Sherry Gillespie

The electronic conduction mechanism in barium strontium titanate thin films

In the literature, the Schottky emission equation is widely used to describe the conduction mechanism in perovskite-type titanate thin films. Though the equation provides a good fit to the leakage current data, the extracted values of the Richardson and dielectric constants are inconsistent with their experimental values. In this work, a modified Schottky equation is applied. This equation resolves the difficulties associated with the standard Schottky equation. Also, the electronic mobility in thin films of barium strontium titanate is reported.

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.

Ferroelectric non-volatile memories for low-voltage, low-power applications

Abstract Non-volatile ferroelectric random access memories (FERAM) offer substantial advantages over conventional floating-gate electrically erasable programmable read only memory (EEPROM) and flash EEPROM devices. FERAMS can be written at high speeds (≈100 ns) and at standard supply voltages 95 V or less) without the use of charge pumps. Switching of ferroelectric capacitors at voltages of 1.5 V or below has been demonstrated. FERAMS also have a high endurance for writes (>1012 cycles demonstrated). The combination of high-speed and low-voltage writing means a FERAM requires far less energy to program than a EEPROM. However, FERAMs use a destructive read followed by a rewrite so that endurance limits also apply to reading. Other reliability concerns include retention (loss of data with time) and imprint (loss of ability to write into the opposite state). We discuss the present ability of ferroelectric materials (lead zirconate titanate and Bi layered perovskites) to meet the electrical requirements and reliability requirements of practical non-volatile memory applications. We also review some of the process challenges in integrating ferroelectrics into typical complementary metal oxide semiconductor (CMOS) integrated circuit fabrication.

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.

Resistance degradation in barium strontium titanate thin films

Experimental and modeling results for resistance degradation in thin Ba0.5Sr0.5TiO3 (BST) film capacitors with platinum (Pt) electrodes are reported. The main experimental results are as follows. Under a constant applied voltage, the current density is observed to increase with time until it reaches a maximum value. Once the maximum value is reached, the current density becomes constant with time. The barrier height at the BST/Pt (cathode) interface is observed to decrease after prolonged electrical stressing. The resistance degradation effect is observed to be reversible, particularly at elevated temperatures. Based on the experimental results, a quantitative model for resistance degradation is proposed. In this model, the increase in the current density is attributed to a decrease in the barrier height at the cathode and this decrease is assumed to have a stretched exponential dependence on time. Using experimentally determined parameters, the model calculates the current density as a function of time a...

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.