Characterization of Nanoscale Domain Structures in Epitaxial Ferroelectric PbTiO3 Capacitors by Reciprocal Space Mapping

In order to test a critical lateral dimension in two dimensional (2D) planar ferroelectric, epitaxial PbTiO3 discrete islands were fabricated by both lithography process and chemical solution deposition process with different lateral size. The evolution of 90 deg. domain structures as a function of film thickness and lateral dimensions was characterized extensively by reciprocal space mapping using the Huber six-circle diffractometer of the synchrotron x-ray diffraction beam line at Pohang Light Source. This method is useful for considering a reciprocal lattice point that is tilted out of the growth plane and has many advantages for the investigation of imperfect heterostructures. As the pattern size decreases in micron scale, the substrate clamping effects is significantly reduced and thus the misfit strain in the films could be relaxed further. As the lateral size of patterns decreases to about 100 nm by e-beam lithography, the quite different domain structures were observed especially in the strain distribution as well as tilting of a-domains. The new type of a-domains appeared in the nanoscale PbTiO3 patterns in the center of the reciprocal space map of tilted PbTiO3 (100) that has been observed in the continuous films. These new a-domains are aligned along the substrate normal without the tilting angle required to maintain an a/c twin boundary relationship. Equilibrium domain structures in the PbTiO3 thin film islands are also analyzed by the finite element simulation and found to be consistent with the experimental observation. In the case of nano islands formed by chemical solution deposition, c-domain abundance increases as both size and thickness decrease identically, which implies that if the size can be reduced below a certain critical size there exists only single c-domain. Compared to the case of patterns with lateral dimension of 100 nm and thickness of 100 nm which has a tendency of reduced c-domain abundance due to the reduction of compressive strain, islands with thickness of 30 nm and 120 nm lateral size have larger c-domain abundance