O. Auciello

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 Ωu2009cm. Retention and imprint characteristics of these capacitors showed no degradation as a function of cumulative waiting times.

Nanoscale investigation of fatigue effects in Pb(Zr,Ti)O3 films

Scanning force microscopy has been used to perform a comparative nanoscale study of domain structures and switching behavior of Pb(ZrxTi1−x)O3 (PZT) thin films integrated into heterostructures with different electrodes. The study revealed a significant difference between polarization state of as‐deposited PZT films on RuO2 and Pt electrodes. The PZT/RuO2 films exhibit polydomain crystallites and show almost symmetric switching behavior, while the PZT/Pt films are mainly in a single polarity state and exhibit highly asymmetric piezoelectric hysteresis loops. Formation of unswitchable polarization within the grains of submicron size as a result of fatigue process was directly observed.

Scanning force microscopy for the study of domain structure in ferroelectric thin films

A piezoresponse technique based on scanning force microscopy (SFM) has been used for studying domain structure in ferroelectric thin films. Studies were performed on Pb(Zrx,Ti1−x)O3(PZT) thin films produced by a sol–gel method. The piezoresponse images of the PZT films were taken before and after inducing polarization in the films by applying a direct current voltage between the bottom electrode and the SFM tip. Polarization induced patterns were written with 20 V pulses and subsequently imaged by the SFM piezoresponse technique. The effect of the film structure on the imaging resolution of domains is discussed.

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.

ORIENTED FERROELECTRIC LA-SR-CO-O/PB-LA-ZR-TI-O/LA-SR-CO-O HETEROSTRUCTURES ON 001 PT/SIO2 SI SUBSTRATES USING A BISMUTH TITANATE TEMPLATE LAYER

We demonstrate the growth of c‐axis oriented La‐Sr‐Co‐O/Pb‐La‐Zr‐Ti‐O/La‐Sr‐Co‐O ferroelectric capacitor heterostructures on a highly [001] oriented Pt film which was grown on a thermally oxidized Si (SiO2/Si) substrate. The [001] growth orientation in the Pt film is induced by depositing it on a thin c‐axis oriented bismuth titanate template layer which is previously grown on the SiO2/Si substrate. The test capacitors show saturation and remnant polarization values comparable with that obtained on single‐crystal LaAlO3 substrates. The use of a Pt layer reduces the sheet resistance of the bottom electrode stack by at least 3–5 times. Since the Pt layer is strongly [001] oriented, it favors the growth of [001] La‐Sr‐Co‐O (LSCO) and lead zirconate titanate layers. The use of the LSCO top and bottom electrodes in contact with the ferroelectric thin film provides the resistance to bipolar fatigue, consistent with our earlier results.

Electrical properties of ferroelectric thin‐film capacitors with hybrid (Pt,RuO2) electrodes for nonvolatile memory applications

For Pb(ZrxTi1−x)O3 (PZT) thin‐film capacitors to be used in ferroelectric nonvolatile memories, they must have low polarization fatigue and low leakage currents. PZT capacitors fabricated in our laboratory with RuO2 electrodes exhibit excellent polarization fatigue characteristics, but they have large and variable leakage currents (typically 10−3–10−4 A/cm2 at 1 V). On the other hand, PZT capacitors with Pt electrodes have low leakage currents (typically <10−9 A/cm2 at 1 V), but they undergo severe polarization fatigue. New (Pt,RuO2) hybrid electrodes which result in PZT capacitors that combine the excellent fatigue behavior of RuO2/PZT/RuO2 with the low leakage currents of Pt/PZT/Pt capacitors have been developed. The hybrid electrodes studied are of two main types: one type consisted of Pt/RuO2 or RuO2/Pt double layers, while the other consisted of a codeposited Pt–RuO2 layer. All capacitors discussed here had an RuO2/PZT/hybrid electrode/MgO heterostructure. It will be shown that capacitors with neglig...

Influence of platinum interlayers on the electrical properties of RuO2/Pb(Zr0.53Ti0.47)O3/RuO2 capacitor heterostructures

Lead zirconate titanate, Pb(ZrxTi1−x)O3 or PZT, thin films grown on RuO2 electrodes by the sol‐gel process have excellent resistance to polarization fatigue, but they generally have two drawbacks. The films have high leakage currents and large property variation. In this letter we show that the use of a thin Pt layer (100 A) deposited on the bottom RuO2 electrode to fabricate RuO2/PZT/Pt/RuO2/(100)MgO capacitors has two important effects. It reduces capacitor leakage by two to four orders of magnitude and it significantly reduces the large property variation. In addition, these capacitors retain their excellent resistance to polarization fatigue which is characteristic of the RuO2/PZT/RuO2 heterostructure.

Contribution of electrodes and microstructures to the electrical properties of Pb(Zr0.53Ti0.47)O3 thin film capacitors

Pb(Zr 0.53 Ti 0.47 )O 3 (PZT) thin film capacitors have been fabricated with four electrode combinations: Pt /PZT/ Pt /SiO 2 Si, RuO 2 /PZT/ Pt /SiO 2 /Si, RuO 2 /PZT/ RuO 2 /SiO 2 /Si, and Pt /PZT/ RuO 2 /SiO 2 /Si. It is shown that polarization fatigue is determined largely by the electrode type (Pt vs RuO 2 ), and microstructure has only a second-order effect on fatigue. If either the top or bottom electrode is platinum, significant polarization fatigue occurs. Fatigue-free capacitors are obtained only when both electrodes are RuO 2 . In contrast, the bottom electrode is found to have a major effect on the leakage characteristics of the PZT capacitors, presumably via microstructural modifications. Capacitors with bottom RuO 2 electrodes show high leakage currents ( J = 10 −3 -10 −5 A/cm 2 at 1 V) irrespective of the top electrode material. Capacitors with Pt bottom electrodes have much lower leakage currents ( J = 10 −8 A/cm 2 at 1 V) irrespective of the top electrode material. At low voltage, the I-V curves show ohmic behavior and negligible polarity dependence for all capacitor types. At higher voltages, the leakage current is probably Schottky emission controlled for the capacitors with Pt bottom electrodes.

Control of structure and electrical properties of lead‐zirconium‐titanate‐based ferroelectric capacitors produced using a layer‐by‐layer ion beam sputter‐deposition technique

It is demonstrated that the structure and electrical properties of Pb(ZrxTi1−x)O3(PZT)‐based capacitors, involving RuO2 electrodes, can be controlled by depositing intermediate PbTiO3 (PT) layers at the PZT/electrode interfaces. Three different PZT‐based capacitor systems were studied, namely: (a) RuO2/PZT/RuO2/(100)MgO, (b) RuO2/PZT/PT/RuO2/(100)MgO, and (c) RuO2/PT/PZT/PT/RuO2/(100)MgO. Electrical characterization of capacitor (a), without a PT layer, shows about 71% reduction in remanent polarization, while those with one (b) or two (c) PT layers present only 34% and 29% decrease in remanent polarization, after 1010 switching cycles. Electrical conduction measurements (current density versus time) have shown that ion beam sputter‐deposited PZT‐based capacitors with or without PT layers present about an order of magnitude (∼6×10−9 A/cm2, at 100 s) smaller dc leakage current, for comparable physical and electrical parameters, than dc leakage characteristic of PZT sol‐gel‐based capacitors with RuO2 electr...

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...