AFM study of the surface morphology of L-CVD SnO2 thin films

In this paper we present the results of Atomic Force Microscopy (AFM) characterisation of the surface morphology of the L-CVD SnO₂ thin films prepared by L-CVD technology and studied after exposure to air, dry air oxidation, and ion beam profiling. The L-CVD SnO₂ thin films after air exposure have a very smooth surface morphology with an average surface roughness (RMS) smaller than 0.5 nm, and average and maximal grain heights of about 1 and 2 nm, respectively. After dry air oxidation the L-CVD SnO₂ thin films exhibit an average surface roughness (RMS), as well as the average and maximal grain height, increased by one order of magnitude. Finally, after the ion beam profiling the L-CVD SnO₂ thin films exhibit an evidently disordered structure with a lot of craters. These experiments showed that the L-CVD SnO₂ thin films exhibit a very high quality surface morphology, what can be useful for solar cells and gas sensors application.     

XPS study of the surface chemistry of L-CVD SnO2 thin films after oxidation

In this paper we present the results of XPS study of the surface chemistry of L-CVD SnO₂ thin films onto Si(100) before and after subsequent additional oxidation. Moreover, the ageing effect was also studied in order to check the influence of ambient oxidation. As-deposited L-CVD SnO₂ thin films exhibit evident nonstoichiometry with the relative concentration [O]/[Sn] equal to 1.29 ± 0.1. After in situ oxidation at high temperature (800 K) the relative concentration [O]/[Sn] increases to 1.95 ± 0.05 which corresponds to the almost stoichiometric SnO₂. Almost the same relative concentration [O]/[Sn] of L-CVD SnO₂ thin films has been obtained after long term exposure to air. The oxidation states of L-CVD SnO₂ thin films in both cases were confirmed by the shape analysis of corresponding XPS O1s and Sn3d_5/2 peaks using the decomposition procedure. For the as-deposited L-CVD SnO₂ thin films a mixture of SnO and SnO₂ was observed, while for the oxidized L-CVD SnO₂ thin films the domination of SnO₂ was determined.     

Experimental studies of O2-SnO2 surface interaction using powder, thick films and monocrystalline thin films

Surface properties of solids and the interactions between molecules and solid surfaces are important for many technical applications. They also involve a range of physical and chemical phenomena of fundamental scientific interest. The importance of oxygen chemistry at SnO₂ surfaces follows from the fact that SnO₂ is used as an active material in gas sensor applications. The operation principle of these sensors is usually based on measurable conductance response of the material, which is understood in terms of reactions of gas molecules with different oxygen species adsorbed onto the surface. The role of the lattice oxygen, but in particular, the bridging oxygen atoms on SnO₂ surfaces, is also active. Detailed understanding of the reaction mechanisms of various oxygen species at SnO₂ surfaces is important, as it offers a way to improve the sensitivity and selectivity of the sensors.Oxygen adsorption-desorption kinetics at the SnO₂ surface is studied experimentally using O₂-temperature-programmed desorption (TPD) method together with conductance measurements in the case of SnO₂ powder and polycrystalline thick films made from the powder. In addition, CO-TPD is studied and the transient behaviour of various oxygen species is considered. Molecular beam epitaxy (MBE) was also used to fabricate polycrystalline and monocrystalline thin films with the SnO₂(101) face on single crystal sapphire substrate. Simultaneous surface potential and conductance measurements during heating and cooling in different ambient atmospheres were used to characterize the monocrystalline SnO₂(101) surface after various surface treatments.     

Local surface morphology and chemistry of SnO2 thin films deposited by rheotaxial growth and thermal oxidation method for gas sensor application

In this paper experimental results of a comparative X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS) study of the crystalline structure, the local morphology, and the surface and in-depth chemistry of SnO₂ thin films obtained by Rheotaxial Growth and Thermal Oxidation (RGTO) method are presented. XRD rules out even a minor presence of a coexisting SnO phase. AFM and SEM show a fractal like morphology of nanograins (20 nm typical size) agglomerated in clusters of crystallites with a bimodal size distribution. XPS shows that the surface of the SnO₂ crystallites is slightly under-stoichiometric as expected from the oxygen deficient termination of their facets. Noteworthy, as evidenced by XPS depth profiles, there are no significant changes of the surface chemistry of the RGTO film with argon ion sputtering.     

NO2 gas sorption studies of Ge33Se67 films using quartz crystal microbalance

A study on the NO₂ gas sorption ability of amorphous Ge₃₃Se₆₇ coated quartz crystal microbalance (QCM) is presented. The thin films have been characterized before and after sorption/desorption processes of NO₂ by energy-dispersive X-ray spectroscopy (EDS), grazing angle X-ray diffraction (GAXRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and atom force microscopy (AFM) measurements. These studies indicated that physisorption occurs when NO₂ gas molecules are introduced into the chalcogenide film and the thin film composition or structure do not change. The mass loading due to NO₂ gas sorption was calculated by the resonator’s frequency shift. At the conditions of our experiment, up to 6.8 ng of the gas was sorbed into 200 nm thick Ge₃₃Se₆₇ film at 5000 ppm NO₂ concentration. It has been established that the process of gas molecules sorption is reversible.     

X-ray photoelectron spectroscopy analyses of titanium oxynitride films prepared by magnetron sputtering using air/Ar mixtures

X-ray photoelectron spectroscopy (XPS) has been employed to investigate titanium oxynitride (TiN_xO_y) films prepared by d.c. magnetron sputtering using air/Ar mixtures, which allows one to perform the deposition at a high base pressure (1.3 × 10^− 2 Pa) and can reduce substantially the processing time. XPS analyses revealed that all the prepared TiN_xO_y films comprised Ti-N, Ti-N-O, and Ti-O chemical states. When the air/Ar ratio was below 0.3, nitrogen-rich TiN_xO_y films were obtained. As the air/Ar ratio was above 0.4, oxygen-rich TiN_xO_y films were formed. XPS depth profile analyses were also performed in selected specimens. It has been found that at relatively low air/Ar ratios, such as 0.5, the oxygen content of the films increased toward the film/substrate interface and when the air/Ar ratio was higher, TiN_xO_y films with large oxygen content with uniform concentrations were then formed.     

Chemical-mechanical polishing of SiO2 thin films studied by X-ray reflectivity

Specular X-ray reflectivity from SiO₂ thin films prepared on silicon substrates by plasma-enhanced chemical vapor deposition showed the films to have a characteristic width of the decay in density at the free surface of 17 Å, to be about three-quarters the density of -quartz, and to have an interfacial layer at the silicon interface that was of the order of 100 Å wide and less dense than the bulk of the film. After chemical-mechanical polishing the characteristic width of the decay in density at the free surface was reduced to 10 Å; furthermore, the near-surface region to a depth of 30 Å had a greater density than the as-deposited film. Off-specular reflectivity confirmed that the decrease in characteristic width at the free surface was due to reduced roughness upon polishing and also revealed that the lateral correlation length in the limit of long wavelengths was the same for both polished and unpolished samples. The compression of the near-surface region during polishing is believed to enhance the dissolution of SiO₂ into the slurry which is necessary to achieve smooth surfaces.     

Influence of Nb dopant on the structural and optical properties of nanocrystalline TiO2 thin films

In this study, preparation of Nb-doped (0-20 mol% Nb) TiO₂ dip-coated thin films on glazed porcelain substrates via sol-gel process has been investigated. The effects of Nb on the structural, optical, and photo-catalytic properties of applied thin films have been studied by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Surface topography and surface chemical state of thin films was examined by atomic force microscope and X-ray photoelectron spectroscopy. XRD and Raman study showed that the Nb doping inhibited the grain growth. The photo-catalytic activity of the film was tested on degradation of methylene blue. Best photo-catalytic activity of Nb-doped TiO₂ thin films were measured in the TiO₂-1 mol% Nb sample. The average optical transmittance of about 47% in the visible range and the band gap of films became wider with increasing Nb doping concentration. The Nb⁵⁺ dopant presented substitutional Ti⁴⁺ into TiO₂ lattice.     

On the correlation between morphology and gas sensing properties of RGTO SnO2 thin films

In this paper, we present the results of studies on optimalisation of morphology of the SnO₂ thin films grown by RGTO technique for application as gas sensor structures. The Sn thin films were grown on Si(111) wafer and Al₂O₃ ceramic plate heated in the range 235-295 °C and subsequently oxidized in dry oxygen atmosphere at high temperature, up to 700 °C. Our studies confirmed that the highest surface coverage of Sn droplets can be reached for the substrate temperature of about 265 °C leading to the highest surface-to-volume ratio of SnO₂ thin films. It was in a good correlation to the optimal gas sensor response and sensor sensitivity of RGTO SnO₂ thin films to nitrogen dioxide NO₂.     

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.     

X-ray photoelectron spectroscopy on implanted argon as a tool to follow local structural changes in thin films

Argon ions were implanted in metallic, semiconducting or insulating substrates, and investigated with X-ray photoelectron spectroscopy. Analysis of the Ar2p core level of argon showed clear differences in binding energy position and width as function of the matrix material, implantation energy, and post-annealing treatment. Although argon is not expected to form chemical bonds with the host matrix, the electronic shells within the gas atom can react to their environment according to different effects. It is shown that the precise determination and correct interpretation of the binding energy levels of the embedded gas atoms provides information about the local environment of the matrix such as amorphization of the crystalline structure, defect healing or gas bubble formation.     

Thickness measurement of SiO2 films thinner than 1 nm by X-ray photoelectron spectroscopy

The thickness measurement of ultra-thin SiO₂ films thinner than 1 nm was studied by X-ray photoelectron spectroscopy (XPS). Amorphous SiO₂ thin films were grown on amorphous Si films to avoid the thickness difference due to the crystalline structure of a substrate. SiO₂ thin films were grown by ion beam sputter deposition under oxygen gas flow and the thickness was measured by in situ XPS. The attenuation length was determined experimentally by a SiO₂ film with a known thickness. The straight line fit between the measured thickness using XPS and the nominal thickness showed a good linear relation with a gradient of 0.969 and a small offset of 0.126 nm. The gradient measured at the range of 3.4–0.28 nm was very close to that measured at sub-nanometer range of 1.13–0.28 nm. This result means that the reliable measurement of SiO₂ film thickness below 1 nm is possible by XPS.     

X-ray photoelectron spectroscopy (XPS) and FTIR studies of vanadium barium phosphate glasses

Barium vanadophosphate glasses, having composition 50BaO–xV₂O₅–(50 − x)P₂O₅, (x = 0–50 mol%), were prepared by conventional melt quench method. Density, molar volume and glass transition temperature (T_g) were measured as a function of V₂O₅ content. Structural investigation was done using XPS and FTIR spectroscopy. First, substitution of the P₂O₅ by the V₂O₅ in the metaphosphate 50BaO–50P₂O₅ glass increases the density and T_g and decreases the molar volume. When the amount of V₂O₅ increases, all these properties show a reverse trend. XPS measurement found in the O1s, P2p, and V2p core level spectra indicate the presence of primarily P–O–P, P–O–V and V–O–V structural bonds, the asymmetry in the P 2p spectra indeed arises from the spin-orbit splitting of P 2p core level, and more than one valence state of V ions being present. IR spectroscopy reveals the depolymerization of the phosphate glass network by systematic conversion of metaphosphate chains into pyrophosphate groups and then orthophosphate groups. Even though metaphosphate to pyrophosphate conversion is taking place due to breaking of P–O–P linkages, formation of P–O–V and P–O–Ba linkages provide cross linking between short P-structural units, which make the glass network more rigid. Above 10–20 mol% V₂O₅ content, network is highly depolymerized due to the formation of orthophosphate units and V–O–V bridge bonds, resulting in poor cross-linking, making the glass network less rigid.     

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.     

Atomic force microscopy and X-ray photoelectron spectroscopy evaluation of adhesion and nanostructure of thin Cr films

Chromium (Cr) thin films were deposited on float glass using electron beam (e-beam) physical vapor deposition and radio frequency (RF) magnetron sputtering techniques. Surface morphology of these Cr films was studied using atomic force microscopy (AFM). The e-beam deposited Cr films consisted of isolated surface mounds while in RF sputtered samples, these mounds combined to form larger islands. Lower surface adhesive properties were observed for e-beam deposited films, as determined from AFM force-distance curves, presumably due to the nanostructural differences. Similar amounts of adsorbed atmospheric carbonaceous contaminants and water vapor were detected on samples deposited using both methods with e-beam deposited samples having additional carbide species, as determined by X-ray photoelectron spectroscopy data. The dominant crystallographic plane in both e-beam deposited and RF sputtered Cr thin films was (110) of body-centered cubic Cr metal structure as determined from X-ray diffraction data. Weak (211) reflection was also observed in RF sputtered samples and was attributed to a different thin Cr film condensation and growth mechanism which resulted in nanostructural differences between films deposited using two different methods.     

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%.     

Preparation and super-hydrophilic properties of TiO2/SnO2 composite thin films

SnO₂-TiO₂ composite thin films were fabricated on soda-lime glass with sol-gel technology. By measuring the contact angle of the film surface and the degradation of methyl orange, we studied the influence of SnO₂ doping concentration, heat-treatment temperature and film thickness on the super-hydrophilicity and photocatalytic activity of the composite films. The results indicate that the doping of SnO₂ into TiO₂ can improve their hydrophilicity and photocatalytic activity, and the composite film with 1-5 mol% SnO₂ and heat-treated at 450°C is of super-hydrophilicity. The optimal SnO₂ concentration for the photocatalytic activity is 10 mol% and larger film thickness is helpful to reduce the contact angle of the composite films.     

Lithographic patterning of benzoylacetone modified SnO2 and SnO2:Sb thin films

The synthesis of directly UV-photopatternable pure and antimony-doped organo-tin materials is presented. UV-photopatternability has been achieved by using the synthesized benzoylacetone modified tin and antimony 2-isopropoxyethoxides. Photopatterned pure and antimony-doped organo-tin films are crystallized by thermal annealing in order to obtain conductive SnO₂ and Sb:SnO₂ thin films. The molar ratio between benzoylacetone and metal alkoxides has to be 2 in order to obtain crack-free, good-quality structures. The effects of UV-irradiation, increasing antimony doping level and benzoylacetone concentration on the electrical properties of the single-layered films are analyzed. The highest obtained conductivity was 20 S/cm. Benzoylacetone concentration and UV-irradiation has only a negligible effect on the film electrical conductivities.     

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.