M. V. Raymond

Voltage shifts and imprint in ferroelectric capacitors

Voltage offsets in the polarization‐voltage characteristics of Pb(Zr,Ti)O3 capacitors can be induced by either thermal or optical processes. The thermally (optically) induced voltage shift occurs by heating (illuminating) the sample under remanence or a saturating bias. Generally speaking, the thermally induced voltage shifts are greater than those obtained optically; this is attributed to the role of oxygen vacancy‐related defect dipoles throughout the film. We find that the inclusion of a dopant element that occupies a portion of the Ti(Zr) sites and has an oxidation state greater than +4 reduces the thermally induced voltage shifts observed in the capacitors. This may result because these particular dopants reduce the oxygen vacancy density and, hence, the defect‐dipole contribution to the voltage shift.

Tunability and Calculation of the Dielectric Constant of Capacitor Structures with Interdigital Electrodes

The voltage dependence of the dielectric constant of ferroelectricmaterials makes them attractive for use as tuning elements in microwavecircuits. In this study, capacitance tuning and loss measurements wereperformed on ferroelectric materials prepared by the sol-gel process and RFmagnetron sputtering. We find that Pb(Zr,Ti)O3 (PZT) thinfilms with interdigital electrodes can be fabricated with reasonably lowloss to make them useful for room temperature tuning applications. Inaddition, it is found that high temperature post-deposition annealing ofboth sputtered SrTiO3 (ST) and sol-gel derivedBaTiO3 (BT) films markedly improves their tuning and lossfactor characteristics. By annealing the samples in the range of1000–1100°C, the tunability was increased by as much as a factorof seven, while the dissipation factors were decreased to values of0.3–0.5%. In addition, it is shown that the permittivity of thefilms in these interdigitated capacitor structures can be calculated usingan analytical model previously described by Farnell et al. [1].

Hysteresis relaxation in (Pb,La)(Zr,Ti)O3 thin film capacitors with (La,Sr)CoO3 electrodes

We report on the thermally activated hysteresis relaxation effects in (La,Sr)CoO3/ (Pb,La)(Zr,Ti)O3/(La,Sr)CoO3 thin film ferroelectric capacitors. Films cooled in oxygen deficient ambients exhibit a marked voltage offset in the hysteresis loops. Upon the application of a dc bias voltage or unidirectional pulses of the same polarity as the offset, the loops become more symmetric. Subsequently, holding the capacitors in the original preferred polarization state leads to a relaxation of the hysteresis loop towards its original voltage offset condition. The relaxation process is described by a stretched exponential and is thermally activated.

Low temperature processing of Nb‐doped Pb(Zr,Ti)O3 capacitors with La0.5Sr0.5CoO3 electrodes

The effect of crystallization temperature on the electrical properties of sol‐gel derived Pb(Zr,Ti,Nb)O3 or PNZT capacitors with La0.5Sr0.5CoO3 (LSCO) electrodes has been investigated. It is demonstrated that LSCO//PNZT(4/30/70)//LSCO capacitors can be fabricated at temperatures as low as 550 °C without significant degradation in their ferroelectric and dielectric properties. Lowering the process temperature to 500 °C resulted in substantial degradation in capacitor properties. Nonetheless, all capacitors processed in the 500 °C to 675 °C range exhibited essentially no fatigue up to 5×109 switching cycles. The low temperature processing is significant as it indicates that this ferroelectric capacitor technology is compatible with high density nonvolatile memory architectures. In other words, these process temperatures are low enough to maintain plug integrity and to prevent degradation of the underlying CMOS circuitry in a high density ferroelectric memory.

La 0.5 Sr 0.5 CoO 3 electrode technology for Pb(Zr,Ti)O 3 thin film nonvolatile memories

Abstract Oxide electrode technology is investigated for optimization of Pb(Zr,Ti)O 3 (PZT) thin film capacitor properties for high density nonvolatile memory applications. PZT thin film capacitors with RF sputter deposited La 0.5 Sr 0.5 CoO 3 (LSCO) electrodes have been characterized with respect to the following parameters: initial dielectric hysteresis loop characteristics, fatigue performance, microstructure and imprint behavior. Our studies have determined that the fatigue of PZT capacitors with LSCO electrodes is less sensitive to B site cation ratio and underlying electrode stack technology than with RuO 2 electrodes. Doping PZT thin films with Nb (PNZT) improves imprint behavior of LSCO//PZT//LSCO capacitors considerably. We have demonstrated that PNZT 4/30/70 // LSCO capacitors thermally processed at either 550°C or 675°C have almost identical initial hysteresis properties and exhibit essentially no fatigue out to approximately 10 10 cycles.

Sputter deposition of SrTiO3 thin films for voltage tunable capacitors

Abstract The electric field dependence of the dielectric constant of ferroelectric and related materials can be used to fabricate voltage tunable filters for microwave applications. In this study, the growth, structure, and dielectric properties of sputter deposited thin films will be discussed. In particular, SrTiO3 thin films were deposited onto (100)pc LaAlO3 and r-cut sapphire substrates using RF magnetron sputtering at elevated temperatures. The films deposited on LaAlO3 were highly (00ι) oriented possessing a cube-on-cube heteroepitaxial structure, while the films deposited directly on sapphire were polycrystalline. We have studied the influence of deposition conditions and post deposition annealing on the crystal quality, microstructure, and electrical properties. We find that post deposition annealing can significantly improve the tunability of heteroepitaxial films. The magnitude and tunability of the dielectric constant will be correlated with film crystalline quality and microstructure.

Mechanism(s) for the suppression of the switchable polarization in PZT and BaTiO{sub 3}

Switchable polarization can be significantly suppressed in ferroelectric (FE) materials by optical, thermal, and electrical processes. The thermal process can occur by either annealing the FE in a reducing environment or by heating it in air to 100 C while impressing a bias near the switching threshold. The optical process occurs while biasing the FE near the switching threshold and illuminating with bandgap light. And the electrical suppression effect occurs by subjecting the FE to repeated polarization reversals. Using electron paramagnetic resonance, polarization-voltage measurements, and charge injection scenarios, the authors have been able to elucidate both electronic and ionic trapping effects that lead to a suppression in the amount of switchable polarization in FE materials. The relative roles of electronic and ionic effects in the same material can depend on the stress condition. For instance, in oxidized BaTiO3 crystals, optical and thermal suppressions occur by electronic domain pinning; electrical fatigue in the BaTiO{sub 3} crystals also appears to involve electronic charge trapping, however, it is suggested that these electronic traps are further stabilized by nearby ionic defects. In sol-gel PZT thin films with either Pt, RuO{sub 2}, or La-Sr-Co-O electrodes it appears that the polarization suppression induced by electrical fatigue, a temperature/bias combination, or a light/bias combination are all primarily due to the trapping of electronic charge carriers to first order.