Self-Assembled Ferroelectric Nanostructures

Integrated Ferroelectrics, 68: 279–286, 2004 C Taylor & Francis Inc. Copyright
ISSN: 1058-4587 print/ 1607-8489 online DOI: 10.1080/10584580490896652

Self-Assembled Ferroelectric Nanostructures I. SZAFRANIAK, S. BHATTACHARYYA,∗ C. HARNAGEA, R. SCHOLZ, and M. ALEXE Max Planck Institute of Microstructure Physics Weinberg 2, 06120 Halle, Germany (Received in final form May 21, 2004) Ferroelectric nanostructures have been fabricated by self-assembly, using the high temperature instability of ultra thin films. Lead zirconate titatnate (PZT) structures, as small as 20 nm lateral size with a height of 9 nm, have been fabricated on (100) SrTiO3 substrates. The shapes and sizes of the ferroelectric structures could be tuned within a small range by varying the initial thickness of the as deposited film and crystallisation annealing. Introduction of intentional defect sites prior to the crystallisation treatment had a pronounced effect on the shape and size of the final nanostructures. The ferroelectric switching of individual nanoislands of 50 nm lateral size and 25 nm height islands was shown by piezoresponse AFM. Keywords: PZT; ferroelectric nanostructure; self-assembly; piezoresponse-AFM

INTRODUCTION Recently, there has been an upsurge in the research on nano-science and nanotechnology, which was initiated by the market demand for high degree of miniaturisation in all new generation devices. It was seen before that the properties of many different functional materials could be exploited even down to the sub-nanometer length scale. Therefore, the next task left was mostly the fabrication of functional material nanostructures, for instance of ferroelectric materials, which have different applications in electronic devices [1, 2]. At the nano-scale level, the existing photolithography techniques already reach their limit [3, 4]. Electron beam lithography is a method to obtain well-tailored ordered nanostructures, but can not produce structures much below 100 nm. The other alternative is the so-called self-assembly ∗ Present address: Materials Research Centre, Indian Institute of Science, Bangalore 560012, India

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route of producing very fine nanostructures (∼10 nm) [5–9]. The final aim of this method is manufacture tailored devices at atomic level but it is a challenge to master the systematic exploitation of inter-molecular forces in a rational manner in order to manipulate the positions of well-isolated structures according to the desired application. In this route, the interaction between the substrate and the nanoparticles and also the interaction between the adjacent nanoparticles decide the microstructural arrangement of them. In this paper, we present the results of our study on low dimensional ferroelectric nanostructures formed by the self-assembly technique. It was already demonstrated before that an ultra-thin film (with thickness below 60 nm) tends to split up into individual islands after heat treatment at very high temperatures [10]. The size of the nanoislands is of a few tens of nanometers, well below the limits of the existing lithography techniques (conventional photolithography or a very sophisticated e-beam lithography technique). We investigated the morphology and properties of final nanocrystals as function of initial film thickness. To gain insight into the role of the substrate on the final microstructure, we have deliberately introduced some defect sites on the substrate prior to deposition, and investigated its effect. Electrical characterisation is also carried out using scanning force microscopy on an individual nanostructures, and the result of this study is also presented in this report.

EXPERIMENTAL PbZr0.52 Ti0.48 O3 (PZT) ultra-thin films of different thickness were obtained by spin-coating a commercial precursor (PZT9906, Chemat Technology, Inc.) onto single crystalline niobium doped SrTiO3 (STO:Nb) (100)-oriented substrates with a Nb concentration of 0.1% (CrysTec GmbH, Berlin). The initial film thickness was set by dilution of the raw precursor in its solvent (butanol), which has ranged from 1:10–1:40, and by adjusting the spinning speed from 3000 to 6000 rpm. The obtained gel film was dried on a hot plate at 80◦ C for 5 min, pyrolized at 300◦ C for 5 min, and finally crystallised at 800◦ C for 1h in a lead oxide atmosphere. During the high temperature treatment, ultra-thin films break up into islands of 20–400 nm lateral size depending on the initial film thickness. In order to study the influence of existing defects in the as-prepared film on the final structure a mechanical imprint technique was applied. Regular arrays of defects were imprinted in the dried film using a Si3 N4 stamp with an array of 260 nm high pyramids. Subsequently, the imprinted film was pyrolized at 300◦ C for 5 min,

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and crystallised at 800◦ C for 1h. The obtained nanostructures were studied by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), piezoresponse force microscopy (PFM) and by X-ray diffraction (XRD).

RESULTS AND DISCUSSIONS The growth of nanostructures was investigated as a function of the initial film thickness. As expected, thicker films transform into films with faceted holes after 1h of crystallisation at 800◦ C. A deposition using a higher dilution results in ultra-thin films, which after the high temperature crystallisation break up into small single-crystal islands. Figure 1(a–c) shows the morphology of the structures obtained after crystallisation of films with different initial thickness. If the initial film thickness is just below a critical value larger islands of irregular shape form, as is shown in Fig. 1(a). Islands have an average height of 15 nm and a wide distribution in lateral sizes up to few hundred nanometres. Thinner layers obtained with lower dilution (1:25) create islands that are both larger and higher (Fig. 1(b)). The distance between close neighbours increases as well, resulting in a low areal density of the islands of about 30/µm2 . Their height increases up to about 25 nm due to the contraction in the lateral size, and the conservation of the total volume. There were still a few places where the film did not completely break up, and formed a few elongated islands. For the highest dilution (1:40) the resulting islands have a height of about 9 nm and lateral dimensions of 40–90 nm with a relatively narrow distribution in size (Fig. 1(c)). The islands are distributed on the substrate with a high density of about 150 crystals on an area of 1 µm2 . In order to investigate the structure of the PZT islands, x-ray diffraction was performed. XRD analysis (Fig. 2) shows an epitaxial relationship between the substrate and top structures. No secondary phase peak was detected, confirming a good quality of perovskite phase formation. Further structure information was obtained from TEM analysis (Fig. 3). Low magnification pictures show that all islands have uniform heights and truncated pyramidal shape with relatively sharp facets that preferably consist of {111} or {110} and {100} faces. The high-resolution pictures confirm that nanoislands are single crystals with almost an atomically flat surface. The crystals are free from volume defects, and the only defects are dislocations concentrated along the interface. They are caused by relaxing of stress inducing by lattice mismatch between STO substrate and PZT islands.

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FIGURE 1 AFM images of PZT nanoislands obtained after deposition of (a) 1:20, (b) 1:25, (c) 1:40 diluted PZT precursor. (Note: vertical and horizontal scales of the (c) image are different) (See color plate XIX).

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FIGURE 2 X-ray diffraction pattern of PZT nanocrystals. The peaks labelled with circles indicate the substrate peaks originating from the remaining CuKβ radiation.

The growth mechanism of nano-sized structures is related to an instability of the ultra-thin film, observed previously for PbTiO3 thin films on (100) SrTiO3 substrate [10]. It was found that, a film with thickness below a critical value (30 nm in the case of PTO/STO) breaks up and form islands after a high temperature heat treatment. The driving force of this process is an excess of the total free energy of the continuous film compared with a film

FIGURE 3 Cross section transmission electron microscope image of a small PZT islands grown on STO substrate. Arrows point to misfit dislocations.

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partially covering the substrate. The free energy is minimized in two ways: 1) the interface energy decreases after islands formation because the interface area is smaller and 2) the free energy of the islands surfaces, appeared after breaking up the film, is lower than free energy of the (100) film surface [10, 11]. As expected, nanostructures formed by this self-assembly method did not show a regular arrangement in their positions. The initial thin film broke up at random places, and formed a collection of irregularly spaced nanocrystals. In order to register them at specific points, we have intentionally introduced defect sites by mechanical imprint at regular spatial intervals. The final structures after imprint and crystallisation (Fig. 4) is apparently different compared with previous structures (Fig. 1(c)). The islands are larger and higher and they have a well-defined rectangular shape. The distances between nearest neighbours are bigger. It seems that introduction of defects can be used as a method for registration of islands on large area scale. The ferroelectric properties of individual crystal islands were probed by piezoresponse force microscopy [12]. Bigger islands as well as structures after imprint show a well-developed piezoelectric hysteresis loop (Fig. 5). Investigations of the smallest self-assembled structures are under way in order to establish the minimum size of the epitaxial nano-sized crystals that are still switching.

FIGURE 4 Scanning electron microscope image of PZT nanostructrures obtained after crystallisation. The thinnest film was imprinted after drying.

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FIGURE 5 The piezoresponse hysteresis loop of an individual nanostructure obtained after imprint process (from Fig. 4).

CONCLUSIONS Nanostructures of ferroelectric PZT have been successfully fabricated onto Nb doped (100) SrTiO3 single crystals by the self-assembly route, exploiting the microstructural instability developed in ultra thin films at high temperatures. X-ray studies indicated a good epitaxial relationship between the substrate and the nanostructures. TEM studies confirmed that, the individual structures were also free from any volume defects. All the nanoislands were found to have a well-defined shape with relatively sharp facets that preferably consist of {111} or {110} and {100} faces. The presence of intentional defects prior to the annealing of the as deposited film had a significant effect on the alignment and the size of the nanostructures.

ACKNOWLEDGMENTS Part of this work has been supported by Volkswagen Stiftung project “Nanosized Ferroelectric Hybrids” (No. 5/77737).

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