Photoconductivity and function properties of polyacetylene films

Photoconductivity and photovoltaic effects of AsF₅-doped and undoped trans-(CH)_x films have been measured at room temperature in the wavelength region from 0.3 to 3.5 μm. The photovoltaic response threshold at 1.48 eV, measured on Schottky barrier junctions with a low work function metal, is interpreted as the single particle band-gap of trans-(CH)_x. I-V and C-V characteristics of the junctions indicate that good Schottky barriers are formed between lightly doped p-type (CH)_x and low work function metals. Evidence for ～ 2 × 10¹⁸ cm^?3 deep traps in both doped and undoped trans-(CH)_x is obtained from analysis of these characteristics.     

Applications of texture in thin films

Thin films are important in a variety of applications, including optical, magnetic,and electronic devices. The origin of microstructure in films formed by physical vapor deposition depends primarily on the homologous deposition temperature, which determines the grain structure and substructure. Texture is coupled to grain structure development through several kinetic parameters, principally surface diffusivity and interfacial energy. The representation of such fiber textures is facilitated by simplified pole intensity versus tilt angle plots rather than entire pole figures. The reliability of thin film interconnection materials such as aluminum alloys is heavily leveraged on both texture and grain structure. The properties of thin magnetic films depend on both the crystallographic texture and the grain shape anisotropy.     

Grain growth in thin metallic films

Grain growth in thin films deposited on a substrate was studied theoretically. The thrust of the model proposed is the effect of vacancy generation accompanying grain growth on the rate of the process. In addition, the magnitude of a tensile stress developing in the film was considered. It was shown that due to the contribution of vacancies to the free energy of the system, discernible grain growth is preceded by an “incubation” period, during which the grain structure can be considered as stable, as the rate of growth is relatively small over this incubation time. During this time, the vacancy concentration remains nearly constant, staying at a level much higher than the thermal equilibrium concentration. Based on numerical analysis, a simple expression for the incubation time in terms of the vacancy sink spacing, temperature and grain boundary characteristics was derived. With this formula, the stability of the grain structure of a thin film can be assessed for given conditions.     

Interfaces and stresses in thin films

A review of the current understanding of the effect of interfaces on the intrinsic stresses in polycrystalline thin films is given. Special attention is paid to the measurement, modeling and application of surface and interface stresses. Mechanisms for generating the compressive and tensile components of the intrinsic stress are assessed. Prospects for future research are presented.     

Texture transformations in Ag thin films

A thickness-dependent texture transformation during annealing of initially (1 1 1) fiber-textured face-centered-cubic metal thin films is phenomenologically well known: sufficiently thin films retain the (1 1 1) texture, while sufficiently thick films transform to a (1 0 0) fiber texture. This transformation has been explained based on minimization of strain and interface energies, but recent work calls into question the roles of both of these driving forces. A high-throughput experimental method for the study of this texture transformation has been developed and applied to thin silver films with and without Ti adhesion layers. More than 150 individual samples spanning a range of thicknesses and interface conditions were prepared in a single deposition run. The texture evolution of these samples was characterized using X-ray diffraction as a function of time and temperature during annealing. The transformation proceeds despite the fact that the stresses are too low according to the strain/interface energy model. For films with Ti adhesion layers, the transformation kinetics and extent of transformation depend on the film thickness in a surprising way with intermediate thickness films showing initially fast transformations and stable mixed textures, while thicker films show an incubation time but transform fully. The results are consistent with reduction in defect energy (e.g. dislocations or point defects) as the driving force for secondary grain growth in an environment in which only (1 0 0) recrystallization nuclei can form. The driving force increases with film thickness so the nonmonotonic variation in transformation rate implies that the density of (1 0 0) nuclei decreases with thickness.     

In doped ZnO thin films

ZnO thin films were deposited by ultrasonic spray technique, zinc acetate was used as starting solution with a molarity of 0.1 M. A set of indium (In) doped ZnO (between 2 and 8 wt%) thin films were grown on glass substrate at 350 °C. The present work is focused on the influence of the doping level on the structural, optical and electrical films properties. Optical film characterization was carried by using UV-visible transmission spectroscopy, the optical gap was deduced from absorption. From X ray diffraction (XRD) analysis, we have deduced that ZnO films are formed with nanocrystalline structure with preferential (0 0 2) orientation. The grain size is increased with In doping from 28 to 37 nm. Electrical characterization was achieved using two-probes coplanar structure, the measured conductivity varies from 2.3 to 5.9 Ω cm⁻¹ when increasing the doping level. However the optical gap is reduced from 3.4 to 3.1 eV.     

Hydrogen behavior in nanocrystalline titanium thin films

Nanocrystalline titanium films of different thicknesses, sputtered on sapphire substrates, were charged electrochemically with hydrogen. Hydrogen absorption and the thermodynamics of the nanocrystalline Ti–H thin film system were studied using electromotive force (EMF) measurements. The phase boundaries obtained from the EMF–pressure–concentration curves were confirmed by X-ray diffraction, complemented by in situ stress measurements during hydrogen charging. The change in the stress increase with hydrogen concentration was found to be in good agreement with the obtained phase boundaries. In comparison to bulk Ti–H system, considerable changes, such as shifted phase boundaries, and narrowed and sloped miscibility gaps, were observed in Ti–H thin films. These changes vary among the films of different crystalline orientation and are attributed to both microstructural effects and stress contributions. The influence of the initial crystallographic growth orientation of Ti films on the measured thermodynamic isotherms, phase transitions and stress development is discussed in detail.     

Microstructural evolution in electroplated Cu thin films

The microstructural evolution of electroplated Cu films (0.89 to 3.0 μm thick) has been studied by texture analysis. Before annealing, the volume fraction of (1 1 1) grains decreases with increasing film thickness, while that of (1 0 0), (1 1 0), and randomly oriented grains increases. Annealing causes a decrease in the 1 1 1 fiber intensity in the thinnest films due to growth of randomly oriented grains and multiple twinning.     

Strain gradient effect in nanoscale thin films

Compared to uniform deformation, gradient-dominant deformation experiments at the micro-scale have consistently shown remarkable strengthening effect. It is proposed in the literature that pronounced strain gradient, on the order of the inverse of a characteristic length scale, adds to the materials strength. The literature contains several formulations that account for the strain gradient in the constitutive equation. However, the role of microstructures in these formulations remains to be investigated. Due to the difficulties in micro/nano scale experimentation, attempts to investigate this mechanism so far have been confined to specimens with dimensions more than 10 microns, or to nano-indentation experiments. In this study, we use MEMS-based testing techniques to explore the effect of strain gradient in 100, 150, 200 and 485 nm thick freestanding Aluminum specimens with average grain size of 50, 65, 80 and 212 nm respectively. Strain gradient plasticity analysis show that the characteristic length scale for Aluminum is about 4.5 μm, which is similar to the values for copper and nickel reported in the literature. Experimental results suggest that the strain gradient effect is fundamentally related to dislocation-based mechanisms, and is absent at extremely small length scales (< 100 nm for Aluminum), where accommodation of dislocations in the crystal grains is not energetically favorable.     

Microtwinning in highly nonstoichiometric VOx thin films

Both pulsed-DC biased and commercial ion-beam sputtered VO_x thin films maintain a face-centered-cubic nanocrystalline phase, even for stoichiometries of x > 1.5, which is well outside the bulk equilibrium solubility range for cubic VO_x. Many of these highly nonstoichiometric films exhibit a high density of microtwins, which give rise to unusual fine structure in the selected-area electron diffraction patterns, namely: an additional defect ring; a significant broadening of the {2 0 0} ring; pairs of parallel rod features which are tangent to the additional defect ring; and additional fine-structure features between the {2 0 0} and {2 2 0} rings. The formation of the microtwins is correlated with the coalescence of vanadium vacancies along the {1 1 1} twin planes in the crystalline lattice.     

Ion-irradiation-induced ferromagnetism in undoped ZnO thin films

We have introduced defects in ZnO epitaxial thin films by swift heavy ¹⁰⁷Ag⁹⁺ ion irradiation and investigated systematically their magnetic, electrical and optical properties. Oxygen annealed ZnO films are epitaxial single crystals that exhibit no long-range magnetic order. However, in this paper it is shown that room-temperature ferromagnetism (RTFM) can be introduced in a controlled manner in these films using ion irradiation and that the magnetization increases with ion dose. This qualitatively agrees with earlier studies which showed that RTFM could be induced in ZnO films through either vacuum thermal annealing or pulsed laser annealing below energy densities that lead to melting. Raman studies of the ion irradiated samples revealed dramatic changes in the vibration modes that correlated with increases in the carrier concentration, indicative of lattice disorder and defect creation. We compare these results with those observed in laser irradiated and vacuum annealed samples, and then discuss these findings in the context of defects and defect complexes created during the high-energy heavy ion irradiation process. We propose a unified mechanism to explain RTFM and n-type conductivity enhancements during irradiation, and laser and vacuum annealing.     

Dislocation interactions in thin FCC metal films

High strength, high hardening rates, and strong Bauschinger-like effects in thin films have been attributed to constraints on dislocation motion and dislocation interactions. To understand these phenomena, dislocation interactions in (1 1 1) and (0 0 1) oriented single crystal FCC films were studied using dislocation dynamics simulations. Interactions on intersecting glide planes resulted in junction formation, annihilation, or attractive non-junction-forming configurations, while dislocations on parallel glide planes formed dipoles. The configurations adopted by interacting dislocations, and thus the strengths of the interactions, were found to be sensitive to the applied strain, film thickness, crystallographic orientation, and boundary conditions. Different interactions thus dominate film behavior in different ranges of film thickness and applied strain. Interactions are stronger on unloading than on loading. Interactions involving three or more dislocations are found to be different from pairwise interactions. The results suggest that simple analytical calculations are unlikely to describe film phenomena but that full 3-D simulations can be used to understand many features of thin film mechanical behavior.     

Progress in oxygen behaviors in two-dimensional thin films

With the development of spintronics,the investigation on the behavior of oxygen in two-dimensional materials has never ceased.On account of its lively nature,oxygen is hard to exist alone in the system.However,it will interact with other atoms and produce complex orbital hybridization effect,which has influenced the performance of the material.Especially for materials in nanoscale,it is inevitable to introduce the oxygen atoms,no matter what in the process of preparation or employ.Therefore,it is necessary to carry on the research about the effect of oxygen behaviors in the two-dimensional thin films.In this paper,it will mainly introduce the effect of oxygen behaviors on the magnetic properties,electrical properties,phase transition,spin-dependent properties and thermal stability,summarize several factors which influence the oxygen behaviors,and generalize the research progress of the mechanism behind the oxygen behaviors.     

Magnetic thin films for information storage

In most storage technologies, the two critical components are the recording head and the recording medium; hence, improvements in storage media go hand in hand with improvements in read/write head technology. Thin films are becoming the dominant medium chosen to meet the burgeoning demand for information storage, and thin-film technology has been transferred to the read/write heads as well. Materials science plays an important role in correlating the structure, properties and processing of thin-film magnetic materials for both applications.     

Dual photoluminescence of polythiophene thin films

Transient photoluminescence of electrochemically prepared thin films of polybithiophene and polyterthiophene was analyzed. It was observed for the first time that time-integrated spectra exhibited dual photoluminescence. In the time interval right after the excitation, a short-lifetime emission corresponding to the bandgap energy was observed, whose lifetime was estimated to be several tens of ps. Moreover, another emission of smaller energy than the bandgap was observed in the following time interval. This emission had relatively longer lifetime: several hundred ps. These results indicated that at least two types of excitons were formed in the films. Both of these could be the self-trapped excitons generated at the recombination sites with different local environments. Polyterthiophene contains many microcrystalline regions in comparison with polybithiophene. Enhancement of the longer-lifetime photoluminescence (PL) component in polyterthiophene suggests that the exciton with longer lifetime is generated in the microcrystalline region.     

Oriented crystallization of thin Pd-Cu films

The phase composition, substructure, and orientation of thin Pd-Cu films obtained by magnetron sputtering of an alloy target and condensation on the (001) surface of phlogopite have been studied by transmission electron microscopy (TEM). Three-orientation epitaxial structures of the β phase with a CsCl structure have been obtained. In epitaxial two-phase solid-solution films, the α and β phases are oriented according to the Nishiyama-Wassermann and Kurdjumov-Sachs orientation relationships. Single-crystal films of α and β solid solutions have been obtained on phlogopite. The plasma effect has been revealed; under the conditions of magnetron sputtering the orientation relationships not typical of epitaxial films of fcc and bcc metals on phlogopite have been realized in the temperature-concentration range of the β ? α phase transformation.     

Characterization of sputter—deposited TiPdNi thin films

TiPdNi thin films were prepared by magnetron sputtering onto unheated glass and silicon substrate.Atomic force microscope,energy-dispersive X-ray microsanalyzer,X-ray diffractometer,differential scanning calorimeter and optican microsope were used to characterize the films.It is found that the surface morphology of the films change during the sputtering process and a shift of about 3%Ti(mole fraction(content from the center to the edge of the substrate occurs.The freestanding as-deposited films undergo crystallization followed by three kinds of cooling conditions.For all these heattreated films,B2→B19→B19‘ two-stage phase transformation takes place.Many Ti2Ni and Ti2Pd type of precipitates are detected in the films.The constraint films on silicon substrate are crystallized at high temperature.After crystallization,the films show a two-way shape memory effect.