Fabrication of Ferroelectric Nanostructure Array and Study on Its Critical Size

Abstract

Ferroelectric materials have gained much attention owing to their unique properties of spontaneous, switchable polarization, piezoelectricity and pyroelectricity. During the past decades, the ferroelectrics of a thin film have been extensively studied both theoretically and experimentally. Although continuous ferroelectric thin film has shown the possibility for various applications such as information storage media and infrared cameras, discrete nanostructures, for instance, nanoislands or nanodots, are ideal because the crosstalk effect invoked by the domain movement or thermal diffusion could be inherently excluded.For nanostructured ferroelectric materials, it is important to find the critical size below which ferroelectricity disappears, because this size determines the ultimate areal density of ferroelectric devices. It was theoretically predicted that a nanodisk with a diameter of 3.2 nm could maintain spontaneous polarization. However, the fabrication of ferroelectric nanostructure at this size level is extremely difficult. Earlier, continuous thin film was carved into discrete nanostructures with controlled size and shape by using focused-ion beam (FIB). But, the crystal structure of nanoislands could be affected during the etching process. Also, the fabrication of nanoislands in a large area is extremely tedious due to a time-consuming process. To overcome these drawbacks, various approaches13-16 have been employed. Szafraniak et al. employed chemical solution deposition (CSD) method to fabricate epitaxial, single-crystal PbZrxTi1-xO3 (PZT) nanoislands. Nanoislands with various sizes (height ranging from 9 to 25 nm and lateral dimension from 20 to 200 nm) were obtained by changing initial precursor film thickness and crystallization temperature. A nanoisland with a relatively large volume (~1.2 × 106 nm3) showed ferroelectric properties. Son et al. fabricated ferroelectric nanoislands with a lateral dimension of 37 nm and a height of 22 nm (thus, a volume of 3 × 104 nm3) by dip-pen lithography and piezoresponse was observed for this nanoisland.Although the previous studies provide useful information on the critical size of ferroelectric nanoislands, the experimental results have been obtained based on a single nanoisland (or nanostructure). To eliminate the possible error in estimating the critical size, ferroelectric behavior should be obtained from a very large number (say over 1010) of nanoislands with uniform shape and narrow size distribution. Several research groups have pursued this objective. Lee et al. fabricated a high-density array of PZT nanoislands with a diameter of 60 nm and a height of 40 nm (volume of 2.8 × 104 nm3) using pulsed laser deposition and aluminum oxide mask. The crystal structure was analyzed by x-ray diffraction (XRD) owing to the uniform size of PZT nanoislands over a large area. They also found that each PZT nanoisland worked as a ferroelectric capacitor. Although this result showed successful fabrication of a high-density array of ferroelectric nanoislands, the lateral dimension of a nanoisland is still larger than the dimension obtained by self-assembly based on CSD. It is noted that CSD could not produce ferroelectric nanostructures with uniform size. We realize that when ferroelectric nanoislands are prepared by block copolymer micelles, the size is comparable to (or smaller than) that obtained by CSD, while maintaining uniform size distribution. Furthermore, the in-plane ordering of the nanoislands on various conducting substrates is good due to the self-assembly of the micelles. In chapter 2, we prepared an ultrahigh density array of ferroelectric PbTiO3 (PTO) nanoislands on Nb-doped SrTiO3 (STO) (100) substrate by using polystyrene-block-poly(4-vinyl pyridine) copolymer (PS-b-P4VP) micelles. The fabricated PTO nanoislands had uniform size distribution in a large area (over cm2 scale), and a good in-plane ordering of the nanoislands was obtained. The average diameter (D) and height (h) of each nanoisland were 22 nm and 7 nm, respectively. Thus, each nanoisland had a volume of ~ 2.6 × 103 nm3 and a scaling ratio (D/h) of 3.3). Well-developed epitaxy of the PTO nanoislands was successfully analyzed by synchrotron x-ray diffraction (XRD) and transmission electron microscopy (TEM). Piezoresponse of the nanoislands was examined by piezoresponse force microscopy (PFM), and all of the nanoislands existing in the entire area showed distinct ferroelectricity. These observations confirm that the ferroelectricity of PTO nanoislands could be maintained at a volume as small as 2.6 × 103 nm3 and high scaling ratio (3.3).In chapter 3, size-dependence of ferroelectric property was studied. As discussed in chapter 2, nanoislands with volume of ~ 2.6 × 103 nm3 had distinctive ferroelectricity. The result implies that ferroelectric critical size would be lower than then this interval. In order to observe size-driven phase change, from ferroelectric to paraelectric state, sizes of nanoisland array were varied and their ferroelectric behavior was observed.