S. Bhaskar

Large ferroelectric response in Bi4−xNdxTi3O12 films prepared by sol–gel process

Neodymium-substituted Bi4Ti3O12 (i.e., Bi4−xNdxTi3O12) were synthesized by sol–gel process for different compositions. Thin films were deposited on Pt (i.e., Pt/TiO2/SiO2/Si) substrate by spin coating. Materials were characterized by x-ray diffraction and Raman spectroscopy. This study indicates that the material makes a solid solution for the compositions: x=0.00, 0.26, 0.46, 0.75, 0.85, 1.00, and 2.00, where an Nd ion replaces the Bi site. The prominent effect of Nd substitution is observed in low-frequency Raman modes. Sol–gel derived thin films of Bi3.54Nd0.46Ti3O12 on a Pt substrate and postannealed at 700 ° C were tested for ferroelectric response which showed high remnant polarization (Pr=25 μC/cm2).

X-ray photoelectron spectroscopy and micro-Raman analysis of conductive RuO2 thin films

Ruthenium oxide (RuO2) was synthesized in thin film and powder forms using the solution chemistry technique. The oxide electrodes on Si substrates were characterized in terms of their structure, composition, stoichiometry, and conductivity. X-ray lattice parameter calculations and micro-Raman analysis revealed the rutile structure in the material. Both films and powders exhibited an unassigned Raman band at about 477 cm−1 in their Raman spectra. Performing peak frequency calculations for B2g and A1g modes of RuO2 using the rigid-ion model, which ruled out the possibility that this band originated from disorder induced symmetry, allowed silent mode. Based on the x-ray photoelectron spectroscopy (XPS) and temperature dependent Raman studies, this band was assigned to hydrated RuO2. XPS characterizations of our samples revealed minute surface contamination of oxygen and chlorine, probably due to the film preparation and high temperature deposition processes. Films with uniform microstructure, low surface rou...

Studies on the structural, microstructural and optical properties of sol–gel derived lead lanthanum titanate thin films

Abstract The La modified lead titanate Pb 1− x La x Ti 1− x /4 O 3 (PLT) ( x =0.0, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) thin films were deposited on sapphire (0001) substrates by the sol–gel process. The X-ray diffraction data and micro-Raman analysis show that with the increase in La content the crystal quality of the PLT films undergoes a tetragonal-to-cubic transformation. X-ray photoelectron spectroscopy analysis shows an excellent surface stoichiometry for all compositions under study. The optical properties of these films were investigated using both transmission and reflection spectra in the wavelength range of 200–900 nm. The appearance of interference fringes is an indication of the thickness uniformity of the film. The low value of extinction coefficient (in the order of 10 −2 ) as observed in our films is a qualitative indication of excellent surface smoothness of the films. Absorption coefficient ( α ) and the band-gap energy ( E g ) are obtained for undoped and La doped films with varying La concentrations. It has been found that the refractive index and packing fraction values decrease with La doping. Lanthanum doping was found to decrease the grain size of the films and improve the densification of individual grains. Increased La content lead to clusterification of smaller grains. The observed variation of band-gap energy with La doping has been correlated with the observed microstructure of these films.

Micro-Raman investigation of stress variations in lead titanate films on sapphire

Using the sol–gel method, PbTiO3 films of 21, 64, 128, 210, 310, and 420 nm thicknesses were obtained on (0001) sapphire. Raman scattering and x-ray diffraction techniques were used to study the correlation between the film thickness and the structural changes on these films. The Raman and x-ray intensities in 21 nm film were too weak to reveal any structural information, while all other films showed tetragonal structure. At room temperature, the variation of lowest E(1TO) phonon mode frequency with film thickness was observed due to compressive stresses in the films. The lattice parameters and the degree of a axis orientation values of tetragonal PbTiO3 have been evaluated as a function of film thickness. The changes in lattice parameters thus obtained were used to estimate the stress at each thickness. An excellent agreement was found between the stress values obtained using Raman and x-ray results. An exponential decrease in stress with increasing film thickness was observed because of the structural c...

Photoluminescence study of deep levels in Cr-doped ZnSe

Single crystals of intrinsic ZnSe were grown by the seeded physical vapor transport method and the diffusion doping was utilized to incorporate Cr in these crystals. The radiative recombinations in these samples with Cr concentration in the range 1.0–10.2×1019 cm−3 were studied by the steady state photoluminescence technique. It was found that the Cr deep centers inhibit the band-to-band emission in Cr-doped ZnSe. Except in undoped single crystals, no emission corresponding to the band-to-band transition was observed from any of the doped samples. Instead, the higher wavelength emissions associated with Cr deep levels were obtained. This capture of photoexcited carriers by deep centers was verified using different excitation wavelengths. The role of chromium impurities in nonradiative recombination processes was also evidenced from the sharp decreases in the deep level emission intensity with increasing Cr concentration.

Diffuse phase transition and relaxor behavior in (PbLa)TiO3 thin films

Room-temperature micro-Raman scattering and temperature-dependent dielectric measurements were carried out on sol-gel-derived ferroelectric Pb1−xLaxTi1−x/4O3 (PLT x=0.05–0.30) thin films. Results indicate that the crystal structure and the electrical properties of PLT films were strongly influenced by the La contents. The dielectric properties of PLT thin films were studied in the temperature range of 80–700 K and frequencies in the range of 1 kHz–1 MHz. Results show that PLT thin films undergo normal-to-relaxor ferroelectric transformation with 30 at. % La contents. The observed relaxor behavior is established in terms of diffuse phase transition characteristics and Vogel–Fulcher relationship.

Critical Issues in Sol-Gel Derived Ferroelectric Thin Films: A Review

In this article we have reviewed our recent research efforts on the synthesis and characterization of a wide spectrum of ferroelectric thin films for a variety of electronic and electro-optic devices including computer memories, tunable microwave devices, infra-red detectors, and electro-optic modulators. The review is separated into three sections; first we have described the process induced modifications of the properties of perovskite films prepared by sol-gel technique. The next section reviews the effect of certain impurity elements (e.g. La, Nd, Ce etc) in modifying the electrical characteristics of perovskite thin films followed by our experimental observations on the synthesis of epitaxially grown perovskite thin films by sol-gel technique. The epitaxial quality of BST thin films has been evaluated by fabricating eight element coupled microstrip phase shifters at NASA Glenn Research Center. Finally, future directions of sol-gel thin film research have been highlighted in view of these experimental observations.

Dielectric and ferroelectric response of sol–gel derived Pb0.85La0.15TiO3 ferroelectric thin films on different bottom electrodes

Sol-gel derived Pb0.85La0.15TiO3 thin films were deposited on Pt, Pt/Si, RuO2/Pt/Si and RuO2/Si bottom electrodes. The structural and microstructural characteristics of the films were studied using X-ray diffraction and atomic force microscopy techniques. Dielectric, ferroelectric and leakage current characteristics were evaluated and depth profile Auger electron spectroscopy was used to obtain direct evidence for reactivity and compositional changes at the film/electrode interface and determine their effect on the ferroelectric and dielectric properties of films. Films deposited on Pt electrode showed a relatively higher dielectric constant of approximately 1300, while the films on RuO2 exhibited lower dielectric constant of only 470. J-t characteristics with leak-age current of approximately 10(-8) A/cm(2) under low biasing field (10 kV/cm) was observed for the films under study. The steady state field dependent de conductivity was examined by the measurement of J-E characteristics. At very low fields (< 30 kV/cm) films followed ohmic behavior and was fitted with a space-charge-limited conduction mechanism in the intermediate fields (30-60 kV/cm). The on-set voltage for the non-linearity was considered as V-TFL using which, the trap concentration estimated for films on RuO2/Si electrode was 1.23 X 10(17) cm(-3). Observed current characteristics have been correlated with large interfacial resistance at the film-electrode boundary. In the case of RuO2 bottom electrodes, the dielectric and ferroelectric properties are correlated with the electrode characteristics and Si diffusion at the film-electrode interface.

Electrical properties of La-graded heterostructure of Pb1−xLaxTiO3 thin films

La-graded heterostructure films were prepared by sol-gel technique on platinum substrates and electrical properties of these films were compared with those of conventional thin films of similar compositions. X-ray diffraction results indicate the pure perovskite polycrystalline structure of these films. Atomic Force Microscopy analysis revealed a finer grain size and relatively lower surface roughness. Relatively higher values of Pm and Pr (69 and 38 ?C cm?2, respectively) and excellent dielectric properties with lower loss (K=1900, tan ?=0.035 at 100 kHz) were observed for La-graded heterostructure films. Also lower leakage current density (not, vert, similar2.5 nA cm?2) and a higher onset field (not, vert, similar50 kV cm?1) of space charge conduction indicated higher breakdown strength and good leakage current characteristics. The ac electric field dependence of the permittivity at sub-switching fields was analyzed in the framework of the Rayleigh dynamics of domain walls. The estimated irreversible domain wall displacement contribution to the total dielectric permittivity was 17 and 9% for conventional 15 at.% La doped PbTiO3 and La-graded heterostructure films, respectively. The improved dielectric and polarization behavior of La-graded heterostructure films may be attributed to homogenous dopant distribution compared to the conventional 15 at.% La doped PbTiO3 films.

Effect of rare earth doping on sol-gel derived PZT thin films

Abstract We have studied the effect of rare earth dopants (Nd, Gd and Ce) on the phase formation behavior and electrical properties of sol-gel derived Pb1.05(Zr0.53Ti0.47)O3 thin films. In all these films the perovskite phase is obtained up to 5 at% doping and beyond that pyrochlore phase was found to coexist with the perovskite phase. Ce and Gd doping(1-2 at%) exhibited improved ferroelectric and dielectric properties as compared to the undoped PZT films. Nd doping (2 at%) was found to be effective to increase the retained switchable polarization of undoped PZT from 63% to 84%. The transition temperature of undoped PZT film was found to be reduced with Nd doping. The Nd doped films also exhibited typical relaxor behavior and a diffuse phase transition, characteristic of the relaxor material. Introduction of Nd into the PZT lattice probably introduces disorder in the B site of ABO3 lattice which causes the observed relaxor behavior