Department of Chemical Engineering
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Item Investigation on the Dielectric and Polarization Behavior of Sol-Gel Derived Erbium Doped Pb(Zr 0.53 Ti 0.47 )O 3 Thin Films(Taylor & Francis, 2002) Roy, BanasriWe have studied the effect of Er 3+ doping on the dielectric and polarization hysteresis behavior of sol-gel derived Pb 1.05 (Zr 0.53 Ti 0.47 )O 3 thin films. Up to 1 at% Er doping, the dielectric constant of undoped PZT increases from 1245 to 1477 (measured at 50 kHz, 500 mV oscillation voltage), whereas the remnant polarization increases from 30 to 41 w C/cm 2 . Under the subswitching external field, the dielectric permittivities follow the Rayleigh law. The Rayleigh coefficient ( f ) was considered as a measure of the ease of domain wall motion. Up to 1 at% Er doping, the increase of f indicates minimal defect-domain wall interaction due to lower defect concentration. Under the switching field, the irreversible part of the switchable polarization was estimated from the C-V and hysteresis measurements. At the saturation field it was observed that up to 1 at% Er doping the irreversible component at switchable polarization increases and reversible component remains low. The improvements of dielectric and ferroelectric properties for up to 1 at% Er doping have been correlated to the relative site occupancy of Er 3+ as a function of Er content in PZT host lattice.Item Effect of neodymium (Nd) doping on the dielectric and ferroelectric characteristics of sol-gel derived lead zirconate titanate (53/47) thin films(AIP, 2001-01-12) Roy, BanasriThe results of the studies on the effect of rare earth Nd doping on the phase formation behavior and electrical properties of sol-gel derived Pb1.05(Zr0.53Ti0.47)O3 (PZT) thin films are presented. The perovskite phase is obtained up to 5 at. % doping and beyond that pyrochlore phase was found to coexist with the perovskite phase in all the films. The transition temperature of undoped lead zirconate titanate (PZT) film was found to be reduced with Nd doping. The Nd doped films also exhibited typical relaxor-type behavior and a diffuse phase transition, similar to that observed in relaxor materials. The introduction of Nd into the PZT lattice probably introduces disorder in the B site of ABO3 lattice, which causes the observed dielectric relaxation. Efforts were made to isolate the irreversible component contributions in low field dielectric and high field polarization switching behavior. REFERENCESItem Effect of acceptor and donor dopants on polarization components of lead zirconate titanate thin films(AIP, 2001-01-11) Roy, BanasriABSTRACT We have compared the magnitudes of reversible and irreversible polarization components of sol–gel-derived Nd3+- and Fe3+-doped PZT (53/47) thin films on platinized silicon substrates. Beyond the switching field, it was found that the reversible component of the polarization remains almost constant both for donor- (Nd3+) and acceptor- (Fe3+) doped PZT films. The irreversible polarization component reduces with the increase in Nd3+ content, whereas it increases until 3 at. % Fe3+-doped PZT thin films. The dielectric behavior of these films at subswitching fields was analyzed in terms of Rayleigh law. The inverse of the Raleigh coefficient (υ) was considered as a measure of the obstacle for the domain-wall motion. In the case of Fe-doped PZT, the inverse of the Raleigh coefficient (υ) shows a declining linearity with Fe content, which may be found exactly opposite to that observed for Nd-doped PZT. The observed results are explained in terms of the nature of the defect-domain-wall interaction of acceptor-and-donor-doped PZT thin films. Studies indicated that in the case of Fe-doped PZT films, the possible defect interaction initiates only above 2 at. % and it was found to be much lower in magnitude in comparison with the Nd doping.Item Effect of rare earth doping on sol-gel derived PZT thin films(Taylor & Francis, 2000-09-04) Roy, BanasriWe 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 behaviorItem Improvement of the degradation characteristics of sol-gel derived PZT (53/47) thin films : Effect of conventional and graded iron doping(Taylor & Francis, 2006-09-03) Roy, BanasriSeveral approaches are evident in the literature to improve the fatigue resistance and lower the leakage current densities of PZT thin films for nonvolatile random access memory (NVRAM) application. We have undertaken an approach of doping (conventional and graded) of PZT (53/47) thin films with Fe cation, which tend to occupy B-site in the AB03 unit cell. Our studies indicated several improvements in the electrical characteristics. The graded doped Fe film of similar dopant concentrations exhibited even better fatigue behavior up to -lo9 cycles. In addition, the Ohmic behavior in I-V characterization extended up to 250 kV/cm for graded doped films, while the non-linearity sets at much lower field (lSOkV/cm) for undoped films at room temperature. Conventional doping improved the electrical behavior as compared to undoped films. However, the graded doped films exhibited even further improved electrical behavior in terms of reducing leakage current by at least an order of magnitude under an applied electric fields of about 150 kV/cm.Item Raman scattering and x-ray diffraction studies of rare earth-doped Pb0.90La0.15TiO3 thin films prepared by the sol–gel method(Wiley, 2000-11-07) Roy, BanasriThe structural modification and phase transition in rare earth (Gd3+ and Ce3+)-doped Pb0.90La0.15TiO3 films, prepared by the sol–gel technique on Pt substrates, were studied utilizing x-ray diffraction and micro-Raman scattering techniques. Lattice parameters calculated from x-ray data indicate an increase in lattice tetragonality with increasing content of Gd and Ce in these films. Raman spectra exhibited features characteristic of bulk PbTiO3, including the observation of the soft mode. Variations in the phonon mode wavenumber, especially that of the lowest soft mode, for Pb0.90La0.15−xCexTiO3 (x = 0.00–0.07) and Pb0.90La0.15−xGdxTiO3 (x = 0.00–0.15) as a function of the composition x and temperature corroborate the increase in tetragonality caused by rare earth doping. As a result, the ferroelectric transition temperature was found to increase almost linearly from 480 to 630 K with increasing Gd content from x = 0.00 to 0.15. In Ce-doped films, the solubility limit was found at about x = 0.05; the transition temperature increases up to 5 doping then it decreases above that composition, probably owing to Ce precipitation.Item Phase transformations in sol-gel prepared PZT and PLT thin films upon isothermal treatments at different temperatures(Taylor & Francis, 1998-08-24) Roy, BanasriPhases formed upon isothermal treatments of sol-gel prepared PZT andPLT films at different temperatures have been determined. In PZT, the sequence of phase formation with increased temperature is found to be: Amorphous → Amorphous + Pyrochlore + PbO → Pyrochlore + PbO → Pyrochlore + PbO + Perovskite → PbO + Perovskite → Perovskite. In the PLT films no PbO phase is found to form and the region in which the pyrochlore phase forms is very limited. TIT diagrams for both the films are proposed.Item Non-vacuum and PLD growth of next generation TCO materials(Elsiever, 2003-12-15) Roy, BanasriTransparent conducting oxides represent a key component in a number of important opto-electronic technologies. The demand for improved materials is increasing. It is increasingly important to develop improved materials and process techniques. We report on the PLD growth of both Mo doped In-O, an n-type material with enhanced mobilities, and p-type Ca–CuInO2 films with improved conductivity. In both cases, very high quality films have been obtained with improved opto-electronic properties. We also report on a solution based route to p-type Cu2SrO2 which has produced phase pure films.Item Rapid thermal annealing of hot wire chemical-vapor-deposited a-Si:H films: The effect of the film hydrogen content on the crystallization kinetics, surface morphology, and grain growth(AIP, 2006-01) Roy, BanasriThe ability to crystallize thin amorphous Si layers into large grain Si can lead to significant improvements in Si solar cells and thin-film transistors. Here we report on the effect of the hydrogen content in as-grown films on the crystallization kinetics, surface morphology, and grain growth for hot wire chemical-vapor-deposited a-Si:H films crystallized by rapid thermal annealing (RTA). At RTA temperatures >750°C for high-hydrogen-content films, we observe the explosive evolution of hydrogen, with a resultant destruction of the film. Little or no damage is observed for films containing low hydrogen content. At a lower RTA temperature (600°C), the films remain intact with similar morphologies. At this same lower RTA temperature, both the incubation time and crystallization time decrease, and the grain size as measured by x-ray diffraction increases with decreasing hydrogen film content. Measurements of the crystallization time versus H evolution time indicate that the vast majority of the hydrogen must evolve from both films before crystallization commences. To examine the relationship between hydrogen evolution and crystallization, a two-step annealing process was utilized. For the high hydrogen content films, the final grain size increases if a large portion of the hydrogen is driven out at temperatures well below the crystallization temperatureItem Two-step annealing of hot wire chemical vapor deposited a-Si:H films(Springer, 2008) Roy, BanasriA two-step annealing process was used to investigate the effect of dehydrogenation on crystallization and grain growth of low and high hydrogen content hot wire chemical vapor deposited (HWCVD) a-Si:H films. A low temperature pre-annealing followed by a rapid thermal annealing step at 600 °C was carried out. For the high hydrogen content film XRD (111) peak narrowed quite a bit, while opposite effect was observed for the low hydrogen content film. According to the grain sizes as calculated from TEM images, grain sizes of both of the two-step annealed high and low hydrogen content films are smaller than that of the single stage annealed film.