lowest resistivity obtained was 4 × 10^?4 Ω cm and the average optical transmission was 90% over the visible region. The structure of the films was examined by X-ray diffraction. The thick films exhibited CdSnO₃ (200) or Cd₂SnO₄ (001) and (130) peaks but most of the thin films were amorphous. The post-deposition annealing of the films in an argon atmosphere affected their electrical and optical properties.