Pulsed rf magnetron sputtered alumina thin films

Alumina thin films were deposited on titanium (Ti) and fused quartz by both direct and reactive pulsed rf magnetron sputtering techniques. X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy and atomic force microscopy were utilized to study the phases and surface morphology of the films. The as-deposited alumina thin films were amorphous. However, after annealing at 500 degrees C in vacuum, the crystalline peaks corresponding to the Theta (0), Delta (8) and Chi ()) alumina phases were obtained. The optical transmittance and reflectance as well as IR emittanc,e data were also evaluated for the thin films. The transmittance, e.g., (similar to 90%) of the bare quartz substrate was not changed even when the alumina thin films were deposited for an hour. However, further increase in deposition time (e.g., 7 h) of the alumina thin films showed only a marginal decrease (e.g., similar to 5%) in average transmittance of the bare quartz substrate. The direct and indirect optical band gaps and extinction coefficient of the alumina films were estimated from the transmittance spectra. The IR emittance of the Ti substrate (e.g., similar to 16%) was almost constant after depositing alumina thin films for an hour. Further increase in deposition time showed only a marginal increase (e.g., similar to 9%) in IR emittance value. Therefore, it is proposed that the alumina films developed in the present work can act as a protective cover for the Ti substrate while retaining the thermo-optical properties of the same. The nanohardness and Young's modulus of the alumina thin films were evaluated by the novel nanoindentation technique. The nanohardness was measured as similar to 6 GPa. Further, Young's modulus was evaluated as similar to 116 GPa. The magnitudes of the nanomechanical properties of the thin films were a little smaller than those reported in the literature. This was linked to the lack of crystalline phases in the as-deposited alumina thin films. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Suppression of grain boundary relaxation in Zr-doped BiFeO3 thin films

Here, we present the results of temperature dependent dielectric studies on chemical solution processed Zr-doped BiFeO3 (BFO) thin films deposited on Pt/Si substrates. We find that in contrast to the undoped BFO films, Zr doping at Fe-site suppresses the low frequency dielectric relaxation originating from the grain boundaries, attributed to the increased dipolar rigidity due to stronger Zr-O bonds. Temperature dependent dc conductivity obtained from impedance and modulus analyses shows two distinct conduction processes occurring inside the grains. At temperature below similar to 423K, conductivity is nearly temperature independent, while in the high temperature regime (above similar to 423K), conduction is governed by the long range movement of oxygen vacancies with an activation energy of similar to 1eV. (C) 2014 AIP Publishing LLC.

Film thickness mediated transition in the kinetics of electric current induced flow of thin liquid metal films

Application of high electric-field between two points in a thin metallic film results in liquefaction and subsequent flow of the liquid-film from one electrode to another in a radially symmetric fashion. Here, we report the transition of the flow kinetics driven by the liquid film thickness varying from 3 to 100 nm. The mechanism of the flow behavior is observed to be independent of the film thickness; however, the kinetics of the flow depends on the film thickness and the applied voltage. An analytical model, incorporating viscosity and varying electrical resistivity with film thickness, is developed to explain the experimental observations. (C) 2014 AIP Publishing LLC.

Fabrication of Poly(Vinylidene Chloride-Co-Vinyl Chloride)/TiO2 Nanocomposite Films and Their Dielectric Properties

Flexible poly(vinylidene chloride-co-vinyl chloride)/TiO2 nanocomposite films were fabricated and their dielectric properties were studied. The structural characterization of the composites was carried out using various spectroscopic and electron microscopic techniques. From the thermal analysis of the composites, an improvement in the thermal properties was observed for the composites, as compared to the neat polymer. An increase in the DC conductivity was also observed in the composites, which was due to the tunneling of charge carriers. Furthermore, it was observed that the optimal loading of titania in the matrix was required, above which the properties of the composites showed deterioration. The study of the dielectric properties of the composites supports their use in microelectronic devices as separator in charge storage devices and in transistors.

Effect of post-deposition annealing on composition and electrical properties of dc reactive magnetron sputtered Al2O3 thin films

We have investigated the effect of post- deposition annealing on the composition and electrical properties of alumina (Al2O3) thin films. Al2O3 were deposited on n-type Si < 100 >. substrates by dc reactive magnetron sputtering. The films were subjected to post- deposition annealing at 623, 823 and 1023 K in vacuum. X-ray photoelectron spectroscopy results revealed that the composition improved with post- deposition annealing, and the film annealed at 1023 K became stoichiometric with an O/Al atomic ratio of 1.49. Al/Al2O3/Si metal-oxide-semiconductor (MOS) structures were then fabricated, and a correlation between the dielectric constant epsilon(r) and interface charge density Q(i) with annealing conditions were studied. The dielectric constant of the Al2O3 thin films increased to 9.8 with post- deposition annealing matching the bulk value, whereas the oxide charge density decreased to 3.11 x 10(11) cm(-2.) Studies on current-voltage IV characteristics indicated ohmic and Schottky type of conduction at lower electric fields (<0.16 MV cm(-1)) and space charge limited conduction at higher electric fields.

Healing time for the growth of thin films on patterned substrates

The healing times for the growth of thin films on patterned substrates are studied using simulations of two discrete models of surface growth: the Family model and the Das Sarma-Tamborenea (DT) model. The healing time, defined as the time at which the characteristics of the growing interface are ``healed'' to those obtained in growth on a flat substrate, is determined via the study of the nearest-neighbor height difference correlation function. Two different initial patterns are considered in this work: a relatively smooth tent-shaped triangular substrate and an atomically rough substrate with singlesite pillars or grooves. We find that the healing time of the Family and DT models on aL x L triangular substrate is proportional to L-z, where z is the dynamical exponent of the models. For the Family model, we also analyze theoretically, using a continuum description based on the linear Edwards-Wilkinson equation, the time evolution of the nearest-neighbor height difference correlation function in this system. The correlation functions obtained from continuum theory and simulation are found to be consistent with each other for the relatively smooth triangular substrate. For substrates with periodic and random distributions of pillars or grooves of varying size, the healing time is found to increase linearly with the height (depth) of pillars (grooves). We show explicitly that the simulation data for the Family model grown on a substrate with pillars or grooves do not agree with results of a calculation based on the continuum Edwards-Wilkinson equation. This result implies that a continuum description does not work when the initial pattern is atomically rough. The observed dependence of the healing time on the substrate size and the initial height (depth) of pillars (grooves) can be understood from the details of the diffusion rule of the atomistic model. The healing time of both models for pillars is larger than that for grooves with depth equal to the height of the pillars. The calculated healing time for both Family and DT models is found to depend on how the pillars and grooves are distributed over the substrate. (C) 2014 Elsevier B.V. All rights reserved.

Influence of sputter power on structural and electrical properties of TiO2 films for Al/TiO2/Si gate capacitors

Titanium dioxide (TiO2) thin films were deposited onto p-Si substrates held at room temperature by reactive Direct Current (DC) magnetron sputtering at various sputter powers in the range 80-200W. The as-deposited TiO2 films were annealed at a temperature of 1023K. The post-annealed films were characterized for crystallographic structure, chemical binding configuration, surface morphology and optical absorption. The electrical and dielectric properties of Al/TiO2/p-Si structure were determined from the capacitance-voltage and current-voltage characteristics. X-ray diffraction studies confirmed that the as-deposited films were amorphous in nature. After post-annealing at 1023K, the films formed at lower powers exhibited anatase phase, where as those deposited at sputter powers >160W showed the mixed anatase and rutile phases of TiO2. The surface morphology of the films varied significantly with the increase of sputter power. The electrical and dielectric properties on the air-annealed Al/TiO2/p-Si structures were studied. The effect of sputter power on the electrical and dielectric characteristics of the structure of Al/TiO2/p-Si (metal-insulator-semiconductor) was systematically investigated. Copyright (c) 2014 John Wiley & Sons, Ltd.

Confinement enhances dispersion in nanoparticle-polymer blend films

Polymer nanocomposites constitute an important class of materials whose properties depend on the state of dispersion of the nanoparticles in the polymer matrix. Here we report the first observations of confinement-induced enhancement of dispersion in nanoparticle-polymer blend films. Systematic variation in the dispersion of nanoparticles with confinement for various compositions and matrix polymer chain dimensions has been observed. For fixed composition, strong reduction in glass transition temperature, T-g, is observed with decreasing blend-film thickness. The enhanced dispersion occurs without altering the polymer-particle interactions and seems to be driven by enhanced matrix-chain orientation propensity and a tendency to minimize the density gradients within the matrix. This implies the existence of two different mechanisms in polymer nanocomposites, which determines their state of dispersion and glass transition.

Effect of silane functionalized alumina on poly(vinyl butyral) nanocomposite films: Thermal, mechanical, and moisture barrier studies

In this work, a hybrid-polymer nanocomposite film, based on polyvinyl butyral/amino-silane functionalized nano alumina, was fabricated by melt processing. The calcium degradation measurements suggest the functionalized nanocomposite films exhibit higher resistance towards moisture penetration as compared to the neat alumina loaded films. Thermal stability, mechanical strength, and contact angle studies of the composites were also conducted to evaluate the performance of the functionalized alumina loaded films. These nanocomposite films were encapsulated over Al/P3HT/ITO Schottky structured device. The changes observed in the current density of the devices to the applied voltage before and after accelerated aging conditions are presented. The nanocomposite with functionalized alumina films exhibits 50% change in current density, which is superior to that attained with neat and non-functionalized films. POLYM. COMPOS., 35:1426-1435, 2014. (c) 2013 Society of Plastics Engineers

Influence of film thickness on the properties of sprayed ZnO thin films for gas sensor applications

Transparent conducting ZnO films were prepared at substrate temperature 400 degrees C with different film thicknesses by nebulizer spray pyrolysis method on glass substrates. XRD studies reveal that the films are polycrystalline in nature having hexagonal crystal structure with preferred grain orientations along (0 0 2) and (1 0 1) directions. The crystallite size increases along (0 0 2) plane with the thickness increase and attains a maximum 109 nm for 913 nm film thickness. Analysis of structural parameters indicates that the films having thickness 913 nm are found to have minimum dislocation density and strain values. The HRSEM measurements show that the surface morphology of the films also changes with film thickness. EDAX estimates the average atomic percentage ratio of Zn and O in the ZnO films. Optical studies reveal the band gap energy decrease from 3.27 to 3.14 eV with increase of film thickness. Room temperature PL spectra show the near-band-edge emission and deep-level emission due to the presence of defects in the ZnO thin films. Impedance spectroscopy analysis indicates that grain boundary resistance decreases with the increasing ammonia concentration up to 500 ppm and the maximum sensitivity is found to be 1.7 for 500 ppm of ammonia. (C) 2014 Elsevier Ltd. All rights reserved.

Impact of spray flux density and vacuum annealing on the transparent conducting properties of doubly doped (Sn plus F) zinc oxide films deposited using a simplified spray technique

In this investigation transparent conducting properties of as-deposited and annealed ZnO:Sn:F films deposited using different spray flux density by changing the solvent volume (10 mL, 20 mL ... 50 mL) of the starting solutions have been studied and reported. The structural analyses of the films indicate that all the films have hexagonal wurtzite structure of ZnO with preferential orientation along (002) plane irrespective of the solvent volume and annealing treatment whereas, the overall crystalline quality of the films is found to be enhanced with the increase in solvent volume as well as with annealing. This observed enhancement is strongly supported by the optical and surface morphological results. From the measurements of electrical parameters, it is seen that, the annealed films exhibit better electrical properties compared to the as-deposited ones. Annealing has caused agglomeration of grains as confirmed by the surface morphological studies. Also, the annealing process has led to an improvement in the optical transparency as well as band gap. It is found from the analyses of the characteristics of the as- deposited and annealed films that the annealed film deposited from starting solution having solvent volume of 50 mL is optimal in all respects, as it possesses all the desirable characteristics including the quality factor (1.60 x 10(-4) (Omega/sq.)(-1)). (C) 2014 Elsevier Ltd. All rights reserved.

Ontology analysis of global gene expression differences of human bone marrow stromal cells cultured on 3D scaffolds or 2D films

Differences in gene expression of human bone marrow stromal cells (hBMSCs) during culture in three-dimensional (3D) nanofiber scaffolds or on two-dimensional (2D) films were investigated via pathway analysis of microarray mRNA expression profiles. Previous work has shown that hBMSC culture in nanofiber scaffolds can induce osteogenic differentiation in the absence of osteogenic supplements (OS). Analysis using ontology databases revealed that nanofibers and OS regulated similar pathways and that both were enriched for TGF-beta and cell-adhesion/ECM-receptor pathways. The most notable difference between the two was that nanofibers had stronger enrichment for cell-adhesion/ECM-receptor pathways. Comparison of nanofibers scaffolds with flat films yielded stronger differences in gene expression than comparison of nanofibers made from different polymers, suggesting that substrate structure had stronger effects on cell function than substrate polymer composition. These results demonstrate that physical (nanofibers) and biochemical (OS) signals regulate similar ontological pathways, suggesting that these cues use similar molecular mechanisms to control hBMSC differentiation. Published by Elsevier Ltd.

Ising model on a random network with annealed or quenched disorder

We study the equilibrium properties of an Ising model on a disordered random network where the disorder can be quenched or annealed. The network consists of fourfold coordinated sites connected via variable length one-dimensional chains. Our emphasis is on nonuniversal properties and we consider the transition temperature and other equilibrium thermodynamic properties, including those associated with one-dimensional fluctuations arising from the chains. We use analytic methods in the annealed case, and a Monte Carlo simulation for the quenched disorder. Our objective is to study the difference between quenched and annealed results with a broad random distribution of interaction parameters. The former represents a situation where the time scale associated with the randomness is very long and the corresponding degrees of freedom can be viewed as frozen, while the annealed case models the situation where this is not so. We find that the transition temperature and the entropy associated with one-dimensional fluctuations are always higher for quenched disorder than in the annealed case. These differences increase with the strength of the disorder up to a saturating value. We discuss our results in connection to physical systems where a broad distribution of interaction strengths is present.

Dielectric Impedance Studies of Poly(vinyl butyral)-Cenosphere Composite Films

In the current study, amino silane functionalized cenosphere particles was used as a reinforcing filler in poly(vinyl butyral) matrix and were made by melt blending. The changes observed in the dielectric performance of the composite films with varying weight percentage of cenosphere particle in the matrix were investigated. The dielectric property and impedance spectroscopy were evaluated as a function of applied frequency in the range of 50 Hz to 5 MHz. It is observed that, because of orientation polarization of the PVB polymer, the permittivity and impedance decrease, whereas conductivity increases. Tangent loss graph indicates that the property of the matrix is associated with geometrical fill factor and the lowest quality factor. Therefore, above 10 kHz, these composites can be considered as dielectric loss-less material. (C) 2013 Society of Plastics Engineers

Solution Processed Cu2CoSnS4 Thin Films for Photovoltaic Applications

Earth abundant alternative chalcopyrite Cu2CoSnS4 (CCTS) thin films were deposited by a facile sol-gel process onto larger substrates. Temperature dependence of the process control of deposition and desired phase formations was studied in detail. Films were analyzed for complete transformation from amorphous to polycrystalline, with textured structures for stannite phase, as reflected from the X-ray diffraction and with nearly stoichiometric compositions of Cu:Co:Sn:S = 2:0:1:0:1:0:4:0 from EDAX analysis. Morphological investigations revealed that the CCTS films with larger grains, on the order of its thickness, were synthesized at higher temperature of 500 degrees C. The optimal band gap for application in photovoltaics was estimated to be 1.4 eV. Devices with SLG/CCTS/Al geometry were fabricated for real time demonstration of photoconductivity under A.M 1.5 G solar and 1064 rim infrared laser illuminations. A photodetector showed one order current amplification from similar to 1.9 X 10(-6) A in the dark to 2.2 x 10(-5) A and 9.8 X 10(-6) A under A.M 1.5 G illumination and 50 mW cm(-2) IR laser, respectively. Detector sensitivity, responsivity, external quantum efficiency, and gain were estimated as 4.2, 0.12 A/W, 14.74% and 14.77%, respectively, at 50 mW cm(-2) laser illuminations. An ON and OFF ratio of 2.5 proved that CCTS can be considered as a potential absorber in low cost photovoltaics applications.