טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentYadlovker Doron
SubjectA Study of High Density Arrays of Ferroelectric
Nano-Domains
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Shlomo Berger
Full Thesis textFull thesis text - English Version


Abstract

The general trend of miniaturization of ferroelectric-based electronic devices had evoked an intensive study of size effects on ferroelectricity in recent years. Experimental studies have shown ferroelectric behavior in ultra-thin films having a thickness of a few nanometers. However, there are only few reports on ferroelectric behavior of nanometer-size rods and wires. Moreover, in these reports, ferroelectricity was only indirectly detected. The need for two-dimensional arrays of nanometer-size ferroelectric particles is high for future high-resolution and high-density devices such as memories, detectors, sensors and actuators.

Our main goal was to achieve a state of the art structure, in which nanometer-size single-crystal rods are present, having uniform crystallographic and polarization directions as well as high-density arrangement.

A highly-dense and homogeneous array of nano-pores with high aspect-ratio was fabricated. Single-crystal rods were grown with preferred orientation inside the pores. In each pore a single-crystal is grown, completely filling its volume. The nucleation and growth processes are demonstrated with three ferroelectric materials.

The presence of nano-ferroelectric domains oriented along the pores is shown in Rochelle salt single-crystals which grew with the [100] direction along the pores. These crystals exhibit ferroelectric behavior having enhanced remnant polarization and higher coercive field. The ferroelectric phase is thermally stable up to the decomposition temperature of Rochelle salt, and there is no upper Curie temperature.

Potassium nitrate crystals grew with the [010] direction aligned along the pores. Field-induced Anti-ferroelectric to ferroelectric transition was observed at room temperature. Such a transition was known to occur only under large hydrostatic pressures.

Certain preferred orientation was achieved even with potassium niobate, which is almost insoluble in water.

A model for prediction of the nucleation site within a cylindrical pore with a cap-shaped bottom based on geometrical parameters of the nucleus and pore was developed. This model indicates a general preference for nucleation on the pore bottom - a necessary condition for growing a single-crystal within each pore. From pure geometrical analysis we conclude that nucleation on the bottom is always preferred. When curvature effects are taken into account, preference for nucleation on the bottom exists only under good wetting conditions.

For all models, the preference for nucleation on the pore bottom increases with decreasing its radius. At a certain pore radius, which depends on the contact angle, and the critical radius for nucleation, the preference for nucleation on the bottom is lost, and a polycrystalline structure is likely to appear.