| Summary: | My thesis will be arranged in four sections: 1. An introduction to ferroelectric memories and materials and my research , on fundamental aspects such as polarisation switching. Some research on novel processing techniques, such as the self patterning of nanoscale capacitors for high density memory applications, will also be presented. 2. Research on electrode screening effects in which the ferroelectric is treated as an ideal insulator. These often neglected effects can explain many of the observed electrical properties of ferroelectric thin films, such as interfacial capacitance and the dependence of the coercive field with thickness. 3. Consideration of semiconductor properties of ferroelectrics. The results of section 2 are valid only when the ferroelectric thin film is an ideal insulator. In this section I incorporate the effects of surface states (in fact, metal-induced gap states) and space charge (mostly oxygen vacancies produced in the processing of the film). The effect of these on the height of the Schottky barrier at the metal-ferroelectric interface is considered. Field and potential distributions for high-K (high dielectric permittivity)dielectrics are ferroelectrics are calculated. It is shown that the electrical properties of these systems bear many similarities with metal-semiconductor-metal punch-through diode materials, but that the standard models of these are flawed. It is shown that the capacitance of the system is suppressed by the capacitance of the electrodes and the space charge in the depletion width, and that this accounts for the difference between the dielectric susceptibility near phase transitions in bulk materials and thin films. Finally we discuss the leakage current in ferroelectric thin film capacitors, and the model we present is able to account for two of the interesting features that are sometimes seen, negative differential resistivity and a positive temperature coefficient of resistivity. 4. Effects due to motion and self-organisation of charged oxygen vacancies. Under applied electric field the oxygen vacancies can migrate towards the electrodes and order themselves into planes which can pin domain walls thus leading to polarisation fatigue under repetitive electrical cycling. The motion of vacancies under the depolarisation field is shown to account well for retention ("shelf-life") failure.
|
|---|
