Synchrotron X-ray diffraction studies of heteroepitaxial ZnO films grown by pulsed laser deposition

Heteroepitaxial ZnO films were grown by pulsed laser deposition on various substrates such as GaN-buffered C-Al₂O₃, C-Al₂O₃, A-Al₂O₃, and R-Al₂O₃. The epitaxy nature of the films was investigated mainly by synchrotron X-ray diffraction. The results showed that the GaN interlayer plays a positive role in growing an unstrained, well-aligned epitaxial ZnO film on the basal plane of Al₂O₃. Importantly, the ZnO film grown on R-Al₂O₃ has two differently aligned domains. The dominant (1 1 0) oriented domain has much better alignment in the in-plane direction than the minor portion of (0 0 1) oriented domain, while in the out-of-plane direction the two domains have almost the same mosaic distribution.     

Electrical properties of Al2O3 film deposited at low temperatures

Amorphous Al₂O₃ films were deposited on p-Si by rf magnetron sputtering to investigate their potential as a gate dielectric in organic thin film transistors (OTFTs). The deposition was performed at room temperature, 200 and 300 °C using Al₂O₃ and Al targets. Achieved Al₂O₃ films have higher capacitance values than thermally grown SiO₂ as characterized by capacitance-voltage measurements. It is also found from current-voltage and roughness measurements that the leakage current and the surface roughness can be least when the films are deposited at room temperature. The capacitance of the film obtained from the Al₂O₃ target appears higher than that of the Al₂O₃ film from the Al target while the results of electrical breakdown are opposite. These room temperature processes are promising for applications to the gate dielectrics of organic TFTs.     

Preparation of single-crystalline ZnO films on ZnO-buffered a-plane sapphire by chemical bath deposition

High-quality zinc oxide (ZnO) films were successfully grown on ZnO-buffered a-plane sapphire (Al₂O₃ (1 1 2¯ 0)) substrates by controlling temperature for lateral growth using chemical bath deposition (CBD) at a low temperature of 60 °C. X-ray diffraction analysis and transmission electron microscopy micrographs showed that the ZnO films had a single-crystalline wurtzite structure with c-axis orientation. Rocking curves (ω-scans) of the (0 0 0 2) reflections showed a narrow peak with full width at half maximum value of 0.50° for the ZnO film. A reciprocal space map indicated that the lattice parameters of the ZnO film (a=0.3250 nm and c=0.5207 nm) were very close to those of the wurtzite-type ZnO. The ZnO film on the ZnO-buffered Al₂O₃ (1 1 2¯ 0) substrate exhibited n-type conduction, with a carrier concentration of 1.9×10¹⁹ cm⁻³ and high carrier mobility of 22.6 cm² V⁻¹ s⁻¹.     

Non-isothermal crystallization kinetics of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>–CaO–SiO<Subscript>2</Subscript> glass containing nucleation agent P<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript>

Fe₂O₃–CaO–SiO₂ glass ceramics containing nucleation agent P₂O₅/TiO₂ were prepared by sol-gel method. The samples were characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The activation energy and kinetic parameters for crystallization of the samples were calculated by the Johnson-Mehi-Avrami (JMA) model and Augis-Bennett method according to the results of DSC. The results showed that the crystallization mechanism of Fe₂O₃–CaO–SiO₂ glass, whose non-isothermal kinetic parameter n = 2.3, was consistent with surface crystallization of the JMA model. The kinetics model function of Fe₂O₃–CaO–SiO₂ glass, f(α) = 2.3(1–α)[–ln(1–α)]^0.57, was also obtained. The addition of nucleation agent P₂O₅/TiO₂ could reduce the activation energy, which made the crystal growth modes change from onedimensional to three-dimensional.     

Effect of TiO2 addition on the chemical durability of Bi2O3–SiO2–ZnO–B2O3 glass system

The effect of the substitution of ZnO for TiO₂ on the chemical durability of Bi₂O₃–SiO₂–ZnO–B₂O₃ glass coatings in hot acidic medium (0.1 N H₂SO₄ at 80 °C) for different times was studied. The thick films produced by a screen-printing method and heat treated at 700 °C/5 min were analyzed by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. The glass from the Bi₂O₃–SiO₂–ZnO–B₂O₃ system developed Zn₂SiO₄ and a glassy phase that were readily attacked by hot 0.1 N sulfuric acid, whereas the heat treated coating from the Bi₂O₃–SiO₂–TiO₂–ZnO–B₂O₃ system presented a finer microstructure with thin interconnected Bi₄Ti₃O₁₂ crystals and a glassy phase more resistant to hot 0.1 N sulfuric acid attack etching.     

Suppression of growth domains in epitaxial ZnO films on structured (0001) sapphire surface

Zinc oxide layers have been grown by magnetron sputtering in an oxygen atmosphere on structured sapphire surfaces. The formation of ZnO islands oriented in two directions (the so-called domains) was observed on (0001) Al₂O₃ surfaces with steps spaced by a distance from several thousands to several tens of thousands of nanometers. The islands formed along steps on (0001) Al₂O₃ surfaces with an ordered terrace-step structure (and, subsequently, ZnO films) have only one orientation. Another method is proposed for suppressing domains during ZnO growth on (0001) Al₂O₃ d.     

Microstructural characteristics and crystallographic evolutions of Ga-doped ZnO films grown on sapphire substrates at high temperatures by RF magnetron sputtering

The microstructural characteristics and crystallographic evolutions of Ga-doped ZnO (GZO) films grown at high temperatures were examined by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The GZO films with various film thicknesses were grown on (0 0 0 1) Al₂O₃ substrates at 750 °C by RF magnetron sputtering using a 2 wt% Ga-doped ZnO single target. The (0 0 0 2) ZnO peaks in the XRD patterns shifted to a higher angle with increasing film thickness and an additional (1 0 1¯ 1) ZnO peak was observed in the final stage of film growth. HRTEM showed the epitaxial growth of GZO films in the initial growth stage and the formation of surface protrusions in the intermediate stage due to elastic relaxation. The surface protrusions consisted of {1 0 1¯ 1}, {1 0 1¯ 3}, and {0 0 0 2} planes. After the surface protrusions had formed, a GZO film with many c-axis tilted grains formed due to plastic relaxation, where the tilted grain boundaries had an angle of 62° to the substrate. The formation of the protrusions and c-axis tilted grains was closely related to the strain status of the film induced by Ga incorporation, high-temperature growth and a high film thickness.     

Structural and optical properties of single-crystal ZnMgO thin films grown on sapphire and ZnO substrates by RF magnetron sputtering

High-quality ZnMgO films were grown by the radio frequency (RF) magnetron sputtering technique in pure oxygen ambient. Single-crystal films were obtained, when the Mg concentration was Zn_0.87Mg_0.13O or lower in the case of ZnMgO/Al₂O₃ and when it was Zn_0.65Mg_0.35O or lower in the case of ZnMgO/ZnO. Polycrystalline films were obtained when the growth temperature was lower than 500 °C, regardless of the Mg concentration. Position of the photoluminescence (PL) ultraviolet (UV) peak of the ZnMgO film shifted with the addition of Mg, from 3.33 eV (ZnO) to 3.51 eV (Zn_0.87Mg_0.13O) and 3.70 eV (Zn_0.65Mg_0.35O). It was also observed that growth of the ZnMgO films at higher temperature resulted in higher band-gap energy. It was proposed that this phenomenon is because concentration of the substitutional Mg atoms occupying Zn site is increased as the growth temperature increases.     

Different substrate materials effect on structure of ta-C films by Raman spectroscopy for magnetic recording sliders

Raman spectra, using visible (514 nm) and ultraviolet (244 nm) excitation, of tetrahedral amorphous carbon (ta-C) films of thickness of 5 nm have been studied as a function of different substrates materials. These materials are Fe-Co (Fe: 67 at.%, Co: 33 at.%) alloy, Fe-Ni alloy (Fe: 18 at.%, Ni: 82 at.%), Au and Al₂O₃-TiC (Al₂O₃: 64 at.%, TiC: 36 at.%), which are mainly used in magnetic recording sliders. The spectra show that the films deposited on Al₂O₃-TiC contain the highest sp³ content, with a lower sp³ content observed in films deposited to Fe-Co and Fe-Ni alloys. The lowest sp³ content was observed in films on the Au substrate. The results also indicate that the anti-wear performance of ta-C film on different substrates varies as Al₂O₃-TiC (the best) > Fe-Co and Fe-Ni alloy > Au (the worst). Also mechanisms are proposed to explain the effect of substrate material on these thin film properties.     

Precipitation of zinc ferrite nanoparticles in the Fe2O3–ZnO–SiO2 glass system

Glass materials in the ZnO–Fe₂O₃–SiO₂ system, containing zinc ferrite nanoparticles, were prepared by the sol–gel method and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer spectroscopy, AC- and DC-magnetization techniques. The gel samples, dried at 130 °C, were further heat treated in air at 500 and 800 °C. At 500 °C zinc ferrite and hematite nanoparticles, with an average size of approximately 24 nm, were precipitated in the brown and opaque 10ZnO–10Fe₂O₃–80SiO₂ and in the ruby colored transparent 5ZnO–5Fe₂O₃–90SiO₂ and 2.5ZnO–2.5Fe₂O₃–95SiO₂ glass matrices. In the 5ZnO–5Fe₂O₃–90SiO₂ sample the nanoparticles exhibited ferro or ferrimagnetic interactions combined with superparamagnetism with a blocking temperature of approximately 14 K. Heating at 800 °C seems to cause partial dissolution of the zinc ferrite and hematite particles in all the investigated compositions. Accordingly at 800 °C the 5ZnO–5Fe₂O₃–90SiO₂ glass shows a paramagnetic behavior down to 2 K.     

Influence of Fe2O3 Doping on TiO2 Electrode for Enhancement Photovoltaic Efficiency of Dye-Sensitized Solar Cells

In this work, we report for the first time the improvement of the photovoltaic characteristics of dye-sensitized solar cells (DSSCs) by doping TiO₂ with Fe₂O₃. DSSCs were fabricated using various percentages of Fe₂O₃-doped TiO₂ composite nanoparticles. The Fe₂O₃-doped DSSCs exhibited a maximum conversion efficiency of 5.76% because of the effective electron transport. DSSCs based on Fe₂O₃-doped TiO₂ films showed better photovoltaic performance than cells fabricated with only TiO₂ nanoparticles. This result was attributed to the prevention of recombination between electrons in the TiO₂ conduction band with the dye or electrolytes. A mechanism was suggested based on impedance results, which indicated improved electron transport at the interface of the TiO₂/dye/electrolyte.     

Microstructure and dielectric properties of La2O3 films prepared by ion beam assistant electron-beam evaporation

Ultrathin La₂O₃ gate dielectric films were prepared on Si substrate by ion assistant electron-beam evaporation. The growth processing, interfacial structure and electrical properties were investigated by various techniques. From XRD results, we found that the La₂O₃ films maintained the amorphous state up to a high annealing temperature of 900 °C for 5 min. From XPS results, we also discovered that the La atoms of the La₂O₃ films did not react with silicon substrate to form any La-compound at the interfacial layer. However, a SiO₂ interfacial layer was formed by the diffusion of O atoms of the La₂O₃ films to the silicon substrate. From the atomic force microscopy image, we disclosed that the surface of the amorphous La₂O₃ film was very flat. Moreover, the La₂O₃ film showed a dielectric constant of 15.5 at 1 MHz, and the leakage current density of the La₂O₃ film was 7.56 × 10⁻⁶ A/cm² at a gate bias voltage of 1 V.     

MOCVD growth of GaN on Si(1 1 1) substrates using an ALD-grown Al2O3 interlayer

GaN thin films have been grown on Si(1 1 1) substrates using an atomic layer deposition (ALD)-grown Al₂O₃ interlayer. This thin Al₂O₃ layer reduces strain in the subsequent GaN layer, leading to lower defect densities and improved material quality compared to GaN thin films grown by the same process on bare Si. XRD ω-scans showed a full width at half maximum (FWHM) of 549 arcsec for GaN grown on bare Si and a FWHM as low as 378 arcsec for GaN grown on Si using the ALD-grown Al₂O₃ interlayer. Raman spectroscopy was used to study the strain in these films in more detail, with the shift of the E₂(high) mode showing a clear dependence of strain on Al₂O₃ interlayer thickness. This dependence of strain on Al₂O₃ thickness was also observed via the redshift of the near bandedge emission in room temperature photoluminescence (RT-PL) spectroscopy. The reduction in strain results in a significant reduction in both crack density and screw dislocation density compared to similar films grown on bare Si. Screw dislocation density of the films grown on Al₂O₃/Si substrates approaches that of typical GaN layers on sapphire. This work shows great promise for the use of oxide interlayers for growth of GaN-based LEDs on Si.     

Decisive role of Au layer on the oriented growth of ZnO nanorod arrays via a simple aqueous solution method

Vertically aligned arrays of ZnO nanorod were synthesized on the Au/SiO₂/Si(1 0 0) substrate by a simple aqueous solution growth process, without pre-prepared ZnO seed layer. For comparison, glass and SiO₂/Si were also used as substrates, and the results show that the Au layer plays a decisive role in orienting the growth of the ZnO nanorod. The effects of other growth parameters, including Zn²⁺ concentration and growth time, on morphology, density, and orientation of the ZnO nanostructure were also studied and with longer reaction time, a new structure namely ZnO nanotip was obtained. Moreover, the growth mechanism of ZnO nanorod arrays grown on the Au/SiO₂/Si substrate was proposed.     

Preparation and characterization of nickel based catalysts on silica,alumina and titania obtained by sol–gel method

This work reports the investigation of the characteristics of catalysts produced by the sol gel process. Our analysis focuses on the properties related to heterogeneous catalysis, such as methane steam reforming, dry reforming, hydrogenation of organic compositions. Alumina, silica and titania based materials doped with Ni were synthesized. The characterization techniques used were: temperature programmed reduction, thermogravimetric analysis, specific surface area and X-ray powder diffraction. The specific surface area values obey the following sequence: Ni–SiO₂ > Ni–Al₂O₃ > Ni–TiO₂. The temperature programmed reduction indicated that in the Ni–SiO₂ sample, the nickel oxide is present in two different forms on the surface; the Ni–Al₂O₃ material presented one peak at high temperature, suggesting the presence of a nickel aluminate form. However, Ni–TiO₂ did not present reduction peaks. The thermogravimetric analysis, which was performed under inert atmosphere, showed the decomposition of the organic residues adsorbed on surface. The X-ray powder diffraction patterns of calcined materials showed two crystalline forms for TiO₂ in Ni–TiO₂ rutile and anatase. In Ni–Al₂O₃, crystalline areas composed of α-Al₂O₃, γ-Al₂O₃ and NiO were observed and finally for Ni–SiO₂, amorphous areas and NiO were found.     

The effect of composition and temperature on the amount and type of nanoferrite particles inserted in Fe2O3–ZnO–MgO–SiO2 glass–ceramics

Glass–ceramics with the composition 2Fe₂O₃.1ZnO.1MgO.96SiO₂ [4ZnMgFe] and 2Fe₂O₃.2ZnO.3MgO.93SiO₂ [7ZnMgFe] (mol%) were prepared using the sol–gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM), electron diffraction (ED) and Mössbauer spectroscopy (MS) were used to investigate the glass–ceramics structure. The samples contain ferrite nanoparticles embedded in a glass matrix. However, zinc ferrite nanoparticles seems to be the preferential crystalline phase formed. The amount of ferrite particles depends on treatment temperature and sample composition. The Mössbauer spectroscopy measurements show that ferrite nanoparticles can exhibit a ferrimagnetic behaviour combined with superparamagnetism.     

Microhardness and softening point of some alumino-borate glasses as flow dependent properties

Vickers indentations in some alumiio-borate glasses were studied. Glass compositions are based on the system B₂O₃Al₂O₃CaO with replacement of part of all the CaO by alkali oxide or divalent oxide, or replacement of part of B₂O₃ by SiO₂, TiO₂ or ZrO₂. Trends in Vickers hardness values are correlated with the Littleton softening points of the same glasses. Observations of the indentations indicate the possibility of a flow mechanism for forming indentations.     

Temperature dependence of growth orientation and surface morphology of Li‐doped ZnO thin films on SiO2/Si substrates

ZnO thin films doped with Li (ZnO:Li) were deposited onto SiO₂/Si (100) substrates by direct‐current sputtering technique in the temperature range from room temperature to 500 °C. The crystalline structure, surface morphology and composition, and optical reflectivity of the deposited films were studied by X‐ray diffraction (XRD), Scanning Electron Microscopy (SEM), X‐ray Photoelectron Spectroscopy (XPS) and optical reflection measurements. Rough surface p‐type ZnO thin film deposition was confirmed. The results indicated that the ZnO:Li films growed at low temperatures show c‐axis orientation, while a‐axis growth direction is preferable at high temperatures. Moreover, the optical reflectivity from the surface of the films matched very well with the obtained results. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation and characterization of ZnO-TiO2 films obtained by sol-gel method

The sol-gel route has been applied to obtain ZnO-TiO₂ thin films. For comparison, pure TiO₂ and ZnO films are also prepared from the corresponding solutions. The films are deposited by a spin-coated method on silicon and glass substrates. Their structural and vibrational properties have been studied as a function of the annealing temperatures (400-750 °C). Pure ZnO films crystallize in a wurtzite modification at a relatively low temperature of 400 °C, whereas the mixed oxide films show predominantly amorphous structure at this temperature. XRD analysis shows that by increasing the annealing temperatures, the sol-gel Zn/Ti oxide films reveal a certain degree of crystallization and their structures are found to be mixtures of wurtzite ZnO, Zn₂TiO₄, anatase TiO₂ and amorphous fraction. The XRD analysis presumes that Zn₂TiO₄ becomes a favored phase at the highest annealing temperature of 750 °C. The obtained thin films are uniform with no visual defects. The optical properties of ZnO-TiO₂ films have been compared with those of single component films (ZnO and TiO₂). The mixed oxide films present a high transparency with a slight decrease by increasing the annealing temperature.     

Preparation and properties of porous glass using fly ash as a raw material

The porous glasses were prepared by a conventional phase separation method using coal fly ash as a raw material, and the properties of these porous glasses were investigated. The composition of coal fly ash is basically composed of SiO₂–Al₂O₃–Fe₂O₃–CaO system and the SiO₂–B₂O₃–Al₂O₃–CaO–Na₂O system of glass was chosen as base glass composition. The pore diameter increases proportional to cube root of heating time (t^1/3), however, the early stage of phase separation is not clear. It is estimated that the rate determining step may be the diffusion process of structural units involving oxygen ions and the phase separation may take place by the nucleation and growth mechanism, and the relatively larger pores of above 1 μm can be obtained easily. The chemical composition of porous glasses is SiO₂–B₂O₃–Al₂O₃(–CaO–Na₂O). A relatively large amount of fly ash (>40%) can be used successfully for the preparation of porous glass.