Fabrication and characteristics of silicon-rich oxide thin films with controllable compositions

Silicon-rich oxide films with controllable optical constants and properties are deposited by the reactive magnetron sputtering method on a Si target. The O/Si atomic ratio x of SiO_x is tuned from 0.12 to 1.84 by adjusting the oxygen flow rate,which is found to be a more effective way to obtain SiO_x films compared with changing the oxygen content [O_2∕(Ar+O_2) ratio]. The optical properties of Si Oxfilms can be tuned from semiconductor to dielectric as a function of ratio x. The structures and components are also investigated by an x ray photoelectron spectroscopy analysis of the Si 2p core levels,the results of which exhibit that the structures of SiO_x can be thoroughly described by the random bonding model.     

Identification of A- and B-site cation vacancy defects in perovskite oxide thin films

Cation vacancies on both sublattices (V(Ti), V(Sr)) have been identified in homoepitaxial pulsed laser deposited SrTiO3 films using high intensity variable energy positron annihilation lifetime spectroscopy (PALS) measurements. Film nonstoichiometry was varied by varying laser fluence. PALS showed that on increasing the fluence above the Ti/Sr～1 value, the concentration ratio [V(Sr)]/[V(Ti)] systematically increased. Reducing the fluence into the Ti-poor region below resulted in additional vacancy cluster defect formation. Vacancy concentrations greater than ～50 ppm were observed in all films.     

Interface effects in perovskite thin films

The control of matter properties (transport, magnetic, dielectric,…) using synthesis as thin films is strongly hindered by the lack of reliable theories, able to guide the design of new systems, through the understanding of the interface effects and of the way the substrate constraints are imposed on the material. The present Letter analyzes the energetic contributions at the interfaces, and proposes a model describing the microscopic mechanisms governing the interactions at an epitaxial interface between a manganite and another transition metal oxide in perovskite structure (as for instance SrTiO3). The model is checked against experimental results and literature analysis.     

Fabrication and characterization of fluorine-doped thin oxide thin films and nanorod arrays via spray pyrolysis

This paper reports the synthesis and characterization of fluorine-doped tin oxide (FTO) thin films via intermittent spray pyrolysis utilizing a solution mixture of tin chloride pentahydrate and ammonia fluoride. Utilizing the same solution, nanorod arrays were fabricated via template-based growth. Uniform and crack-free FTO films over 20×20 mm with a thickness up to 900 nm have been routinely achieved; such FTO films demonstrate electrical resistivity as low as 2.2×10^-4 Ω cm as well as good optical transparency ranging from 75 to 85%. In addition, FTO nanorods were fabricated using template-filling methods at a temperature of 440 °C. The nanorods have a diameter of ∼160–250 nm, appear to be comprised of small nanoparticles 5–10 nm in size, and have a resistivity value of 4×10^-1 Ω cm. PACS 81.07.-b; 73.61.-r; 81.16.Be     

Phase-to-phase perovskite growth transformation in thin PZT films

The structure and dielectric properties of thin lead zirconate titanate films grown ex situ on Pt/SiO₂/Si substrates were studied. It was shown that with a crystallization temperature increase in the range of 540 to 580°C, the mechanism of perovskite phase growth changes and the ferroelectric parameters change according to this transformation.     

Kinetics of cation site exchange in mixed garnets

The redistribution rate of cations between the a- and d -sites in quasibinary mixed yttrium—iron gallium and—iron aluminum garnet single crystals has been investigated for temperatures between 773 and 1523 K. The frozen-in cation distributions of quenched and of slow cooled samples were derived from magnetization measurements and are described by a phenomenological kinetic equation. The activation energies derived for the thermally activated process range from 1.3 to 4eV and are related to the anion vacancy concentration of the crystals. The zero defect intrinsic a-dbarrier height is 4.5 eV as inferred from the observed linear relation between the frequency factor and the activation energy of the Arrhenius equation. This empirical relation, known as the compensation law, has been further applied to interpret reported intra-sublattice site exchange effects of cations on the c-sites and cation self-diffusion data of YIG. The inferred c-c barrier height of 5.5 eV agrees within the error limits of ± 0.1eV with the enthalpy of formation, 5.4 eV, and the enthalpy of solution, 5.3 eV, derived with equilibrium models based on the ionic fluid (Temkin) model and using experimental results of the kinetics, the cation distribution and the solubility of garnets. Apparently the c-c bonds form the strongest binding scaffold of the garnet structure, which is manifested by a relation to the melting point. While premelting around the anion vacancies assisted by their thermal fluctuations reduces the effective barrier height for cation migration, the apparently uneffected melting point of the compound itself is reflected by the compensation law.     

Photoconductivity of thin organic films

Thin organic films were deposited on silicon oxide surfaces with golden interdigitated electrodes (interelectrode gap was 2 μm), and the film resistivities were measured in dark and under white light illumination. The compounds selected for the measurements include molecules widely used in solar cell applications, such as polythiophene (PHT), fullerene (C₆₀), pyrelene tetracarboxylic diimide (PTCDI) and copper phthalocyanine (CuPc), as well as molecules potentially interesting for photovoltaic applications, e.g. porphyrin-fullerene dyads. The films were deposited using thermal evaporation (e.g. for C₆₀ and CuPc films), spin coating for PHT, and Langmuir-Schaeffer for the layer-by-layer deposition of porphyrin-fullerene dyads. The most conducting materials in the series are films of PHT and CuPc with resistivities 1.2 × 10³ Ω m and 3 × 10⁴ Ω m, respectively. Under light illumination resistivity of all films decreases, with the strongest light effect observed for PTCDI, for which resistivity decreases by 100 times, from 3.2 × 10⁸ Ω m in dark to 3.1 × 10⁶ Ω m under the light.     

Pulsed laser deposition of perovskite relaxor ferroelectric thin films

Structural, dielectric and ferroelectric properties of thin films of La-doped lead zirconate titanate (PLZT) and sodium bismuth titanate-barium titanate (NBT-BT) perovskite relaxor ferroelectric have been investigated. PLZT films were deposited on Pt/Si substrates in oxygen atmosphere by pulsed laser deposition (PLD) and radio frequency (RF) discharge-assisted PLD, using sintered targets with different La content and Zr/Ti ratio, near or at the boundary relaxor ferroelectric. The films are polycrystalline with perovskite cubic or slightly rhombohedral structure. A slim ferroelectric hysteresis loop, typical for relaxors, has been measured for all film sets. Dielectric characterization shows a large value of capacitance tunability and low dielectric loss. However, common problems related to lead diffusion into the metallic electrode layer do not allow one to obtain high capacitance values, due to the formation of an interface layer with low dielectric constant. Lead-free NBT-BT thin films have been deposited on single crystal (1 0 0)-MgO substrates starting from targets with composition at the morphotropic phase boundary between rhombohedral and tetragonal phase. Films deposited by PLD are polycrystalline perovskite with a slight (1 0 0) orientation. Capacitance measurements were performed using interdigital metallic electrodes deposited on the film's top surface and showed high relative dielectric constant, on the order of 1300.     

Ultrasonically sprayed-on perovskite solar cells-effects of organic cation on defect formation of CH3NH3PbI3 films

Methylammonium lead iodide (CH₃NH₃PbI₃) based perovskite having low degrees of the disorder is of great interest for optoelectronic and photovoltaic applications. In this work, a layer of CH₃NH₃PbI₃ was successfully prepared using an ultrasonically sprayed-nebulous method. Changes in structural and optical properties alongside with photo-induced charge separation and transportation behavior were systematically studied. The surface photovoltage spectra reveal a significant reduction of the density of deep defect states as the organic content was increased. It was observed that the measured values of Urbach energies decrease from 33.36 to 28.24 meV as the amount of organic content was increased to an optimum value. The best perovskite solar cells obtained using the sprayed-on approach exhibited a J_sc of 16.54 mA/cm², a V_oc of 0.99 V, and a FF of 62.4, resulting in an overall PCE of 10.09%.     

Activated carbon as back contact for HTM-free mixed cation perovskite solar cell

Carbon materials have potential applications in perovskite solar cell because of their excellent electronic properties and low cost. In this paper, we report, for the first time, activated carbon as a back contact for hole transport layer-free mixed halide perovskite solar cells. The ability of activated carbon to form conducting chain-like structure when dispersed in a polymeric solution makes it a possible candidate for back contact. A composite of activated carbon and PEMA was optimized with varying concentration. Mixed cation was used as a perovskite absorber and was analysed for its structural and optical properties. The fabricated devices were studied for their electrical performance. They were also subjected to stability study and showed promising results.     

Cathodic electrodeposition of mixed oxide thin films

Cathodic electrodeposition is a rather unexplored route for the synthesis of mixed oxides. Two different mechanisms are generally reported which lead to oxide thin film formation: (i) the direct reduction of the oxidation state of the metallic elèment, (ii) an interfacial pH increase and local supersaturation followed by oxide precipitation. We emphasize the major requirements for mixed thin film formation via electrosynthesis and present results obtained with two different systems of special interest. The first, ZnO/Eu(OH)_x, is based on the second mechanism. The second, TiO₂/WO₃, is a mixed mechanism process. We describe the conditions under which europium oxide can be deposited cathodically. This oxide is obtained by co-deposition with zinc oxide. After a heat treatment at 400 °C, X-ray diffraction shows that the films are mainly made of monoclinic Eu₂O₃. The successful WO₃–TiO₂ mixed film electrodeposition in a wide compositional range is also reported.     

Orientation dependence of ferroelectricity in pulsed-laser-deposited epitaxial bismuth-layered perovskite thin films

Thin films of bismuth-layered perovskites such as SrBi₂Ta₂O₉, Bi₄Ti₃O₁₂, and BaBi₄Ti₄O₁₅ with preferred orientations were grown by pulsed laser deposition on epitaxial conducting LaNiO₃ electrodes on single-crystalline (100) SrTiO₃ or on top of epitaxial buffer layers on (100) silicon. A morphology and structure investigation by X-ray diffraction analysis, scanning probe microscopy, and scanning and transmission electron microscopy showed that the films consisted of both c-axis-oriented regions and mixed (110)-, (100)-, and (001)-oriented regions. The regions with mixed orientation featured rectangular as well as equiaxed crystalline grains protruding out of a smooth c-oriented background. A closer examination revealed that the regions with mixed orientation actually consisted of a c-axis-oriented sublayer growing directly on the epitaxial LaNiO₃ electrode, on top of which the growth of either (110)-, (100)-, or (001)-oriented grains took place. Macroscopic as well as microscopic measurements of the ferroelectric properties of regions with pure c-orientation and of regions with mixed orientations showed a clear relationship between their ferroelectric properties and their morphology and crystallographic orientation. In the regions with mixed orientation, the films exhibited saturated ferroelectric hysteresis loops with well-defined remnant polarisation P_r and coercive field E_c. The regions having c-axis orientation with a smooth surface morphology in contrast exhibited a linear P-E curve with no hysteretic behaviour for SrBi₂Ta₂O₉ and BaBi₄Ti₄O₁₅ and a weak ferroelectric behaviour for Bi₄Ti₃O₁₂. This clearly showed that the ferroelectric properties of bismuth-layered ferroelectric oxides depended on the crystalline orientation of the film and that the observed ferroelectric hysteresis loops in SrBi₂Ta₂O₉ and BaBi₄Ti₄O₁₅ films were solely due to the (100)- and (110)-oriented grains. The size of the (110)- and (100)-oriented grains being of the order of 100 nm and spontaneous polarisation having been observed and switched in a controlled manner is a demonstration that ferroelectricity can exist in structures of submicrometer size. These results might have a technological impact due to the relevance of bismuth-layered ferroelectric oxides for the fabrication of non-volatile FeRAM memories.     

Electrical conductivity studies of mixed phthalocyanine thin films

Thin films of mixed of Copper Phthalocyanine (CuPc) and Nickel Phthalocyanine (NiPc) are deposited onto a pure glass substrate by a simultaneous thermal evaporation technique at room temperature. The material D.C. electrical conductivity of films at room temperature and also films annealed at 523 K has been investigated. The optical absorption and band gaps of the films are also measured. The results show that the electrical resistance is lower for the mixed films compared with the pure samples and also the optical band gap decreases for the mixed samples compared to the pure samples.     

MAPLE applications in studying organic thin films

Thin films of various organic materials have been created by the matrix assisted pulsed laser evaporation (MAPLE) technique. The principles, advantages, and difficulties of deposition are discussed. The focus is on target preparation, solvents, studied materials, and growth rate. Measured solvent transmissions and the results obtained are reported, and an overview of MAPLE applications is presented.     

New concept of failure of thin organic films

A critical (steady state) value of the thermal expansion coefficients of different coatings was determined by a nondestructive technique (NDT) known as laser shearography. The behavior of organic coatings, i.e., ACE premium-grey enamel, a yellow acrylic lacquer, and a gold nail polish on a metallic alloy, i.e., a carbon steel, was investigated over a temperature range of 20–60 °C. The value of the thermal expansion coefficients of coatings was derived from the slope of the plot of the thermal deformation (strain) versus the applied temperature. The integrity of the coatings with respect to time was assessed by comparison the measured coefficients of thermal expansion (CTE) to the critical (steady state) or asymptotic value of CTE. By shearography, measurement of coating properties could be performed independent of parameters such as UV exposure, humidity, presence of chemical species, and other parameters which may normally interfere with conventional methods of the assessing of the integrity of coatings. Therefore, one may measure CTE of coatings, regardless of the history of the coating, in order to assess the integrity of coatings. Also, the obtained shearography data were found to be in a reasonable trend with the data of electrochemical impedance spectroscopy (EIS) in 3%NaCl solution.     

Quantitative ferroelectric characterization of single submicron grains in Bi-layered perovskite thin films

The local polarization state and the electromechanical properties of ferroelectric thin films can be probed via the converse piezoelectric effect using scanning force microscopy (SFM) combined with a lock-in technique. This method, denominated as piezoresponse SFM, was used to characterize at the nanoscale level ferroelectric SrBi₂Ta₂O₉ and Bi₄Ti₃O₁₂ thin films, grown by pulsed laser deposition. Two types of samples were studied: polycrystalline films, with grains having random orientations, and epitaxial films, consisting of (100)_orth- or (110)_orth-oriented crystallites, 100 nm to 2 7m in lateral size, which are embedded into a (001)-oriented matrix. The ferroelectric domain structure was imaged and the piezoelectric response under different external conditions was locally measured for each type of sample. Different investigation procedures are described in order to study the ferroelectric properties via the electromechanical response. A distinct ferroelectric behavior was found for single grains of SrBi₂Ta₂O₉ as small as 200 nm in lateral size, as well as for 1.2 7m쏿 nm crystallites of Bi₄Ti₃O₁₂. By probing separately the crystallites and the matrix the investigations have demonstrated at the nanoscale level that SrBi₂Ta₂O₉ has no spontaneous polarization along its crystallographic c-axis, whereas Bi₄Ti₃O₁₂ exhibits a piezoelectric behavior along both the a- and c-directions. The electrostriction coefficients were estimated to be 3᎒^-2 m⁴/C² for polycrystalline SrBi₂Ta₂O₉ and 7.7᎒^-3 m⁴/C² for c-orientedBi₄Ti₃O₁₂. Quantitative measurements at the nanoscale level, within the experimental errors give the same values for remanent polarization and coercive field as macroscopic ferroelectric measurements performed on the same samples.     

Fabrication and electrical characterisation of Zr-substituted BaTiO3 thin films

Thin films of zirconium-substituted barium titanate were deposited by chemical solution deposition on platinum-coated silicon substrates at a temperature of 700 °C. The films showed a polycrystalline perovskite structure. The grain size was found to decrease with increase of Zr substitution. The effect of Zr substitution on the dielectric constant and the leakage was studied. It was found that with increasing Zr content the phase transition becomes diffuse and relaxor-like. The dielectric constant was also found to decrease with increasing amounts of Zr after an initial increase. The hysteresis loops became thinner and the remanent polarisation was found to decrease. The leakage behaviour was explained by Schottky theory. The barrier heights of the different films were calculated and found to vary between 1.12 eV and 1.19 eV. PACS 68.55.-a; 81.20.Fw; 77.84.Dy