R. Dat

Pulsed laser ablation synthesis and characterization of layered Pt/SrBi2Ta2O9/Pt ferroelectric capacitors with practically no polarization fatigue

Pulsed laser ablation deposition was used to synthesize polycrystalline SrBi2Ta2O9 layered ferroelectric thin films on platinized silicon substrates. Top electrodes were produced by dc magnetron sputter deposition to fabricate capacitors for electrical tests. The polarization electric field hysteresis loops showed saturation in the 2–4 V range with a coercive field of 25 kV/cm. The capacitors showed practically no polarization fatigue up to 1011 switching cycles. The resistivity of the SrBi2Ta2O9 for a coercive field of 100 kV/cm was approximately 2×1011 Ω cm. Retention and imprint characteristics of these capacitors showed no degradation as a function of cumulative waiting times.

Polycrystalline La0.5Sr0.5CoO3/PbZr0.53Ti0.47O3/ La0.5Sr0.5CoO3 ferroelectric capacitors on platinized silicon with no polarization fatigue

Pulsed laser ablation‐deposition is used to produce fatigue‐free La0.5Sr0.5CoO3(LSC)/ PbZr0.53Ti0.47O3 (PZT)/LSC ferroelectric capacitors on oxidized (100) Si substrates coated with a bilayer of Pt/Ti. These capacitors utilize a unique bottom electrode combination of LSC on Pt, where the LSC (a conducting oxide) acts as a template to promote the ferroelectric perovskite phase of PZT and to minimize polarization fatigue, while Pt is used for its high electrical conductivity and high temperature stability. We have used the hybrid Pt/LSC electrode discussed in this letter to integrate PZT‐based capacitors with Si substrates. X‐ray diffraction analysis shows that the PZT film is polycrystalline and is entirely perovskite phase. Devices show no significant degradation of the switchable polarization after 3×1010 switching cycles. Aging tests show that the rate of loss of switchable polarization may allow useful memory retention for times up to 1010 s.

Investigation of the ablated flux characteristics during pulsed laser ablation deposition of multicomponent oxides

The ablated flux characteristics of PbZr0.52Ti0.48O3 (PZT), La0.5Sr0.5CoO3 (LSC), and MgO ceramic targets have been studied as functions of the ablation time, the ablation energy, and the chamber gas pressure. The time dependence of the ablation rate shows an initial exponential decay, reaching a steady‐state value at longer times. The energy dependence of the ablation rate (in vacuum) reveals a distinct ablation threshold energy for MgO ablation, while for PZT and LSC no ablation threshold is evident. The differences in the ablation characteristics of these materials are explained mainly by differences in their melting points, thermal conductivities, and absorption coefficients. Upon adding O2 gas, a visual change in the color and shape of the PZT ablation plume is evident. The color change indicates a gas phase reaction of the ablated species with the O2 gas, while the shape change implies a change in the angular distribution of the ablated species. We have measured a narrowing of the ablated flux distr...

Effect of electrodes on the ferroelectric properties of pulsed-laser ablation-deposited PbZrXTi1-xO3 thin film capacitors

Abstract PbZrXTi1-xO3 (PZT) thin films have been deposited on a variety of electrode and substrate materials using pulsed-laser ablation-deposition (PLAD). Growth of PZT films on epitaxial (100)Pt/MgO, fiber-textured (111)Pt/Ti/SiO2/Si, epitaxial (110)RuO2/MgO2 or polycrystalline RuO2/SiO2/Si resulted in mixtures of perovskite and pyrochlore phases. Under identical growth conditions, films deposited on epitaxial (001)YBa2Cu3O7−δ/MgO and (100)La0.5CoO3/MgO, or polycrystalline La0.5Sr0.5CoO3/MgO/Si, contain only the ferroelectric, perovskite phase. The YBa2Cu3O7–δ (YBCO) and La0.5Sr0.5CoO3 (LSC) surfaces stabilize the nucleation of the ferroelectric phase under the PLAD growth conditions. Capacitors with sputter-deposited Pt top electrodes all show significant polarization fatigue by 108 switching cycles, although the (001) PZT films on (100)LSC/MgO and (001)YBCO/MgO show the least degradation. By using PLAD to deposit both top and bottom (100) LSC electrodes, we have obtained LSC/PZT/LSC capacitors on (100...

A review of composition-structure-property relationships for PZT-based heterostructure capacitors

Abstract Studies performed by our group on composition-structure-property relationships of Pb(ZrxTi1−x)O3 (PZT)-based heterostructure capacitors are reviewed. The work discussed is related to the synthesis and characterization of ferroelectric PZT and conductive Pt, RuO2, and La0.5Sr0.5CoO3 layers and their integration into heterostructure capacitors suitable for non-volatile memories. The main objective of our research was to determine the influence of deposition parameters and layer processing on the composition, structure, and properties of PZT-based capacitors, with the goal of controlling fatigue, retention, and imprint effects. The work discussed relates mainly to the synthesis of films by ion beam sputter-deposition (IBSD) and pulsed laser ablation deposition (PLAD), where the heterostructures are grown in-situ without exposing the interfaces to uncontrollable atmospheric conditions. Limited comparisons are presented between structural characteristics and properties of PZT capacitors produced by IB...

Imprint testing of ferroelectric capacitors used for non-volatile memories

Abstract A set of criteria is presented, based on the polarization versus electric field hysteresis characteristics, which can be used to reveal a tendency towards imprint failure in ferroelectric capacitors. A series of pulsed voltage waveforms, generated by the RT66A ferroelectric tester, are utilized to validate our established criteria for the tendency towards imprinting. The tests have been performed on hetero-structure Pb(ZrxTi1−x)O3(PZT)-based capacitors produced by a pulsed laser ablation depositon technique (PLAD). The loss of retained charge in the ferroelectric capacitors is considered in light of the imprint test methodology. Rates of retention loss and imprint are extracted from the experimental data and treated as separate processes that lead to device failure. The rate of approach towards imprint failure is found to be influenced by the magnitude of the read pulse.

Pulsed laser ablation-deposition of La0.5Sr0.5CoO3 for use as electrodes in nonvolatile ferroelectric memories

La{sub 0.5}Sr{sub 0.5}CoO{sub 3} (LSCO) thin films have been deposited on (100) MgO substrates using pulsed laser ablation-deposition (PLAD). The crystallographic orientation of LSCO was found to be dependent on the surface treatment of (100) MgO prior to deposition. PLAD deposition parameters were optimized to yield LSCO films with an RMS surface roughness of 40{endash}50 A. A smooth surface morphology was reproduced as long as the oxygen content of the LSCO target was preserved. Otherwise, {open_quote}{open_quote}splashing{close_quote}{close_quote} occurred which resulted in the transfer of condensed particles from molten spherical globules of LSCO from the target to the substrate. Splashing was subsequently eliminated and smooth surface quality was restored after annealing the LSCO target at 550{degree}C in oxygen for 3 h. Optical emission spectroscopy (OES) of the LSCO{close_quote}s plume identified excited atomic cobalt neutrals, excited singly ionized strontium and lanthanum, and excited molecular LaO species. Oxygen interaction with the plume produced no new species. Furthermore, the OES data suggest that the observed LaO molecules were not created by the chemical reaction between La and O{sub 2} during ablation, but were ejected directly from the target during the PLAD process. {copyright} {ital 1996 Materials Research Society.}

Lead zirconate titanate ferroelectric capacitors produced on sapphire and gallium arsenide substrates

Abstract Pb(ZrxTi1−x)O3 (PZT) based ferroelectric capacitors have been produced at 600°C on R-cut sapphire and Si-doped (100) GaAs substrates using a pulsed laser ablation deposition (PLAD) technique. La0.5Sr0.5CoO3 (LSCO) conducting electrodes deposited using PLAD serve as top and bottom electrodes. X-ray diffraction results show that the PZT film is polycrystalline and phase-pure in both cases. Electrical characterization of the films show remanant polarization in excess of 20 μC/cm2. Results of long term properties show that these capacitors: are very tolerant to extremely large numbers of switching cycles; retain charges over very long periods of time; and do not show a strong tendency for their dipoles to be imprinted in a preferred direction.

Surface roughness of PbZrxTi1−xO3 thin films produced by pulsed laser ablation-deposition

Abstract The surface roughness of PbZr x Ti 1− x O 3 (PZT) produced by pulsed laser ablation-deposition (PLAD) was evaluated as a function of the deposition temperature, oxygen pressure, laser energy density and target-to-substrate distance. Quantitative and qualitative results were obtained by scanning the surface of PZT with an atomic force microscope. The normalized root-mean-square roughness decreases as the magnitudes of the four process variables are increased. The improvement in the surface roughness with an increase in all the process parameters, except the laser energy density, can be explained qualitatively in terms of the particle energetics required for surface activation to occur. The decrease in roughness with increasing laser energy density implies that partially dissociated ejecta are completely vaporized as the laser energy density is increased.

Effects of temperature on the ferroelectric properties of PbZr0.53Ti0.53O3-based capacitors prepared by pulsed laser-ablation deposition

Abstract The effects of elevated temperature on the long and short term ferroelectric properties of La0.5Sr0.5Co3(LSCO)/PbZr0.53 Ti0.47O3(PZT)/La0.5Co3(LSCO) heterostructure capacitors are presented in this paper. Capacitors are evaluated in the temperature range of 25°C to 100°C. Polarization values (switched and non-switched), coercive voltages, and PZT resistivity are measured as a function of temperature. Rate of fatigue, tendency to imprint, and retention loss are also investigated in the 25–100°C temperature range. An activation energy for the process responsible for loss of remanent polarization is determined from the fatigue data.