Preparation, structural and optical characterization of BaWO4 and PbWO4 thin films prepared by a chemical route

Polycrystalline BaWO4 and PbWO4 thin films having a tetragonal scheelite structure were prepared at different temperatures. Soluble precursors such as barium carbonate, lead acetate trihydrate and tungstic acid, as starting materials, were mixed in aqueous solution. The thin films were deposited on silicon, platinum-coated silicon and quartz substrates by means of the spinning technique. The surface morphology and crystal structure of the thin films were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction, and specular reflectance infrared Fourier transform spectroscopy, respectively. Nucleation stages and surface morphology evolution of thin films on silicon substrates have been studied by atomic force microscopy. XRD characterization of these films showed that BaWO4 and PbWO4 phase crystallize at 500 degreesC from an inorganic amorphous phase. FTIR spectra revealed the complete decomposition of the organic ligands at 500 degreesC and the appearance of two sharp and intense bands between 1000 and 600 cm(-1) assigned to vibrations of the antisymmetric stretches resulting from the high crystallinity of both thin films. The optical properties were also studied. It was found that BaWO4 and PbWO4 thin films have Eg = 5.78 eV and 4.20 eV, respectively, of a direct transition nature. The excellent microstructural quality and chemical homogeneity results confirmed that soft solution processing provides an inexpensive and environmentally friendly route for the preparation of BaWO4 and PbWO4 thin films. (C) 2003 Elsevier Ltd. All rights reserved.

Electroluminescence of zinc oxide thin-films prepared via polymeric precursor and via sol-gel methods

Zinc oxide (ZnO) is an electroluminescent (EL) material that can emit light in different regions of electromagnetic spectrum when electrically excited. Since ZnO is chemically stable, inexpensive and environmentally friendly material, its EL property can be useful to construct solid-state lamps for illumination or as UV emitter. We present here two wet chemical methods to prepare ZnO thin-films: the Pechini method and the sol-gel method, with both methods resulting in crystalline and transparent films with transmittance > 85% at 550 nm. These films were used to make thin-film electroluminescent devices (TFELD) using two different insulator layers: lithium fluoride (LiF) or silica (SiO2). All the devices exhibit at least two wide emission bands in the visible range centered at 420 nm and at 380 nm attributed to the electronic defects in the ZnO optical band gap. Besides these two bands, the device using SiO2 and ZnO film obtained via sol-gel exhibits an additional band in the UV range centered at 350 nm which can be attributed to excitonic emission. These emission bands of ZnO can transfer their energy when a proper dopant is present. For the devices produced the voltage-current characteristics were measured in a specific range of applied voltage. (C) 2007 Elsevier B.V. All rights reserved.

Influence of ambient gas on the growth and properties of porous tin-doped indium oxide thin films made by pulsed laser deposition

The laser ablation method was used for depositing porous nanocrystalline indium-tin oxide thin films for gas sensing applications. Samples were prepared at different pressures using three gases (O-2, 0.8N(2):0.2O(2), N-2) and heat-treated in the same atmosphere used for the ablation process. X-ray diffraction results show that the films are not oriented and the grain sizes are in the range between 15 and 40 nm. The grains are round shaped for all samples and the porosity of the films increases with the deposition pressure. The degree of sintering after heat treatment increases for lower oxygen concentrations, generating fractures on the surface of the samples. Film thicknesses are in the range of I pm for all gases as determined from scanning electron microscopy cross-sections. Electrical resistance varies between 36.3 ohm for the film made at 10 Pa pressure in N-2 until 9.35 x 10(7) ohm for the film made at 100 Pa in O-2. (C) 2007 Elsevier B.V. All rights reserved.

Electrical conduction mechanism and phase transition studies using dielectric properties and Raman spectroscopy in ferroelectric Pb0.76Ca0.24TiO3 thin films

We have studied the phase transition behavior of Pb0.76Ca0.24TiO3 thin films using Raman scattering and dielectric measurement techniques. We also have studied the leakage current conduction mechanism as a function of temperature for these thin films on platinized silicon substrates. A Pb0.76Ca0.24TiO3 thin film was prepared using a soft chemical process, called the polymeric precursor method. The results showed that the dependence of the dielectric constant upon the frequency does not reveal any relaxor behavior. However, a diffuse character-type phase transition was observed upon transformation from a cubic paraelectric phase to a tetragonal ferroelectric phase. The temperature dependency of Raman scattering spectra was investigated through the ferroelectric phase transition. The soft mode showed a marked dependence on temperature and its disappearance at about 598 K. on the other hand, Raman modes persist above the tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive above the phase transition temperature. The origin of these modes must be interpreted in terms of a local breakdown of cubic symmetry by some kind of disorder. The lack of a well-defined transition temperature suggested a diffuse-type phase transition. This result corroborate the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in the thin film. The leakage current density of the PCT24 thin film was studied at elevated temperatures, and the data were well fitted by the Schottky emission model. The Schottky barrier height of the PCT24 thin film was estimated to be 1.49 eV. (C) 2003 American Institute of Physics.

Effect of thermal treatment temperature on the crystallinity and morphology of LiTaO3 thin films prepared from polymeric precursor method

Lithium tantalate thin films (LiTaO3) with (50:50) stoichiometry were prepared by spin coating method using a polymeric organic solution. The films were deposited on silicon (100) substrates with 4 layers. The substrates were previously cleaned and then the solution of lithium tantalate was deposited by adjusting the speed at 5000 rpm. The thin films deposited were thermally treated from 350 to 600degreesC for 3 hours in order to study the influence of the thermal treatment temperature on the crystallinity, microstructure, grain size and roughness of the final film. X-ray diffraction (XRD) results showed that the films are polycrystalline and secondary phases free. The thickness of films was observed by scanning electron microscopy (SEM). The atomic force microscopy (AFM) studies showed that the grain size and roughness are strongly influenced by thermal treatment.

Theoretical and experimental study of the relation between photoluminescence and structural disorder in barium and strontium titanate thin films

Thin films of barium and strontium titanate (BST), synthesized by the polymeric precursor solution and spin coated on [Pt (140nm)/Ti (10 nM)/SiO2(1000 nm)/Si] substrates were found to be photoluminescent at room temperature when heat treated below 973 K, i.e. before their crystallization. First principles quantum mechanical techniques, based on density functional theory (DFT) were employed to study the electronic structure of two periodic models: one is standing for the crystalline BST thin film and the other one for the structurally disordered thin film. The aim is to compare the photoluminescence (PL) spectra of the crystalline and disordered thin films with their UV-vis spectra and with their computed electronic structures. The calculations show that new localized states are created inside the band gap of the crystalline model, as predicted by the UV-vis spectra. The study of the charge repartition in the structure before and after deformation of the periodic model shows that a charge gradient appears among the titanate clusters. This charge gradient, together with the new localized levels, gives favorable conditions for the trapping of holes and electrons in the structure, and thus to a radiative recombination process. Our models are not only consistent with the experimental data, they also allow to explain the relations between structural disorder and photoluminescence at room temperature. (c) 2005 Elsevier Ltd. All rights reserved.

Influence of temperature on the microstructure and electrical properties of BBT thin films

The BBT films were prepared by a spin-coating process from the polymeric precursor method (Pechini process). In order to study the influence of the temperature on the BBT microstructure and electrical properties, the films were deposited on platinum coated silicon substrates and annealed from 700degreesC to 800degreesC for 2 hours in oxygen atmosphere. The crystallinity of the films was examined by X-ray diffraction while the surface morphology was analysed by atomic force microscope. The dielectric properties and dissipation factor of BaBi2Ta2O9 films at 1 MHz were observed. The polarization-electric field hysteresis loops revealed the ferroelectric characteristics of BaBi2Ta2O9 thin films.

Transparent and conductive ZnO : Al thin films prepared by sol-gel dip-coating

Aluminum doped zinc oxide polycrystalline thin films (AZO) were prepared by sol-gel dip-coating process. The sol was prepared from an ethanolic solution of zinc acetate using lithium hydroxide or succinic acid as hydrolytic catalyst. The quantity of aluminum in the sol was varied from 1 to 10 mol%. The structural characteristics studied by X-ray diffractometry were complemented by resistivity measurements and UV-Vis-NIR spectroscopy. The films are transparent from the near ultraviolet to the near infrared, presenting an absorption cut-off at almost 290 nm, irrespective of the nature of the catalyst and doping level. The best conductors were obtained for the AZO films containing 3 mol% of aluminum, prepared under acidic and basic catalysis and sintered at 450 degreesC. Their optical band-gap of 4.4 eV calculated from the absorption cut-off is larger than the values for band-gap widening predicted by the standard model for polar semiconductors. These polycrystalline films are textured with preferential orientation of grains along the wurtzite c-axis or the (100) direction. (C) 2003 Elsevier Ltd. All rights reserved.

Growth and characterization of BaBi2Nb2O9 thin films made by RF-magnetron sputtering

The RF-magnetron sputtering technique has been used to deposit polycrystalline thin films of layered-structured ferroelectric BaBi2Nb2O9 (BBN). The XRD patterns for the films annealed at 700degreesC for 1 hour show the presence of the BBN phase as well as the BaNb2O6 secondary phase. A better crystallization of the BBN phase and an inhibition of the secondary phase is obtained with the increase of temperature. The surface of the prepared films was rather dense and smooth with no cracks. The 300 nm thick BBN thin films exhibited a room-temperature dielectric constant of about 779 with a dissipation factor of 0.09 at a frequency of 100 kHz.

A Raman and dielectric study of a diffuse phase transition in (Pb1-xCax)TiO3 thin films

Dielectric and Raman scattering experiments were performed on polycrystalline Pb1-xCaxTiO3 thin films (x=0.10, 0.20, 0.30, and 0.40) as a function of temperature. The results showed no shift in the dielectric constant (K) maxima, a broadening with frequency, and a linear dependence of the transition temperature on increasing Ca2+ content. on the other hand, a diffuse-type phase transition was observed upon transforming from the cubic paraelectric to the tetragonal ferroelectric phase in all thin films. The temperature dependence of Raman scattering spectra was investigated through the ferroelectric phase transition. The temperature dependence of the phonon frequencies was used to characterize the phase transitions. Raman modes persisted above the tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of a breakdown of the local cubic symmetry due to chemical disorder. The lack of a well-defined transition temperature and the presence of broad bands in some temperature interval above the FE-PE phase transition temperature suggested a diffuse-type phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in these thin films.

Ferroelectric properties and leakage current characteristics of Bi3.25La0.75Ti3O12 thin films prepared by the polymeric precursor method

The ferroelectric properties and leakage current mechanisms of preferred oriented Bi3.25La0.75Ti3O12 (BLT) thin films deposited on La0.5Sr0.5CoO3 by the polymeric precursor method were investigated. These films showed excellent ferroelectric properties in terms of large remnant polarization (2P(r)) of 47.6 mu C/cm(2) and (2E(c)) of 55 kV/cm, fatigue-free characteristics up to 10(10) switching cycles, and a current density of 0.7 mu A/cm(2) at 10 kV/cm. X-ray diffraction and scanning electron microscope investigations indicate that the deposited films exhibit a dense, well-crystallized microstructure having random orientations and with a rather smooth surface morphology. The improved ferroelectric and leakage current characteristics can be ascribed to the platelike grains of the BLT films, which make the domain walls easier to be switched under external field.

Synthesis and electrical characterization of CaBi2Nb2O9 thin films deposited on Pt/Ti/SiO2/Si substrates by polymeric precursor method

We report the successful deposition of CaBi2Nb2O9 (CBN) thin films on platinum coated silicon substrates by polymeric precursor method. The CBN thin films exhibited good structural, dielectric and CBN/Pt interface characteristics. The leakage current of the capacitor structure was around 0.15 A cm(-2) at an applied electric field of 30 kV cm(-1). The capacitance-voltage measurements indicated good ferroelectric polarization switching characteristics. The typical measured small signal dielectric constant and the dissipation factor at a frequency of 100 kHz were 90 and 0.053, respectively. The remanent polarization and the drive voltage values were 4.2 C cm(-2) and 1.7 V at an applied voltage of 10 V. No significant fatigue was observed at least up to 10(8) switching cycles. (c) 2005 Elsevier B.V. All rights reserved.

Low temperature synthesis and electrical properties of PbTiO3 thin films prepared by the polymeric precursor method

Ferroelectric PbTiO3 thin films were successfully prepared on a Pt(111)Ti/SiO2/Si(100) substrate for the first time by spin coating, using the polymeric precursor method. X-ray diffraction patterns of the films indicate that they are polycrystalline in nature. This method allows low temperature (500 degrees C) synthesis and high electrical properties. The multilayer PbTiO3 thin films were granular in structure with a grain size of approximately 110-120 nm. A 380-nm-thick film was obtained by carrying out four cycles of the spin-coating/heating process. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses showed the surface of these thin films to be smooth, dense and crack-free with low surface roughness (=3.4 nm). At room temperature and at a frequency of 100 kHz, the dielectric constant and the dissipation factor were, respectively, 570 and 0.016. The C-V characteristics of perovskite thin film prepared at low temperature show normal ferrolectric behavior. The remanent polarization and coercive field for the films deposited were 13.62 mu C/cm(2) and 121.43 kV/cm, respectively. The high electrical property values are attributed to the excellent microstrutural quality and chemical homogeneity of thin films obtained by the polymeric precursor method. (C) 2000 Elsevier B.V. S.A. All rights reserved.

Preparation, microstructural and electrical characterization of SrTiO3 thin films prepared by chemical route

Polycrystalline SrTiO3 thin films having a cubic perovskite structure were prepared at different temperatures by the polymeric precursor method on platinum-coated silicon substrate. Crystalline films with uniform composition and thickness were prepared by spin-coating and the post-deposition heat treatment was carried out at different temperatures. The film showed good structural, dielectric, and insulating properties, Scanning electron microscopy (SEM) micrographs showed no occurrence of interdiffusion between the bottom electrode (platinum) and the film during post-annealing, indicating a stable interface between the SrTiO3 and the bottom electrode. The dielectric constant and dissipation factor at a frequency of 100 kHz were 250 and 0.01, respectively, for a 360 nm thick film annealed at 600 degreesC. The capacitance versus applied voltage characteristics showed that the capacitance was almost independent of the applied voltage. The I-V characteristics were ohmic in low fields and a Schottky emission and/or Poole-Frenkel emission were postulated in high fields. Room temperature leakage current density was found to be in the order of 10(-7) A/cm(2) for a 360 nm thick film in an applied electric field of about 100 kV/cm. The charge storage density of 36 fC/mum(2) was obtained in an applied electric field of about 100 kV/cm. (C) 2001 Published by Elsevier B.V. Ltd. All rights reserved.