Preparation and magnetic properties of the CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> thin films on Si substrate by sol-gel technique

A stable precursor for CoFe₂O₄ thin film was prepared by sol-gel technique from the aqueous solution of FeCl₃·6H₂O and CoCl₂·6H₂O. Sol was deposited on a naturally oxidized silicon-substrate by spinning technique (2000 rpm) and heat treated at different temperatures ranging from 700 to 1100 °C. Thickness of the films was controlled in the range of 400–500 nm and all the films were characterized by using XRD and SEM. The effects of temperature and the composition on the formation of CoFe₂O₄ thin film were also studied. Films obtained at relatively lower temperature showed multi-phases of α-Fe₂O₃, CoFe₂O₄ and CoO while the formation of CoFe₂O₄ phase increases with increasing temperature. Furthermore, the composition of the solution in mol% has great role on the formation of CoFe₂O₄ films and the film containing 50 mol% of Co²⁺ exhibited CoFe₂O₄ mono-phase. Surface morphology of the films was studied by scanning electron microscope (SEM). Magnetic properties of the films, studied by using vibrating sample magnetometer (VSM), showed relatively high saturation magnetization (8.04–22.21 kWb/m²) as well as high coercivity (44.59–63.30 kA/m). Saturation magnetization also increases with increasing heat treatment temperature.     

Tungsten-oxide thin films as novel materials with high sensitivity and selectivity to NO2, O3, and H2S. Part II: Application as gas sensors

The electrical response of tungsten-oxide thin films as-deposited by electron-beam deposition and annealed (at 350–800 °C for 1–3 h in O₂) to NO₂, O₃ and H₂S was studied both experimentally and theoretically. In order to interpret the kinetic characteristics of tungsten-oxide thin films on exposure to different gases, a model based on surface adsorption/desorption processes coupled with bulk diffusion was used. A link between the geometrical and chemical heterogeneities of the tungsten-oxide film surfaces and their performance characteristics as gas sensors was established. It was shown that the nature and amount of surface-adsorption sites in the different nonstoichiometric phases (W _n O_3n–2 or W _n O_3n–1) and WO₃ as well as their conduction mechanisms are defined from the phase composition of the film, the crystallographic and electronic structures of the phases, the orientation of the crystallites within the film and the geometrical shape and dimensions of the crystallites. All tungsten-oxide thin films investigated in this work are suitable for detection of very low concentrations of NO₂ (0.05–0.5 ppm in N₂ and synthetic air), ozone (25–90 ppb) and H₂S (3–15 ppm in N₂ and synthetic air) at very low working temperatures (80–160 °C). The films annealed at 400 °C for 1–2 h are very selective to ozone at 120–160 °C; the films annealed at 400 °C for 1–3 h and at 800 °C for 1 h are very sensitive to NO₂ (in N₂).     

Influence of different process parameters on physical properties of fluorine dopes indium oxide thin films

Fluorine-doped indium oxide films were prepared by the spray pyrolysis technique. The physical properties of these films were investigated with respect to various process parameters, namely variation of dopant concentration (in the solution), deposition temperature (T _s), carrier gas (air) flow rate and the thickness of the film. The best films had a Hall mobility of the order of 28 cm²V^–1 s^–1 and a carrier density of 2.7 × 10²⁰ cm^–3. These films were deposited at T _s=425 °C at an air flow rate of 71 min^–1 for an atomic ratio of fluorine to indium of 72%. The electrical resistivity of these films was of the order of 10^–4 cm and the average transmission in the visible range was found to be 80–90%. The films were polycrystalline, n-type semiconductors with [400] as a preferred orientation. The preferred orientation changes from [400] to [222] depending upon the process parameters.     

Structural and compositional properties of sol-gel formed Ni, Co and Ni-Co oxide films

Sol-gel (SG) formed oxide films (between 60 and 800 nm in thickness) were deposited on Pt foil substrates by dip-coating from Ni, Co and mixed Ni-Co sols. After withdrawal at a constant rate, the films were dried at temperatures between 100 and 400°C for various periods of time (15 min to 1 hour). Electron diffraction and transmission electron microscopy studies have indicated that the as-formed SG films are nanocrystalline oxide materials, consisting of cubic units of NiO and/or CoO (and Co₃O₄, in pure Co SG films) in crystallites which are 1–4 nm in diameter, the size depending on the oxide composition and the drying conditions employed. Oxide films formed at 200°C are highly hydrous in nature, and drying at higher temperatures than this results in the loss of water, the formation of more compact films and an increase in the crystallite size. Based on the observed charge efficiencies (ca. 75% for pure Ni oxide and ca. 60% for 50:50 Ni:Co oxide films), it is suggested that only metal sites on the outer surfaces of the crystallites, with easy access to the solution ions and water, participate in the Ni-Co oxide redox reaction in alkaline solutions.     

Structural and electrochemical studies of Co oxide films formed by the sol-gel technique

Two different types of Co oxide films, each having a distinct electrochemical signature correlated with the film drying temperature, were formed using the sol-gel (SG) technique. Two different states of gelation of the film precursor were also explored. Cyclic voltammograms, collected in alkaline solutions for the low temperature films, displayed two pairs of peaks corresponding to the Co(II) to Co(III) and Co(III) to Co(IV) transitions, centered at 1.2 and 1.4 V, respectively, while the high temperature films underwent only the Co(III) to Co(IV) redox process at 1.4 V. The charge densities obtained for the lower temperature films (particle sizes 2–10 nm in diameter) ranged between 40 and 70 mC/cm²; charge densities for the higher temperature films (particle sizes of 5–40 nm), otherwise formed identically, were between 10 and 20 mC/cm². The more viscous Co oxide gels led to significantly higher charge densities than less viscous gels, as well as greater film stability during electrochemical cycling. Using a wide range of film characterization techniques, it was shown that Co oxide films formed at > 180∘C are composed mainly of Co³O⁴ spinel, while films formed at < 180∘C consist predominantly of CoO.     

Highly Crystalline Y3Fe5O12 Thin Films by Cictric Spray Pyrolysis

Ethylene glycol solution of mixed-metal Y(III)-Fe(III) citric complexes has been used as a starting material for a spray pyrolysis deposition of Y₃Fe₅O₁₂ (YIG) thin films on silica and on single crystal Y₃Al₅O₁₂ (YAG) substrates heated up to 500°C. O₂ was used as a carrier gas. A post-deposition thermal annealing at 800–1100°C was applied. Films of 0.1–1.2 m in thickness were obtained. Their composition, binding energies, crystal structure, morphology, and magnetic properties were studied. Phase homogeneous polycrystalline or highly textured YIG films were obtained on silica or YAG substrates at annealing temperatures of 860° and 960°C, respectively. The crystallite grain sizes of both type of layers were 80 nm along the (400) plane. A columnar structure of the films with the column diameter of 400–500 nm might be supposed following the atomic force microscopy data. Saturation and permanent magnetizations along the easy axis of 1545 Gs and 816 Gs, respectively, and coercive force along the same axis of 4 Oe were measured.     

Study on microstructure and magnetic domain structure in sputtered (Ni66Fe22Co12) x C1−x nanocomposite films

Composition and annealing temperature dependence of microstructure and magnetic domain structure in sputtered (Ni₆₆Fe₂₂Co₁₂) _x C_1–x nanocomposite films with x=10–75 at % were studied by X-ray diffraction and magnetic force microscopy (MFM). Films with x20 at % showed amorphous structures, and no domain structure could be found due to the disappearance of magnetocrystalline anisotropy. For the films with x=30–55 at %, face-centered cubic (fcc) NiFeCo nanocrystals encapsulated in graphite-like carbon could be found in the samples annealed beyond 400 °C, and stripe domains with typical dimension of 120–150 nm were observed. For the films with x62 at %, the as-deposited films went through a meta-stable stage at which a rhombohedral Ni₃C phase and fcc NiFeCo co-existed after annealing to a temperature between about 300–400 °C (dependent on composition). Upon further annealing to a sufficiently high temperature between about 350–500 °C, the carbide phase decomposed into fcc NiFeCo and graphite. While short-range domain structures were observed in the samples before the formation of carbide phase, long-range domain structure with dispersed domains in the meta-stable stage were observed. After the decomposition of carbide, large domains with typical size of 500–700 nm were observed due to the formation of large grain aggregators.     

Conductivity measurements on cobalt and nickel oxides in highly-enriched oxygen atmospheres

The pressure dependencies of the conductivities of nickel and cobalt monoxides have been investigated in the range 10^–2 to 7.5 × 10² atmospheres at temperatures in the region of 1000° C. Conductivity saturation occurs in CoO due to the formation of Co₃O₄, and the experimental data correlates well with a model for the separation of the higher phase. NiO exhibits no saturation effect in the pressure range investigated and the p^1/5 dependence of the conductivity at 950° C with this oxide above 0.1 atm is interpreted in terms of the formation of singly ionised vacancies.     

Sol–gel derived Co<Subscript>3</Subscript>O<Subscript>4</Subscript> thin films: effect of annealing on structural,morphological and optoelectronic properties

Nanocrystalline Co₃O₄ thin films were prepared on glass substrates by using sol–gel spin coating technique. The effect of annealing temperature (400–700 °C) on structural, morphological, electrical and optical properties of Co₃O₄ thin films were studied by X-ray diffraction (XRD), Scanning Electron Microscopy, Electrical conductivity and UV–visible Spectroscopy. XRD measurements show that all the films are nanocrystallized in the cubic spinel structure and present a random orientation. The crystallite size increases with increasing annealing temperature (53–69 nm). These modifications influence the optical properties. The morphology of the sol–gel derived Co₃O₄ shows nanocrystalline grains with some overgrown clusters and it varies with annealing temperature. The optical band gap has been determined from the absorption coefficient. We found that the optical band gap energy decreases from 2.58 to 2.07 eV with increasing annealing temperature between 400 and 700 °C. These mean that the optical quality of Co₃O₄ films is improved by annealing. The dc electrical conductivity of Co₃O₄ thin films were increased from 10⁻⁴ to 10⁻² (Ω cm)⁻¹ with increase in annealing temperature. The electron carrier concentration (n) and mobility (μ) of Co₃O₄ films annealed at 400–700 °C were estimated to be of the order of 2.4–4.5 × 10¹⁹ cm⁻³ and 5.2–7.0 × 10⁻⁵ cm² V⁻¹ s⁻¹ respectively. It is observed that Co₃O₄ thin film annealing at 700 °C after deposition provide a smooth and flat texture suited for optoelectronic applications.     

Growth and Characterization of PrBa2Cu3O7?x Thin Film Used as Template Layer on SrTiO3 Substrate

We have grown PrBa₂Cu³O_7–x (PBCO) thin films on (100) SrTiO₃ substrates using pulsed laser deposition (PLD). X-ray diffraction (XRD) studies indicate that the orientation of PBCO films varied with increasing deposition temperature: b axis oriented films can be grown at 680°C, and a axis oriented films at the temperature between 692°C and 705°C. Atomic force microscopy (AFM) reveals that a good flatness of the films was obtained with surface mean roughness of less than 24 Å, indicating that it is suitable for use as template layers in a axis oriented epitaxial YBa₂Cu₃O_7–y /PBCO and YBCO/tetragonal–YBCO/PBCO multilayer structures.     

Chemical deposition of ZnO films for gas sensors

Zinc oxide (ZnO) thin films were prepared following a chemical, deposition technique using a sodium zincate bath. Structural characterizations by scanning electron microscopy (SEM) and X-ray diffraction (XRD) indicate the formation of ZnO film with a preferred c-axis orientation. The electrical conductance of the ZnO films became stable and reproducible in the 300–500 K temperature range with two activation energy barrier values of 0.3 eV and 0.8 eV in the low temperature (300–420 K) and high temperature (430–500 K) ranges, respectively. The ZnO films prepared by this method are highly resistive, indicating the presence of a large density of oxygen adsorbed acceptor-like trap states (O ₂ ^- , O_-, etc.). Palladium sensitized ZnO films were exposed to hydrogen (H₂) with air as a carrier gas at different operating temperatures ranging between 150–375°C and the response is evaluated.     

Preparation of fatigue-free SrBi2Ta2O9 thin films by r.f. magnetron sputtering and their ferroelectric properties

Fatigue-free bismuth-layered SrBi₂Ta₂O₉ (SBT) films were deposited on Pt/Ti/SiO₂/Si substrates by r.f. magnetron sputtering at room temperature. The variation of structure and electrical properties were studied as a function of annealing temperatures from 750–850 °C. The films annealed at 800 °C had a composition ratio of Sr:Br:Ta = 0.7:2.0:2.0. X-ray photoelectron spectroscopy signals of bismuth show an oxygen-deficient state within the SBT films. The films annealed at 800 °C have a thickness of 200 nm and a relatively dense microstructure. The remanent polarization (2P _r), and the coercive field (2E _c), obtained for the SIBT films, were 9.1 C cm^–2 and 85 kV cm^–1 at an applied voltage of 3 V, respectively. The films showed fatigue-free characteristics up to 10¹⁰ cycles under 5 V bipolar square pulses. The leakage current density was about 7 × 10^–7 A cm^–2 at 150 kV cm^–1. The SBT films prepared by r.f. magnetron sputtering were attractive for application to non-volatile memories.     

Sputter deposited nanocrystalline Ni-25Al alloy thin films and Ni/Ni3Al multilayers

Thin films of nominal composition Ni-25at%Al have been sputter deposited from a target of the intermetallic compound Ni₃Al at different substrate deposition temperatures. The film deposited on an unheated substrate exhibited a strongly textured columnar growth morphology and consisted of a mixture of metastable phases. Nanoindentation studies carried out on this film exhibited a strong strain hardening tendency. In contrast, the film deposited at 200 °C exhibited a recrystallized non-textured microstructure consisting of grains of a partially ordered Ni₃Al phase. At higher deposition temperatures (∼400 °C), larger grains of the bulk equilibrium, long-range ordered, Ll₂ Ni₃Al phase were observed in the film. Unlike the film deposited on an unheated substrate, the films deposited at elevated temperatures did not exhibit any dependence of the hardness on the indentation depth and, consequently no strain hardening. The average hardness of the film deposited at 200 °C was higher than the one deposited at 400 °C. In addition to monolithic Ni-25Al thin films, multilayered Ni/Ni3Al thin films were also deposited. Multilayers deposited non-epitaxially on unheated substrates exhibited a strong {111} fiber texture while those deposited epitaxially on (001) NaCl exhibited a {001} texture. Free-standing multilayers of both types of preferred orientations as well as of different layer thicknesses were deformed in tension untill fracture. Interestingly, the {111} oriented multilayers failed primarily by a brittle fracture while the {001} multilayers exhibited features of ductile fracture.     

Microstructure and secondary phases in epitaxial LaBaCo_2O_(5.5 + δ) thin films

Aberration-corrected scanning transmission electron microscopy was employed to investigate the microstructures and secondary phases in LaBaCo_2O_(5.5 + δ)(LBCO) thin films grown on SrTiO_3(STO) substrates. The as-grown films showed an epitaxial growth on the substrates with atomically sharp interfaces and orientation relationships of [100]_(LBCO)//[100]_(STO)and(001)_(LBCO)//(001)_(STO). Secondary phases were observed in the films, which strongly depended on the sample fabrication conditions. In the film prepared at a temperature of 90℃, nano-scale CoO pillars nucleated on the substrate, and grew along the [001]direction of the film. In the film grown at a temperature of 1000℃, isolated nano-scale Co_3O_4 particles appeared, which promoted the growth of {111} twinning structures in the film. The orientation relationships and the interfaces between the secondary phases and the films were illustrated, and the growth mechanism of the film was discussed.     

Characteristics of PbTiO3 thin films on Pt/Ti/SiO2/Si by continuous cooling process

In order to introduce a new deposition process for ferroelectric thin film, the deposition temperature was continuously cooled down from 580°C to 400°C during the deposition which we call continuous cooling process (CCP). X-ray diffraction patterns showed that the PbTiO₃ thin films deposited by the CCP and at 480°C had polycrystallinity, but at substrate temperatures of 400°C and 580°C had poor crystallinity. Scanning electron microscopy of the CCP-deposited film surface showed larger granular-like micrograins than that of the film deposited at 480°C and smaller than that of the film at 580°C. While there was no other phase formation at the PbTiO₃-Pt interface in the CCP-deposited film, resulting in a sharp interface, there was severe interface reaction at the PbTiO₃-Pt and the Pt-Si in the film deposited at 580°C, resulting in an abrupt interface. Atomic force microscopy under ambient conditions showed smoother surface of the film by the CCP than that of the films at 580°C. Furthermore, the film by the CCP had higher packing density than that of the film at 480°C. Besides enhancement of the structural properties, the CCP deposition appeared to have improved the electrical properties such as dielectric constant, dissipation factor, leak current density and polarization. In the case of the film by the CCP, polarization-electrical field measurement showed the saturation polarization of 27 Ccm^–2, remanent of 14 Ccm^–2 and coercive of 150 kV. These results indicate that the CCP in metalorganic chemical vapour deposition has a possibility for fabrication of PbTiO₃ ferroelectric thin films.     

Chemically deposited zinc oxide thin film gas sensor

Zinc oxide (ZnO) thin films were prepared by a low cost chemical deposition technique using sodium zincate bath. Structural characterizations by X-ray diffraction technique (XRD) and scanning electron microscopy (SEM) indicate the formation of ZnO films, containing 0.05–0.50 m size crystallites, with preferred c-axis orientation. The electrical conductance of the ZnO films became stable and reproducible in the 300–450 K temperature range after repeated thermal cyclings in air. Palladium sensitised ZnO films were exposed to toxic and combustible gases e.g., hydrogen (H₂), liquid petroleum gas (LPG), methane (CH₄) and hydrogen sulphide (H₂S) at a minimum operating temperature of 150 °C; which was well below the normal operating temperature range of 200–400 °C, typically reported in literature for ceramic gas sensors. The response of the ZnO thin film sensors at 150 °C, was found to be significant, even for parts per million level concentrations of CH₄ (50 ppm) and H₂S (15 ppm).     

Effects of annealing temperature on microstructure and electrical properties of Mn-Co-Ni-O thin films

Mn_1.85Co_0.3Ni_0.85O₄ (MCN) thin films were prepared on Al₂O₃ substrates by chemical solution deposition method at different annealing temperature (650, 700, 750 and 800 °C). Effects of annealing temperature on microstructure and electrical properties of MCN thin films were investigated. The MCN thin film annealed at 750 °C is of good crystallization and compact surface. It shows lower resistance (4.8 MΩ) and higher sensitivity (3720.6 K) than those of other prepared films. It also has small aging coefficient (3.7%) after aging at 150 °C for 360 h. The advantages of good properties make MCN thin film very promising for integrated devices.     

Effect of growth temperature on the structural and transport properties of magnetite thin films prepared by pulse laser deposition on single crystal Si substrate

Magnetite (Fe₃O₄) thin films are prepared by pulsed laser deposition using an α-Fe₂O₃ target on silicon (111) substrate in the substrate temperature range of 350 °C to 550 °C. X-ray diffraction (XRD) measurement shows that the film deposited at 450 °C is a single phase Fe₃O₄ film oriented along [111] direction. However, the film grown at 350 °C reveals mixed oxide phases (FeO and Fe₃O₄), while the film deposited at 550 °C is a polycrystalline Fe₃O₄. X-ray photoelectron spectroscopy study confirms the XRD findings. Raman measurements reveal identical spectra for all the films deposited at different substrate temperatures. We observe abrupt increase in the resistivity behavior of all the films around Verwey transition temperature (T_V) (125 K-120 K) though the transition is broader in the film deposited at 350 °C. We observe that the optimized temperature for the growth of Fe₃O₄ film on Si is 450 °C. The electrical transport behavior follows Shklovskii and Efros variable range hopping type conduction mechanism below T_V for the film deposited at 450 °C possibly due to the granular growth of the film.     

Effect of the substrate temperature on the properties of Ga-doped ZnO films for photovoltaic cell applications deposited by a pulsed DC magnetron sputtering with a rotating cylindrical target

Effect of substrate temperature on the properties of Ga-doped ZnO (GZO) films was investigated by pulsed DC magnetron sputtering with a rotating cylindrical target with an aim to establish suitable process conditions for their photovoltaic (PV) cell applications. Without formation of undesirable secondary oxide phases such as Ga₂O₃ and ZnGa₂O₄, the GZO film having mixed orientation at lower deposition temperature evolved into the c-axis oriented one with increasing deposition temperature to 230 °C, which accompanied morphological evolution to vertically oriented dense columnar structure and improved doping efficiency. Correlated with this, crater-like surface texturing was possible only on the sample deposited at 230 °C. Electrical resistivity and diffuse surface reflectance over the spectral range of 200-1200 nm of this GZO film after surface texturing were 8.73 × 10⁻⁴ Ω cm and 3.32%, respectively, indicating that the film has application potential as anti-reflection coating and front electrode of PV cells. Morphological features, surface texturing behavior, electrical and optical properties of the GZO films in this study suggest that this novel technique would be applicable to the fabrication of anti-reflection coating and front electrode of PV cells only when substrate temperature is sufficiently high.     

TiOx interlayer characterization for sol–gel derived Pb(Zr, Ti)O3 thin films on titanium foil

Pb(Zr, Ti)O₃ films were prepared on titanium foil using sol–gel processing. The films were of large area, crack-free, uniform, with perovskite structure and exhibiting strong adhesion to the titanium foil substrate. Films and the interface region between the film and the substrate were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and dielectric property measurements. The measurements revealed the formation of a TiO_x layer at the interface between the film and the foil. The thickness of the TiO_x interlayer increases with increasing annealing temperature, and was amorphous when annealed below 600 °C. The dielectric properties of films depend on the thickness of the TiO_x interlayer. Films on Ti foil with dielectric constant of 200–400, dielectric loss <5%, leakage current of <1×10^–7 A cm^–2 at 100 kV cm^–1 and breakdown field strength of 0.6–1.13 MV cm^–1 were demonstrated. The TiO_x interlayer resulted in asymmetric C–V hysterisis behavior attributed to trapped charge in the vicinity of the TiO_x interlayer and to elastic mismatch strain.