Ferroelectric Domain Breakdown: Application to Nanodomain Technology

The major trends in ferroelectric photonic and electronic devices are based on development of materials with nanoscale features. Piezoelectric, electrooptic, nonlinear optical properties of FE are largely determined by the arrangement of ferroelectric domains. A promising way is a modification of these basic properties by means of tailoring nanodomain and refractive index superlattices. The physical size of spontaneously polarized regions and modern technological requirements of lateral domain dimensions scale down to 100 nm in millimeter thick ferroelectric bulk crystals; this implies a development of a new approach both in instrumentation and physics of polarization reversal. We review in this paper a new and novel tool for nanodomain tailoring and technology in ferroelectric bulk crystals - high voltage atomic force microscopy. It is shown that application of the switching voltage in the kV range to a nanometer size switching electrode leads to new physical mechanism of polarization-reversal ferroelectric domain breakdown. In this process string-like domains of nanometer radius penetrate into hundreds of micrometers depth of ferroelectric crystals. Experimental results on nanodomain reversal and theory of breakdown phenomenon are analyzed and discussed in details. Pronounced domain breakdown phenomenon has also been observed in LiNbO₃ bulk crystals subjected to high-energy electron beams. C--polar face of the LiNbO₃ crystal coated by thick amorphous dielectric layers protects penetration of the beam electrons into the ferroelectric crystal resulting in formation of immobile electron drops. The electron charge localized in a small volume creates high intensity electric fields and causes domain reversal in a nanometer scale with a very high ratio between the domain radius and its length. A domain shape invariant is defined; this allows obtaining unambiguous criteria for stringlike nanodomain generation resulting in a figure of merit of a ferroelectric crystal applicable for nanodomain technology. It is shown that such a FE should possess high spontaneous polarization, low density of domain wall surface energy and small lateral dielectric permittivity. We report on the application of atomic force microscopy tip arrays for nanodomain engineering in ferroelectric crystals. Using a ten-tip array, it is shown that nanodomain writing results in a regular domain grating that penetrates throughout the bulk crystal as in the case of single tip writing. The developed nanodomain reversal techniques and understanding of physical mechanisms paves the way to new technologies for fabrication of various nanodomain configurations for advanced optoelectronic and microelectronic devices.