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.     

XPS and AFM studies of surface chemistry and morphology of In2O3 ultrathin films deposited by rheotaxial growth and vacuum oxidation

In this paper, the results of XPS and AFM studies of the surface chemistry and morphology of In₂O₃ nanolayers obtained by rheotaxial growth and vacuum oxidation (RGVO) technology are presented. The ultrathin In films were deposited under UHV by thermal evaporation of indium pellets on the well defined Si substrate maintained at different temperatures. Optimal conditions to obtain the smallest grains and highest surface coverage have been determined, which was controlled by AFM, whereas the cleanness of deposited In nanolayers was controlled by XPS method. The ultrathin films of In₂O₃ (nm scale) were obtained in two ways, i.e. by oxidation of ultrathin films of In after their deposition, as well as by oxidation of In ultrathin films already during the deposition process. The XPS experiments showed that in both cases the obtained ultrathin films of In₂O₃ were almost stoichiometric. In turn, the AFM studies confirmed that only ultrathin films obtained during the simultaneous In deposition and oxidation exhibit almost flat surface morphology with average roughness at the level of about 0.85 nm.     

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.     

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.     

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.     

X-ray photoelectron spectroscopy and thermal desorption spectroscopy comparative studies of L-CVD SnO2 ultra thin films

In this paper we present the results of comparative studies of the chemical stability of L-CVD SnO₂ ultra thin films (20 nm) deposited on the atomically clean Si(100) substrate after their subsequent in situ hydrogenation and oxidation, and then after air exposure. For the control of surface chemistry of these films we used in a comparative way the X-ray Photoemission Spectroscopy (XPS) combined with ion depth profiling (DP XPS) and Thermal Desorption Spectroscopy (TDS). Our XPS experiments showed that the L-CVD SnO₂ ultrathin films after subsequent in situ hydrogenation and oxidation consist of strongly nonstoichiometric layer at the top of Si dioxide substrate. After subsequent air exposure they were covered with undesired 3 monolayers of C contamination and various forms of oxygen. During the TDS procedure a two-step desorption of molecular hydrogen (H₂), water vapor (H₂O), carbon dioxide (CO₂) and atomic oxygen (O) at the temperatures of ~ 530 K and 600 K was observed, respectively. The TDS results were in a good correlation with evident decreasing of the relative concentration of C contaminations, as well as variation of nonstoichiometry of the L-CVD SnO₂ ultra thin films as determined by XPS combined with ion depth profiling.     

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.     

Response to oxygen and chemical properties of SnO2 thin-film gas sensors

The measurements of the response—in terms of the conductance changes—to oxygen adsorption of tin dioxide (SnO₂) thin-film-based gas sensors were performed. The sensing SnO₂ layers were obtained by means of the rheotaxial growth and thermal oxidation (RGTO) method. The sensor responses were measured under a dry gas flow containing oxygen in nitrogen, within the range of temperature from 25 to 540 °C. For comparison, similar studies were performed for a commercial SnO₂ thick-film (TGS 812) gas sensor.The in-depth profiles of the chemical composition of the RGTO SnO₂ layers were determined from the scanning Auger microprobe experiment. The changes in concentration ratios [O]/[Sn] and [C]/[Sn] from the near-surface region towards the grain bulk were shown.     

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.     

Influence of surface Pd doping on gas sensing characteristics of SnO2 thin films deposited by spray pirolysis

In this article the analysis of steady state and transient gas sensing characteristics of undoped and Pd surface doped SnO₂ films, deposited by spray pyrolysis, is described. The influence of parameters such as air humidity (2-50% RH), operation temperature (25-500 °C) and Pd surface concentration (0-1% ML Pd) on gas response to CO and H₂ (0.1-0.5%), response time, shape of sensitivity S(T) curves and activation energy of τ(1/kT) dependencies are discussed. A mechanism based on a chemisorption model is proposed to explain how Pd influences the gas sensing characteristics of SnO₂ films.     

Interfacial characteristics and dielectric properties of Ba0.65Sr0.35TiO3 thin films

Ba_0.65Sr_0.35TiO₃ (BST) thin films were deposited on Pt/Ti/SiO₂/Si substrates by radio frequency magnetron sputtering technique. X-ray photoelectron spectroscopy (XPS) depth profiling data show that each element component of the BST film possesses a uniform distribution from the outermost surface to subsurface, but obvious Ti-rich is present to BST/Pt interface because Ti⁴⁺ cations are partially reduced to form amorphous oxides such as TiO_x (x < 2). Based on the measurement of XPS valence band spectrum, an energy band diagram in the vicinity of BST/Pt interface is proposed. Dielectric property measurements at 1 MHz reveal that dielectric constant and loss tangent are 323 and 0.0095 with no bias, while 260 and 0.0284 with direct current bias of 25 V; furthermore, tunability and figure of merit are calculated to be 19.51% and 20.54, respectively. The leakage current density through the BST film is about 8.96 × 10^− 7 A/cm² at 1.23 V and lower than 5.66 × 10^− 6 A/cm² at 2.05 V as well as breakdown strength is above 3.01 × 10⁵ V/cm.     

Pyrite (FeS2) thin films deposited by sol-gel method

Nanocrystalline pyrite (FeS₂) films were achieved by the sol-gel dip-coating process and sulfurization treatment. The microstructural, optical and electrical characteristics were investigated and the effect of sulfurization time on film properties was discussed. The XRD spectra show that FeS₂ film can be obtained for 1 h sulfurization and no other phase appears. The morphology of the precursor Fe₂O₃ films shows a porous and loose structure. However, with the sulfurization time increasing, the precursor films completely transformed into the pyrite films which have a compact and smooth structure. The pyrite films with a different sulfurization time have the optical absorption edges changed in the range of 0.90-0.99 eV. With the increase of sulfurization time, the carrier concentration increases and the carrier mobility decreases. It is speculated that crystallographic defects in the films could play an important role in film properties.     

Structural and optical properties of the SiO2-P2O5 films obtained by sol-gel method

A comparative study of the sol-gel films prepared in the SiO₂-P₂O₅ system starting with triethylphosphate, triethylphosphite and phosphoric acid as P precursors was performed. The work addresses basic aspects of physics, chemistry, and engineering of oxide films for applications in microelectronics, sensing, nano-photonics, and optoelectronics by establishing the influence of different precursors on the composition, structure and optical properties of the obtained films. The influence of the type of substrate (glass and indium tin oxide coated glass) and of the thermal treatment (200 and 500 °C) on their structure and properties was studied. By spectroscopic ellipsometry, X-Ray photoemission spectroscopy and atomic force microscopy measurements the high vaporization of the phosphorous during the densification of the films by thermal treatment was noticed when P-alkoxides were used. The phosphoric acid that forms chemical bond with silica network during the sol-gel process leads to better incorporation of P in the silica network as compared to the P-alkoxides.     

Electrochemical deposition and oxidation of CuFe2 alloy: a new method to deposit CuFe2O4 thin films at room temperature

The objective of this research work is to develop an electrochemical process to deposit polycrystalline copper iron oxide (CuFe₂O₄) films on different conducting substrates at room temperature (27°C). Cathodic Electrodeposition of CuFe₂ was carried out on various conducting substrates in galvanostatic mode. The composition of alloy was determined using atomic absorption spectroscopy (AAS) technique. The electrodeposited CuFe₂ alloy films were electrochemically oxidized (anodized) in aqueous KOH electrolyte at room temperature. The structural studies of oxide films were carried out using X-ray diffraction and IR absorption techniques. For surface morphological studies, optical microscopy and scanning electron microscopy (SEM) techniques were used.     

Effects of physical growth conditions on the structural and optical properties of sputtered grown thin HfO2 films

HfO₂ thin films were prepared by reactive DC magnetron sputtering technique on (100) p-Si substrate. The effects of O₂/Ar ratio, substrate temperature, sputtering power on the structural properties of HfO₂ grown films were studied by Spectroscopic Ellipsometer (SE), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrum, and X-ray photoelectron spectroscopy (XPS) depth profiling techniques. The results show that the formation of a SiO_x suboxide layer at the HfO₂/Si interface is unavoidable. The HfO₂ thickness and suboxide formation are highly affected by the growth parameters such as sputtering power, O₂/Ar gas ratio during sputtering, and substrate temperature. XRD spectra show that the deposited films have (111) monoclinic phase of HfO₂, which is also supported by FTIR spectra. XPS depth profiling spectra shows that highly reactive sputtered Hf atoms consume some of the oxygen atoms from the underlying SiO₂ to form HfO₂, leaving Si-Si bonds behind.     

Undoped and Cr-doped TiO2 thin films obtained by spray pyrolysis

Undoped and chromium doped titanium oxide thin films were fabricated by spray pyrolysis by using a solution of titanium tetrachloride and ethyl alcohol. The films have been deposited on heated glass substrates at 373 K. After annealing for 90 min at 723 K, the initially amorphous films became polycrystalline with a predominant anatase structure and average crystallite sizes depending on dopant (Cr) concentration. The repartition of chromium impurities in the matrix of titanium oxide films, analyzed by electron paramagnetic resonance and X-ray photoelectron spectroscopy showed that the entrance of chromium into the anatase structure is mainly achieved by substitution. A decrease in unit cell parameters ratio (c/a) with the increase of chromium content sustains this assertion. The wetting properties of the titanium oxide films were evaluated from contact angle measurements between de-ionized water and films surface during- and post-irradiation with UV light. The correlation between the concentration of the dopant, film structure, surface morphology and wettability characteristics is discussed.     

Influence of the reactive N2 gas flow on the properties of rf-sputtered ZnO thin films

Nitrogen (N)-doped ZnO thin films were RF sputtered with different N₂ volume (ranging from 10% to 100%) on sapphire (001) substrates. The influence of N₂ vol.% on the properties of ZnO films was analyzed by various characterization techniques. The X-ray diffraction studies showed that the films grow along the preferential (002) crystallographic plane and the crystallinity varied with varying N₂ vol.%. The films sputtered with 25 vol.% N₂ showed better crystallinity. The transmittance was decreased with increasing N₂ volume until 25% and was almost constant above 25%. A maximum optical band gap (2.08 eV) obtained for 10 vol.% N₂ decreased with increasing N₂ volume to reach a minimum of 1.53 eV at 100%. The compositional analysis confirmed the incorporation of N into ZnO films, and its concentration increased with increasing N₂ volume to reach a maximum of ∼ 3.7 × 10²¹ atom/cm³ at 75% but then decreased slightly to 3.42 × 10²¹ atoms/cm³. The sign of Hall coefficient confirmed that the films sputtered with ≤ 25 vol.% N₂ possess p-type conductivity which changes to n-type for > 25 vol.% N₂.     

Hydrophilic properties of nano-TiO2 thin films deposited by RF magnetron sputtering

Microstructure and hydrophilicity of nano-titanium dioxide (TiO₂) thin films, deposited by radio frequency magnetron sputtering, annealed at different temperatures, were studied by field emission scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and water contact angle methods. It is found that the crystal phase transforms from amorphous to rutile structure with increase of annealing temperature from room temperature to 800 °C. It is also indicated that the organic contaminants on the surface of the films can be removed and the oxygen vacancies can be reduced by the annealing treatment. Annealed at the temperature below 300 °C, amorphous TiO₂ thin films show rather poor hydrophilicity, and annealed at the temperature range from 400 to 650 °C, the super hydrophilicity anatase of TiO₂ thin films can be observed. However, when the annealing temperature reaches 800 °C, the hydrophilicity of the films declines mainly derived from the appearance of rutile.     

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.