Microstructural study of SnO2 thin layers deposited on sapphire by sol-gel dip-coating

Tin oxide (SnO₂) films have been grown onto (006) sapphire substrates by sol-gel dip-coating using tin alkoxide solutions. It is shown, using grazing-incidence X-ray diffraction, reciprocal space mapping and atomic force microscopy, that thermal annealing at 500 °C induces the crystallization of SnO₂ in the rutile-type phase. Further annealing treatments at temperatures lower than 1100 °C give rise to slow grain growth controlled by surface diffusion, whereas rapid grain growth (controlled by an evaporation-condensation mechanism) takes place at temperatures higher than 1100 °C. Concomitantly, the film splits into isolated islands and a fibre texture occurs at higher temperatures.     

SnO2:Ga thin films as oxygen gas sensor

Ga-doped SnO₂ thin films deposited by spray pyrolysis were investigated as oxygen gas sensors. Gallium was added to the films to enhance the catalytic activity of the surface’s film to oxygen. Film resistance was studied in an environment of dry air loaded with oxygen in excess at partial pressures in the range from 0 to 8.78×10³ Pa. The best sensitivity lies close to partial pressures of 133.3 Pa. Film sensitivity reach a maximum at 350 °C. For this temperature and a doping concentration of 3 at.% of Ga in the starting solution, a sensitivity up to 2.1 was obtained.     

Structural, electrical and optical characteristics of SnO2:Sb thin films by ultrasonic spray pyrolysis

Antimony-doped tin oxide (SnO₂:Sb) thin films were fabricated by an ultrasonic spray pyrolysis method. The effect of antimony doping on the structural, electrical and optical properties of tin oxide thin films were investigated. Tin(II) chloride dehydrate (SnCl₂·2H₂O) and antimony(III) chloride (SbCl₃) were used as a host and a dopant precursor. X-ray diffraction analysis showed that the non-doped SnO₂ thin film had a preferred (211) orientation, but as the Sb-doping concentration increased, a preferred (200) orientation was observed. Scanning electron microscopy studies indicated that the polyhedron-like grains observed for the non-doped SnO₂ thin films became rounder and decreased in size with the Sb-doping concentration. The lowest resistivity (about 8.4 × 10^− 4 Ω·cm) was obtained for the 3 at.% Sb-doped films. Antimony-doping led to an increase in the carrier concentration and a decrease in Hall mobility. The transmittance level in the near infrared region was lowered with the Sb-doping concentration.     

氧化锡薄膜场致发射研究

利用直流磁控反应溅射法,制备氧化锡薄膜,利用扫描电镜等方法对氧化锡薄膜微观结构进行分析。在低真空下,对不同厚度的氧化锡薄膜进行场致发射测试,结果显示,在氧化锡薄膜厚度为60nm时,场致发射性能最佳,当电流密度为10μA/m2时,开启电压为4.5 V/μm,阴阳两极电场为7 V/μm时,有较佳的场发射密度,同时发光亮度达到2180 cd/m2,结果表明,氧化锡薄膜在场发射平板显示及真空电子器件方面具有较好的应用潜力。     

Initial growth of SnO2 thin film on the glass substrate deposited by the spray pyrolysis technique

Spray pyrolysis of di-n-butyltin(IV) diacetate (DBTDA) has led to the deposition of [200]-oriented SnO₂ film on a glass substrate. In order to clarify growth mechanism of the preferential orientation the sprayed SnO₂ thin film has been investigated by using the atomic force microscopy and the X-ray photoelectron spectroscopy. The results have suggested that the sprayed solution forms the SnO₂ small particles on the glass substrate and they spread overall relatively soon. At the very early stage each particle grows with almost the same rate and only its density increases with no change in a surface roughness.     

Role of surface properties of MoO3-doped SnO2 thin films on NO2 gas sensing

The role of surface morphology of MoO₃-doped SnO₂ thin film on the gas sensing properties is analyzed. SnO₂ thin films doped with 1, 3, 5 and 10 wt% MoO₃ are prepared by sol-gel spin coating process. Structural and morphological properties are studied using glancing angle X-ray diffractometer, atomic force microscopy, transmission electron microscopy and high resolution transmission electron microscopy. Energy dispersive X-ray analysis and X-ray photoelectron spectroscopy studies are used for chemical analysis. A good correlation is found between the characteristics of the surface and gas sensing properties of these films. MoO₃ addition is found responsible for increase in acidic nature of films which in turn increases their sensitivity and selectivity towards NO₂ gas.     

H2S sensing property of porous SnO2 sputtered films coated with various doping films

Pure SnO₂ films and Ag-, Cu-, Pt-, and Pd-doped SnO₂ films were investigated for H₂S sensing properties. SnO₂ films were deposited by DC magnetron sputtering at various substrate temperatures and discharge gas pressures. As the discharge gas pressure increased and the substrate temperature decreased, the film became porous. Doping with Cu or Ag film improved the sensitivity, and the highest sensitivity was obtained in the porous SnO₂ film coated with an Ag film 16 nm thick. According to the X-ray diffraction (XRD) pattern, Ag deposited on SnO₂ film transformed to Ag₂S upon exposure to H₂S. When the Ag-doped film sensor was operated at a low temperature, the sensitivity was extremely high, but the recovery was insufficient. By increasing the operation temperature, the recovery was improved but the sensitivity decreased.     

The influence of the precursor concentration on the properties of SnO2 thin films

The paper presents a comparative approach concerning the properties of SnO₂ thin layers obtained via spray pyrolysis deposition (SPD) using precursors with different molar concentrations. The photocatalytic efficiency of the samples was tested in the photodegradation of methylene blue. The sample obtained from 0.001 M precursor exhibit higher electrical conductivity (current-voltage curves) and photocatalytic properties compared with the samples obtained from precursors with lower molar concentrations. Regardless of the molar concentrations the samples present tetragonal structure (X-ray diffraction) and granular morphology (atomic force microscopy).     

Effect of density and thickness on H2-gas sensing property of sputtered SnO2 films

Undoped and Pd-doped SnO₂ films were deposited under various conditions for the investigation of the effect of Pd doping, porosity, and thickness on their H₂ gas sensing properties. The temperature of the substrate and the pressure of the discharge gas were varied. All films formed were composed of columns with thicknesses between 20 and 30 nm. The film density decreased as the discharge gas pressure increased and the substrate temperature decreased. It showed values between 4.2×10³ and 7.0×10³ kg/m³ depending on the deposition condition. Low film density and Pd doping resulted in high sensitivity and fast response. The largest sensitivity was observed for a Pd-doped film with a low density of 4.7×10³ kg/m³ and a thickness of 20 nm.     

Study of structural, electrical, optical, thermoelectric and photoconductive properties of S and Al co-doped SnO2 semiconductor thin films prepared by spray pyrolysis

In this paper, the effect of S and Al concentrations on the structural, electrical, optical, thermoelectric and photoconductive properties of the films was studied. The [Al]/[Sn] and [S]/[Sn] atomic ratios in the spray solutions were varied from 10 at.% to 40 at.% and 0 to 50 at.%, respectively. X-ray diffraction analysis showed the formation of SnO₂ cassiterite phase as a main phase and the numerous sulfur phases including S, SnS, SnS₂ and Sn₂S₃ in SnO₂:Al films. Scanning electron microscopy studies showed that in the absence of S, increasing the Al content results in a smaller grain size and with the addition of S, the films appear to contain small cracks and nodules. The minimum resistance of 0.175 (kΩ/□) was obtained for S-doped SnO₂:Al (40 at.%) film with 20 at.% S-doping. From the Hall effect measurements, the majority carrier concentration was obtained in order of 10¹⁷-10¹⁸ cm^− 3. The thermoelectric measurements showed that majority carriers change from electrons to holes for S-doping in SnO₂:Al (40 at.%) thin films. The maximum Seebeck coefficient of + 774 μV/K (at T = 370 K) was obtained for S-doped SnO₂:Al (10 at.%) film with 50 at.% S-doping. The band gap values were obtained in the range of 3.8-4.2 eV. The S-doped SnO₂:Al (40 at.%) films have shown considerably photoconductivity more than S-doped SnO₂:Al (10 at.%) with increasing S-doping. The best photoconductive property was obtained for co-doped SnO₂ thin film with 40 at.% Al and 5 at.% S concentration in solution.     

TiO2 coated SnO2 nanosheet films for dye-sensitized solar cells

TiO₂-coated SnO₂ nanosheet (TiO₂-SnO₂ NS) films about 300 nm in thickness were fabricated on fluorine-doped tin oxide glass by a two-step process with facile solution-grown approach and subsequent hydrolysis of TiCl₄ aqueous solution. The as-prepared TiO₂-SnO₂ NSs were characterized by scanning electron microscopy and X-ray diffraction. The performances of the dye-sensitized solar cells (DSCs) with TiO₂-SnO₂ NSs were analyzed by current-voltage measurements and electrochemical impedance spectroscopy. Experimental results show that the introduction of TiO₂-SnO₂ NSs can provide an efficient electron transition channel along the SnO₂ nanosheets, increase the short current density, and finally improve the conversion efficiency for the DSCs from 4.52 to 5.71%.     

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.     

Transmission electron microscopy investigation of the effect of deposition conditions and a platinum layer in gas-sensitive r.f.-sputtered SnO2 films

R.f. magnetron sputtering is used to grow nanocrystalline tin oxide (SnO₂) thin films on polycrystalline alumina substrates. Films are prepared at the substrate deposition temperature (T_dep) of 20 and 250 °C in the presence of 10% of oxygen in the argon atmosphere. Non-reactive depositions are also realized at 250 °C. The same technique is used to deposit a thin layer of platinum on the surface of the film or sandwiched between two layers of SnO₂ at both temperatures. Specimens are investigated by scanning, conventional and high-resolution transmission electron microscopy. Microstructural observations of SnO₂ and Pt/SnO₂ thin films are discussed in relation to their electrical behavior and to their responses in gas-sensing applications (to CO and C₂H₅CHO). In particular, this paper enlightens the effect of an intercolumnar porosity on the gas-sensing behavior.     

Effect of Li doping on the structural, optical and electrical properties of spray deposited SnO2 thin films

Studies on spray deposited transparent conducting Li doped SnO₂ thin films are scarce. Li (0 to 5 wt.%) doped SnO₂ thin films spray deposited onto glass substrates at 773 K in air from chloride precursors were studied for their structural, optical and temperature dependent electrical behaviors. X-ray diffraction patterns indicated single phase with polycrystalline nature. Systematic variation in surface morphology on Li doping was examined by scanning electron microscopy and atomic force microscopy. Film thickness, optical band gap (direct and indirect), sheet resistance and figure of merit were computed from spectral transmittance and temperature dependent resistivity data. Lithium doping was found to decrease the value of sheet resistance by an order in magnitude. Activation energy was computed from temperature dependent electrical resistivity data measured in the range 300 to 448 K. The 4 wt.% Li doped SnO₂ film was found to have a high value of figure of merit among other films. The results are discussed.     

Nanocrystalline SnO2 and In2O3 as materials for gas sensors: The relationship between microstructure and oxygen chemisorption

The correlations between microstructure of nanocrystalline TCO SnO₂ and In₂O₃ and parameters of oxygen chemisorption are analyzed. Nanocrystalline SnO₂ and In₂O₃ were prepared by wet chemical method. The sample's microstructure was characterized by TEM, XRD and low-temperature nitrogen adsorption. Electrical properties of TCO were studied at 200-400 °C depending on the oxygen partial pressure. Increase of TCO grain size leads to the increase of the fraction of atomic forms of chemisorbed oxygen at the fixed temperature. It could be due to the decrease of surface barrier resulting in the decrease of activation energy of dissociation of molecular ion O_2(ads)⁻.     

Large-area SnO2: F thin films by offline APCVD

In this paper, we reported the successful preparation of fluorine-doped tin oxide (FTO) thin films on large-area glass substrates (1245 mm × 635 mm × 3 mm) by self-designed offline atmospheric pressure chemical vapor deposition (APCVD) process. The FTO thin films were achieved through a combinatorial chemistry approach using tin tetrachloride, water and oxygen as precursors and Freon (F-152, C2H4F2) as dopant. The deposited films were characterized for crystallinity, morphology (roughness) and sheet resistance to aid optimization of materials suitable for solar cells. We got the FTO thin films with sheet resistance 8-11 Ω/□ and direct transmittance more than 83%. X-ray diffraction (XRD) characterization suggested that the as-prepared FTO films were composed of multicrystal, with the average crystal size 200-300 nm and good crystallinity. Further more, the field emission scanning electron microscope (FESEM) images showed that the films were produced with good surface morphology (haze). Selected samples were used for manufacturing tandem amorphous silicon (a-Si:H) thin film solar cells and modules by plasma enhanced chemical vapor deposition (PECVD). Compared with commercially available FTO thin films coated by online chemical vapor deposition, our FTO coatings show excellent performance resulting in a high quantum efficiency yield for a-Si:H solar cells and ideal open voltage and short circuit current for a-Si:H solar modules.     

SnO2 thin-films prepared by a spray-gel pyrolysis: Influence of sol properties on film morphologies

Nanostructured tin oxide films were prepared by depositing different sols using the so-called spray-gel pyrolysis process. SnO₂ suspensions (sols) were obtained from tin (IV) tert-amyloxide (Sn(t-OAm)₄) or tin (IV) chloride pentahydrate (SnCl₄·5H₂O) precursors, and stabilized with ammonia or tetraethylammonium hydroxide (TEA-OH). Xerogels from the different sols were obtained by solvent evaporation under controlled humidity.The Relative Gelling Volumes (RGV) of these sols strongly depended on the type of precursor. Xerogels obtained from inorganic salts gelled faster, while, as determined by thermal gravimetric analysis, occluding a significant amount of volatile compounds. Infrared spectroscopic analysis was performed on raw and annealed xerogels (300, 500 °C, 1 h). Annealing removed water and ammonium or alkyl ammonium chloride, increasing the number of Sn-O-Sn bonds.SnO₂ films were prepared by spraying the sols for 60 min onto glass and alumina substrates at 130 °C. The films obtained from all the sols were amorphous or displayed a very small grain size, and crystallized after annealing at 400 °C or 500 °C in air for 2 h. X-ray diffraction analysis showed the presence of the cassiterite structure and line broadening indicated a polycrystalline material with a grain size in the nanometer range. Results obtained from Scanning Electron Microscopy analysis demonstrated a strong dependence of the film morphology on the RGV of the sols. Films obtained from Sn(t-OAm)₄ showed a highly textured morphology based on fiber-shape bridges, whereas the films obtained from SnCl₄·5H₂O had a smoother surface formed by “O-ring” shaped domains.Lastly, the performance of these films as gas sensor devices was tested. The conductance (sensor) response for ethanol as a target analyte was of the same order of magnitude for the three kinds of films. However, the response of the highly textured films was more stable with shorter response times.     

Cathodoluminescence study of SnO2 powders aimed for gas sensor applications

In this paper we report on cathodoluminescence (CL) spectra of SnO₂ powders, synthesized using the wet chemical route. The analysis of influence of the modes of calcination (T_an-450–800 °C), and doping by both Pd and Pt (0.01–10.0 wt.%) on CL spectra was made. It was found that the measurement of CL spectra could be an effective research method of nanostructured metal oxides, aimed for gas sensor applications. It was established that in nanocrystalline SnO₂ the same system of energy levels, associated with radiative recombination, as in single crystalline and polycrystalline SnO₂, is retained. It was found that doping by both Pd and Pt modifies the structural properties of SnO₂ grains. Also, there is an optimum doping; near 0.1–0.2 wt.%, at which a maximum intensity of cathodoluminescence is reached. It was concluded that for low concentrations of both Pd and Pt additives in SnO₂ an improvement of the material's crystal structure is promoted, and is associated with a decrease in the non-radiating recombination rate.     

SnO2-nanowires grown by catalytic oxidation of tin sputtered thin films for formaldehyde detection

The aim of the present work is to test the sensing behaviour of tin dioxide nanowires, which have been grown directly onto a sensing device. This device consists in an alumina substrate provided with platinum interdigitated microelectrodes and a Pt heater on the reverse side. The nanowire growing process based on a vapour-liquid-solid method consists of three steps: deposition of a tin thin film by DC sputtering, a 5 nm-thick Au layer deposition and an annealing treatment in the presence of oxygen for the growth of the SnO₂ nanowires.These samples have been tested under different concentrations of formaldehyde (HCHO), showing a high sensitivity and very short response and recovery times even at low operating temperatures (130 °C).     

Estimation of compensation ratio by identifying the presence of different hopping conduction mechanisms in SnO2 thin films

The electrical properties of undoped SnO₂ thin films prepared by the sol-gel technique were investigated by conductivity measurements in a temperature range of 50-200 K. Structural characterizations of the films were performed by atomic force microscopy and X-ray diffraction. Optical properties of the samples were also characterized by optical absorption spectroscopy. The different hopping models were used to investigate the characteristics of electrical conduction by hopping in employed temperature range. It was shown that three types of behavior can be expected, nearest-neighbour hopping at high temperatures, the Mott variable-range hopping at low temperatures and Efros-Shklovskii variable-range hopping at lower temperatures. The criteria for the observation of these three regions were established and the transitional behavior of the conductivity was determined. The experimentally determined critical transition temperatures were at the orders of magnitudes with what could be expected based on hopping conduction calculations. Under these analyses, the compensation ratio of the films was determined.