Peir Chu

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.

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.

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.

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.

Effect of the sensing capacitance in a ‘sawyer-tower’ set-up on hysteresis loops

Abstract It is well known that, for a given ferroelectric capacitor, the tilting of the hysteresis loops and the amount of remanent polarization measured in a ‘Sawyer Tower’ set-up are dependent on the sense capacitance (or load). However, it is less intuitive that a small sense capacitance will cause horizontal loop shifts and that these shifts depend on the state of the ferroelectric capacitor before the measurement. Since typical ferroelectric non-volatile memories operate with small sense-to-bit capacitance ratios, it is important to have models that can predict such effects. Complicated experimentation is necessary to implement the load and initial state effects properly in a load-line model. In contrast, the numerical ferroelectric capacitor model does not require any special experimentation to predict such effects. Experimental results from SrBi2Ta2O9 are compared with both models.

Electrode Materials for Ferroelectric Capacitors: Properties of Reactive DC Sputtered IrO 2 Thin Films

Due to its low resistivity and excellent thermal stability, IrO 2 has attracted attention as an alternative for electrode material in ferroelectric integrated circuit applications. In this work, IrO 2 films deposited using reactive DC magnetron sputtering were studied. Film properties such as resistivity, crystallinity and morphology were examined as a function of deposition condition. Optimum process parameters to obtain high quality IrO 2 thin films are suggested.

Physical and chemical characterization of barium strontium titanate thin films

Thin films of barium strontium titanate (BST) are under consideration as a replacement for silicon dioxide as the dielectric in advanced memory devices. The major attraction of BST is its very high dielectric constant compared to SiO2. Integral to the development and implementation of a new materials system such as BST is physical and chemical characterization. To achieve optimal electrical performance with a robust process requires an in-depth understanding of the relationships between materials/process variables, physical/chemical properties and the electrical behavior of the BST films. This paper covers specifics of many of these inter-relationships.