Fabrication of epitaxial nanostructured ferroelectrics and investigation of their domain structures
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Nanostructured ferroelectrics are important objects for studies on ferroelectric size effects as well as for applications to memory devices with ultra-high memory density. In the present article, we introduce several approaches for the synthesis of confined ferroelectrics with sizes in and below the hundreds of nanometer range, including top-down processes like e-beam lithography, self-assembly methods like chemical solution deposition, and growth by pulsed laser deposition using stencil masks. Furthermore, the ferroelectric domain structure of part of these nanostructures is investigated by means of synchrotron X-ray diffraction, and its contribution to the ferroelectric properties is discussed.
KeywordsPiezoelectric Property Misfit Strain Ferroelectric Thin Film Chemical Solution Deposition Piezoresponse Force Microscopy
Ferroelectric materials have been and are still investigated intensively due to their wide range of electronic device applications making use of the exceptional physical properties, for example, dynamic random access memories (DRAMs), nonvolatile ferroelectric random access memories (NV-FERAMs) based on ferroelectric materials with high permittivity and polarization, respectively [1, 2, 3]. Requirements for the increase of data storage density and the decrease of production costs have driven the miniaturization of ferroelectrics as small as they can preserve the ferroelectricity [4, 5, 6, 7, 8]. For example, in order to achieve a memory density of 1 Tb/inch2, one has to realize ferroelectric bits with sizes of less than 25 × 25 nm2. Accordingly, intense efforts for the fabrication of nanostructured ferroelectrics have been contributed by many researchers to understand the ferroelectric size effects in low-dimensional nanostructures, in which ferroelectricity could be altered or even totally vanish at a certain critical size. The fabrication of low-dimensional ferroelectrics and their characterization, for example, ultra-thin films [9, 10, 11, 12, 13, 14], nanotubes [15, 16], and nanowires [17, 18, 19], have well developed during last decades. Promising theoretical predictions as well as many interesting experimental observations of ferroelectricity by means of scanning force microscopy and synchrotron X-ray diffraction have been reported [9, 11, 20, 21]. These observations confirmed that ferroelectricity could be preserved in films of very low thickness due to the reduction of the depolarization energy by forming nanoscale 180° stripe domain patterns on insulating substrates or due to the presence of conducting metallic electrodes, resulting in a stable polarization state at a thickness of several unit cells. In a theoretical work it was reported that the interface between an ultra-thin ferroelectric thin film and a paraelectric substrate could affect the ferroelectric properties due to correlations of atomic displacements through the interface . In real systems, however, structural defects such as interfacial dislocations [23, 24] or imperfect screening by conducting electrodes  may affect the ferroelectric properties. Overall one can say that the fundamental limit of thickness in ferroelectric films has been overcome.
Contrary to the thickness limit of ferroelectricity, the size effect resulting from a lateral size limit, i.e., the question how small a ferroelectric nanostructure can become without loosing the ferroelectric properties, is still unveiled due to difficulties in the fabrication of uniform and reproducible ferroelectric nanostructures of very small sizes. There were several theoretical studies on the size effect showing that ferroelectric and piezoelectric properties could be altered due to the increased influence of the surfaces and to reduced substrate constraints [6, 25, 26, 27]. Recently, experimental [28, 29, 30, 31] and theoretical  observations suggested that confined ferroelectrics could have unique properties. Nagarajan et al.  showed that the ferroelectric and piezoelectric properties were increased in discrete ferroelectric films owing to a reduced substrate clamping effect. Lee et al. prepared PZT patterns with morphotropic phase boundary (MPB) composition utilizing the strain relaxation in ferroelectric islands . Bühlmann et al.  explained that the increase of piezoresponse in nanoislands originated from a reduced share of a-domains. Naumov et al.  found by ab initio studies that the ferroelectrics could have toroidal polarization states in zero-dimensional ferroelectric particles with a size of 3.2 nm, which would principally enable a memory density of 60 Tb/inch2.
Various experimental approaches to obtain nanostructured ferroelectrics with small size, good ordering, and high quality have been recently developed, including so called top-down [33, 34, 35, 36, 37] and bottom-up methods [38, 39, 40, 41, 42]. Top-down methods, for example, e-beam direct writing (EBDW), e-beam patterning, focused ion-beam (FIB) milling, and nano imprint lithography are useful approaches in view of precise size control and ordering of nanofeatures. However, the large-scale operation of these methods is expensive, time-consuming and above all, the ferroelectric properties are frequently affected by mechanical damage or e-beam or ion-beam damage of the surface-near areas of the nanostructures [34, 43]. On the other hand, bottom-up methods such as metal-organic vapor deposition (MOCVD), chemical solution deposition (CSD), and hydrothermal methods may enable the preparation of even smaller ferroelectric nanostructures and to study intrinsic size effects. Roelofs et al.  showed that no piezoresponse was detected in PbTiO3 nanograins with sizes below 20 nm, which can be the size limit where ferroelectricity can still exist. Shimizu et al. observed that PbTiO3 nanoislands with 38 nm lateral size still show a piezoresponse when studied by atomic force microscopy . Although bottom-up methods permit the investigation of extremely miniaturized regions of ferroelectrics, they have other limitations. Not only are there difficulties in precisely locating each bit (each nanostructure) in a well-ordered array, but also in controlling the size of the latter exactly. For taking advantages of both top-down and bottom-up approaches, selective growth using various mask or seed patterns is being investigated, for example, nanosphere- , metal nanotube membrane- [46, 47], and template-based growth [48, 49]. Due to the uniform size and well-ordered arrays of ferroelectric nanostructures in these cases, the authors were able to measure the structural orientation by laboratory-based XRD, and determine their piezoelectric properties . Most recently, novel techniques to synthesize unique structures of ferroelectrics have been employed. Zhu et al.  showed that using AAO templates could be the way to realize 5-nm sized ferroelectric ring structures which have a potential to demonstrate the vortex polarization suggested by Naumov. Evans et al.  fabricated self-assembled nanocapacitors with nanosized Pt electrodes by means of AAO.
Despite the multifarious activities for studies of nanostructured ferroelectrics, there have been no detailed reports which would give a clear understanding of domain structures in ferroelectric nanoislands. Since the ferroelectric properties and device performances are a function of the crystallographic orientations and the domain structures in ferroelectric thin films, a wide range of work has been devoted to understand the formation mechanisms of domains, the role of domain boundaries, and the controlling factors of the domain structure [52, 53, 54]. Jesse et al.  revealed by switching spectroscopy PFM (SS-PFM) at the nanometer scale that 90° domain boundaries can be nucleation centers for 180° polarization switching. Jia et al.  found in atomic-scale high-resolution TEM investigations of PZT films that charged 180° domain boundaries with head-to-head or tail-to-tail configuration of the electric dipoles are about 10 times broader than non-charged 180° boundaries. Thus, one may assume that the charged domain boundaries are more mobile than the uncharged ones, so that the 180° domains quickly grow in the polarization direction rather than showing lateral growth. However, more understanding of domain structures and their effects onto ferroelectric and piezoelectric properties in nanosized ferroelectrics is desirable, because the domain structures can be significantly changed by the different strain state in nanostructures compared to bulk or thin film ferroelectrics.
In this article, we discuss recent results of our work, mainly focusing on the fabrication methods of nanostructured ferroelectrics by e-beam lithography, self-assembly, laser interference lithography and template masks, as well as investigations of domain structures and their effects on the electrical properties. The domain structure was investigated by reciprocal space mapping techniques with synchrotron XRD, and the electrical properties were measured by AFM in the piezoresponse mode.
Fabrication methods for nanostructured ferroelectrics
Electron-beam lithography was performed in a scanning electron microscopy (SEM) system (JEOL, Japan) equipped with a beam blanker (Deben, UK) controlled by the Elphy Plus software (RAITH, Germany). MMA-MAA EL6 (Microchem, USA) films with a thickness of about 150 nm were used as e-beam sensitive resist. For patterning, an accelerating voltage of 29 kV, an e-beam spot size of 10 nm, and an area dose of 200 μC/cm2 were used. Platinum with a thickness of 100 nm was deposited by e-beam evaporation to both serve as etching mask and top electrode after developing the e-beam pattern. Then, rinsing in acetone for lift-off and in methanol, followed by deionized water was carried out. An inductively coupled plasma (ICP) advanced oxide etcher (AOE) (STS, UK) was used for the etching process. The Pt top electrode was etched by Ar and He with a flow of 15 and 10 sccm, respectively. Then, C4H4, C2H2, Ar, and O2 with 5, 10, 5, and 10 sccm, were used for the etching of 100-nm-thick PbTiO3 patterns. Patterned samples were annealed in oxygen ambient at 600 °C for 1 h to remove any etching-related damages.
Chemical solution deposition (CSD)
For CSD, commercial PbTiO3 polymeric precursors with a concentration of 0.3 mol/L and 10% excess PbO were dissolved in 2-methoxyethanol. The initial thickness was varied by varying the dilutions of the raw precursor in the range of 1:5 to 1:120. PbTiO3 solutions were spin-coated on selected substrates at 5000 rpm for 30 s, and the obtained ultra-thin gel film was kept in air for at least 30 min. As-deposited films were pyrolized at 450 °C for 1 h in a box furnace; then the specimens were heated to high temperatures, for example, to 650–800 °C, and kept for 1 h to crystallize the pyrolized film. The sizes of the nanoislands were controlled by the initial thickness of the as-deposited film, by the annealing temperature and the annealing time.
Ultra-thin anodic alumina mask (UTAAM)
Self-ordered UTAAM with uniform channels could be synthesized by two-step anodization of surface-finished pure aluminum plates [57, 58]. Initially, the aluminum plate was electropolished in a mixture of HClO4 and C2H5OH = 1:3, then the first anodization was carried out in electrolytes under a constant cell potential for 12 h. After the first anodization, the anodic alumina was completely etched in chromic acid (mixture of H2CrO4, H3PO4, and deionized water). By making use of self-ordered structural arrays of pores on the aluminum surface, porous anodic alumina with uniform channels could be formed after the second anodization. For the use of porous alumina as a hard mask, the thickness of AAO was strictly controlled. For example, the thickness of the AAO after oxalic acid anodization was 400 nm in order to obtain an aspect ratio of about 6, which is sufficient for PZT deposition through the pores. After the AAO membrane was synthesized, the remaining aluminum and the barrier oxide layer at the pore bottom were removed. The key step for the fabrication of the UTAAM is polystyrene (PS) coating. The PS coating layer protects the ultra-thin hard mask from cracking during the etching step of the aluminum and the barrier layer. A solution of PS in CHCl3 was spin-coated at 3000 rpm for 30 s, and then the solvent was evaporated at 80 °C. The rest of aluminum was etched in a mixed solution of CuCl2 · 2H2O, deionized water, and HCl, followed by HNO3-rinsing for the complete removal of the residual copper on the surface of the mask. Barrier layers of AAO prepared in phosphoric, oxalic, and sulfuric acid were removed by 10 wt% of H3PO4 at 45 °C for 60 min, 5 wt% of H3PO4 at 30 °C for 30 min and 5 wt% of H3PO4 at 30 °C for 15 min, respectively. During the etching of the barrier layer, the pore walls were protected from the etching solution due to the PS in the pores. The PS layer makes the UTAAM float on the deionized water and on the H3PO4 solution due to its hydrophobic nature. The mask was taken out of the deionized water, floating it directly onto the substrate for deposition, and the PS was removed by CHCl3 or oxygen plasma etching, resulting in a hard mask for deposition.
Laser interference lithography (LIL)
First, an antireflection coating (ARC, XHRiC-11, Brewer Science, US) layer with a thickness of 150 nm was spin-coated at 2500 rpm and baked at 180 °C for 60 s. Then a SiO2 mask layer was deposited on the ARC by sputtering. A 180 nm thick negative photoresist (PR, TSMR-iN027, OHKA, Japan) was spin-coated at 600 rpm and pre-baked at 90 °C for 90 s. The PR was post-baked at 110 °C for 90 s after laser exposure. For interference lithography, a HeCd laser with a wavelength of 325 nm and a Lloyd’s mirror interferometer were used. Two coherent beams generate periodic interference patterns on the PR, and the patterns were transferred to the SiO2 mask layer by reactive ion etching (RIE). CHF3 gas was used to open the SiO2 layer and the subjacent ARC layer during the RIE step. The residual PR on top of the SiO2 layer was also removed by O2 plasma under operating conditions of 10 mTorr and 100 W in the RIE step. The lift-off process of the ARC layer was carried out by a RCA-1 (H2O:NH4OH:H2O2 = 5:1:1) cleaning solution at 70 °C for 1 h in the lift-off step after pulsed laser deposition. By this lift-off of the ARC layer, the SiO2 mask was simultaneously taken away from the substrate.
Pulsed laser deposition (PLD)
The ferroelectric material was deposited onto the substrates by PLD with a KrF eximer laser (λ = 248 nm) at an energy fluence of 400–600 mJ/cm2. The background pressure was below 1 × 10−6 Torr to give a high energy to the particles emanating from the target, so that they can reach the substrate surface through the SiO2 and AAO masks.
High-resolution synchrotron X-ray diffraction (XRD)
The domain structure of ferroelectric nanoislands was observed by high-resolution synchrotron XRD installed on the Huber six-circle diffractometer of 3C2 and 10C1 beam lines at the Pohang Light Source (PLS). The incident beam size was less than 1 mm2 and the energy was 8 keV. Reciprocal space mapping was used for the investigation of the final domain structure in the ferroelectric nanoislands.
Transmission electron microscopy (TEM)
Conventional TEM investigations were carried out in a Philips CM 20 Twin (Philips, Netherlands) microscope, whereas HRTEM investigations were performed in a Jeol 4010 (Jeol, Japan) microscope. Some of the thin samples for TEM investigations were prepared by standard methods of mechanical polishing and ion milling. The other samples were prepared by FIB milling using a gallium ion beam in an FEI Nova 600 NanoLab system.
Piezoresponse force microscopy (PFM)
Finally, PFM was employed for the measurement of the electrical properties, such as switching behavior, and piezoresponse of each nanoisland under an external electric field. A CP-Research atomic force microscope was used in contact mode to record the piezoelectric signals from the nanoislands, using a conducting AFM tip with an elastic constant of 2.5 Nm and a lock-in amplifier.
Results and discussions
Non-tilted a-domains in nanostructured ferroelectrics
When the lateral size of a ferroelectric nanostructure decreases while the thickness is kept constant, the degree of misfit strain relaxation increases. The misfit strain relaxation is strongly dependent on the scaling ratio and occurs predominantly in the edge region of discrete patterns. In the nanopatterns, the misfit strain at the edge regions of the patterns is fully relaxed , resulting in the appearance of non-tilted a-domains. We had initially assumed that single-domain structures would appear in nanosized ferroelectrics. However, in reality the a-domain abundance rather increases and the described new type of a-domains appears in nanosize islands. It remains to be explained how the new and the conventional a-domains are distributed in the small features. If the size of the ferroelectric islands decreases further, then there might be a critical size below which the periodic c/a/c/a domain structures with 45° twin boundaries are unfavorable. Therefore, one needs to prepare even smaller sized ferroelectrics and investigate their domain structures. However, it occurred to be difficult to fabricate ferroelectric nanostructures with a lateral size of only several tens of nanometers by a top-down method such as e-beam lithography. Thus, we employed a bottom-up approach, i.e., CSD, for the next step of the investigations.
Piezoresponse and a-domains in nanostructured ferroelectrics
By the structural analysis and electrical measurements, the abundance of a-domains and the piezoresponse were investigated as a function of the size of the ferroelectric nanostructures. However, the sizes of the ferroelectric nanostructures and their standard deviations were too wide to be accurate and practical. Another fabrication method to achieve rather uniformly sized ferroelectric nanostructures is still required. In the next session, we introduce a novel method for fabrication of even smaller ferroelectric nanocapacitors by means of a stencil mask.
Domain structures and piezoelectric properties in ferroelectric nanocapacitors
In the previous sections, ferroelectric nanostructures with various sizes were prepared and relations between their domain structures and piezoelectric properties were discussed. The reduction of substrate clamping effects and strains lead to the presence of non-tilted a-domains and an increase of the piezoresponse in the nanostructures. In this section, we would like to report on a novel method for the fabrication of ferroelectric nanocapacitors and on the investigation of their domain structures.
Ferroelectric nanodisks and nanorings
In order to investigate the size effect and the domain structures in ferroelectric nanostructures, we fabricated PbTiO3 and Pb(Zr0.2Ti0.8)O3 nanoislands and nanocapacitors employing a top-down method (e-beam lithography), a bottom-up technique (chemical solution deposition), and selective growth through stencils. The latter were obtained by LIL and by an ultra-thin anodic alumina process. The 100-nm sized PbTiO3 islands were achieved by e-beam lithography for X-ray diffraction analysis. The CSD method enabled us to grow epitaxial PbTiO3 nanoislands on Pt (001)/MgO (001) and vary the size of ferroelectric nanoislands between 450 and 55 nm by controlling the dilution of the solutions. Using AAO masks and pulsed-laser deposition, we were able to synthesize 64-nm sized ferroelectric nanoislands and multilayered Pt/PZT/Pt nanocapacitors on MgO (001) and Nb-doped SrTiO3 substrates, taking advantage of the good thermal stability of this ceramic stencil material. LIL is a most useful approach to obtain uniform wafer-scale arrays of ferroelectric nanodisks and nanorings by controlling the experimental conditions. The domain structure of all these ferroelectric nanostructures was characterized by reciprocal space mapping using high-resolution synchrotron XRD and AFM in the piezoresponse mode. When the size of the nanostructured ferroelectrics decreases down to 100 nm with scaling ratio of 1, a new type of a-domains with a tilting angle Δω = 0 was observed and thus periodic c/a/c/a polydomain structures became unstable. This appearance of non-tilted a-domains is a clear evidence of misfit strain relaxation in the ferroelectric nanostructures. The piezoelectric properties could be enhanced by the reduced a-domains and substrate clamping effects in nanostructured ferroelectrics. PZT nanocapacitors with 64-nm size showed three domain configurations of c-domains, tilted a-domains, and non-tilted a-domains. PFM measurements confirmed that the piezoelectric constant of the nanocapacitors was higher than that of a ferroelectric continuous medium. a-domains could be switched under an external electric field due to the reduced misfit strain in nanostructured ferroelectrics. The twinned 90° domain boundaries observed in PFM images also can play a role as nucleation sites of 180° domain switching and can be a possible reason for the increase of the piezoelectric properties in PZT nanocapacitors. The questions “how small can nanostructured ferroelectrics become while still preserving ferroelectricity” and “will a single-domain state be stable in such nanostructures” have still not yet been answered conclusively. To investigate these questions, further new approaches for the fabrication of even smaller-sized ferroelectrics will be required.
The authors thank Y. J. Park, J.-Y. Choi (Pohang Light Source, Pohang, Korea), S. Lee, Y. H. Jeong (Pohang University of Science and Technology, Pohang, Korea), R. Hillebrand, R. Ji, S. K. Lee, A. Lotnyk, M. A. Schubert, S. Senz, and U. Gösele (Max Planck Institute of Microstructure Physics, Halle, Germany), as well as K. Nielsch (Institute of Applied Physics, Hamburg, Germany) for many fruitful discussions and for experimental and analytical contributions. H. Han is grateful for the award of a fellowship of the German Academic Exchange Service (DAAD) and for support by DFG (446 KOR 113/215/0-1) and KRF. Financial support from the Volkswagen Foundation (Project I/80897), the German Ministry of Education and Research (BMBF, FKZ 03N8701) and from the Brain Korea 21 Project are also acknowledged. The experiments at PLS were supported by the MEST (Ministry of Education, Science, and Technology) and POSTECH.
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