Deborah J. Taylor

Oxygen vacancy mobility determined from current measurements in thin Ba0.5Sr0.5TiO3 films

The current density is measured as a function of time for thin (⩽1000 A) barium strontium titanate (BST) capacitors with platinum electrodes. The current density curve shows a peak prior to the onset of resistance degradation. The peak position on the time axis varies with applied voltage and temperature. The data are explained by the theory for space-charge-limited (SCL) current transients, and the measured current is identified as ionic current associated with oxygen vacancies. Using the SCL analysis, the mobility of the oxygen vacancies is measured as a function of temperature. The mobility obtained from current measurements is shown to be compatible with the Einstein relation for mobility and diffusivity. In summary, the ionic current associated with oxygen vacancies is shown to be an important component of the measured current in thin BST films.

Characterization of sputtered barium strontium titanate and strontium titanate-thin films

Sputtered Ba1−xSrxTiO3 (BST) and SrTiO3 (STO) films and capacitors made with these dielectrics have been characterized with respect to physical and electrical properties. Specific capacitance values included a high of 96 fF/μm2 for BST films deposited of 600 °C and a high of 26 fF/μm2 for STO films deposited at 400 °C. Leakage current densities at 3.3 V for the most part varied from mid 10−8 to mid 10−6 A/cm2. All of the dielectrics are polycrystalline, although the lowest temperature STO films have a nearly amorphous layer which impacts their capacitance. Grain size increases with deposition temperature, which correlates to higher dielectric constants. The lattice parameter of the BST films is larger than that of bulk samples. Capacitance, leakage, breakdown, and lifetime results are reported.

Investigation of bulk and interfacial properties of Ba0.5Sr0.5TiO3 thin film capacitors

In this letter, we report the results for capacitance versus frequency measurements on a set of Ba0.5Sr0.5TiO3 (BST) capacitors with platinum electrodes (Pt) and varying BST film thicknesses. The study shows that Pt/BST interfacial capacitance is independent of frequency whereas the bulk dielectric constant has a power law dependence on frequency. Also, the bulk dielectric constant is observed to decrease whereas the interfacial capacitance increases with increasing temperature. In addition, we report the dependence of dielectric dispersion on BST film thickness and temperature. Calculations are performed which provide insights into the observed dispersion effects.

Characteristics of spin-on ferroelectric SrBi 2 Ta 2 O 9 thin film capacitors for ferroelectric random access memory applications

We report on the properties and characterization of SrBi 2 Ta 2 O 9 (SBT, or Y − 1) thin film capacitors for ferroelectric random access memory (FERAM) applications. The films were prepared by spin-coating from carboxylate precursors. The remanent polarization ( P r ) was 5−6 μ C/cm 2 and the coercive field was ∼55 kV/cm. Excellent fatigue endurance was observed up to 10 11 cycles. Auger analysis indicates bismuth diffusion through the Pt electrode after capacitor anneal which might require excess Bi in the precursor solution for stoichiometry control. No detectable amount of α emission was found from SBT films, which reduces the possibility of soft error when used in the memory devices.

Investigation of hydrogen induced changes in SrBi2Ta2O9 ferroelectric films

The effect of hydrogen on strontium bismuth tantalate (SrBi2Ta2O9; SBT) ferroelectric capacitors is investigated. Using several analytical techniques such as x-ray diffraction, electron diffraction, Auger electron, scanning and transmission electron microscopies, the structural and compositional changes in the ferroelectric film are studied as a function of annealing gas and temperature. The mechanism for hydrogen induced damage to the capacitor is identified. Measurements show that the hydrogen induces both structural and compositional changes in the ferroelectric film. Hydrogen reacts with the bismuth oxide to form bismuth and the reduced bismuth diffuses out of the SBT film causing the electrodes to peel.

Electrical properties of SrBi2Ta2O9 thin films and their temperature dependence for ferroelectric nonvolatile memory applications

This letter addresses the temperature dependence of some of the electrical properties of 0.2‐μm‐thick SrBi2Ta2O9 capacitors with platinum electrodes on silicon wafers for nonvolatile memory applications. Investigations of the remanent polarization, the nonvolatile polarization and the coercive field with pulse amplitude were made at different temperatures. The endurance of the polarization as a function of temperature is reported. Up to 125 °C, the capacitors show less than a 10% reduction in polarization from their initial values following 1×1012 cycles.

Memory applications based on ferroelectric and high-permittivity dielectric thin films

Abstract Several metal oxide perovskites and Bi layered perovskites are of substantial interest for integrated circuit memories. Strontium titanate, barium strontium titanate, and lead zirconate titanate based paraelectrics are of particular interest for dynamic memory applications since they have high charge storage densities, low leakage currents, and resistance against time dependent dielectric breakdown sufficient to achieve gigabit densities and beyond. The high remanent polarization of ferroelectric lead zirconate titanate and strontium bismuth tantalate hold out the promise of low-voltage, high-speed, non-volatile memories. Advances are being made in the reliability of ferroelectric memories.

Performance of srbi2ta2o9 for low-voltage, non-volatile memory applications

Abstract The performance of SrBi2Ta2O9 for ferroelectric non-volatile memory applications is investigated. The temperature dependence of the hysteresis loops, small signal capacitance-voltage measurements, and fatigue resistance are reported. Increasing temperature accelerates fatigue, but excellent fatigue resistance to greater than 1012 cycles is found for temperatures of 125 °C and below. The difference between current-time curves for switched and unswitched capacitors using high-speed pulse measurements indicated the availability of 5.7 μC/cm2 for 3 V memory operation.

Stability of reactive DC-sputtered Ir and IrO2 thin films in various ambients

Abstract Ir and IrO2 are potential electrode materials for ferroelectric thin film capacitors. However, some process related issues need to be considered before integrating ferroelectric capacitors into memory cells. This paper presents the effects of post-deposition annealing on the characteristics of DC-magnetron sputtered Ir and IrO2 thin films. Film properties such as composition, resistivity, crystallinity, adhesion, and microstructure were examined before and after annealing.

A new electrode technology for high-density nonvolatile ferroelectric (SrBi/sub 2/Ta/sub 2/O/sub 9/) memories

New electrode materials (Ir and IrO/sub 2/) are proposed for high-density nonvolatile ferroelectric random access memories (NVFERAMs). These electrodes are used in order to integrate ferroelectric (SrBi/sub 2/Ta/sub 2/O/sub 9/) capacitors with standard CMOS technology. Excellent electrical characteristics (e.g. low leakage and high polarization), good fatigue resistance and good mechanical properties (i.e. excellent adhesion to SiO/sub 2/ without using a glue layer) were obtained for the new capacitor structures.