Microstructure and electrical properties of textured Sr0.51Ba0.48La0

Appl. Phys. A (2000) / Digital Object Identifier (DOI) 10.1007/s003390000407

Applied Physics A Materials Science & Processing

Microstructure and electrical properties of textured Sr0.51 Ba0.48La0.01Nb2O6 thin films Z. Song1 , C. Lin1 , L. Wang1 , J. Huang1 , D. Hesse2 , N.D. Zakharov2 , H. Xu3 , M. Okuyama3 1 State

Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050, P.R. China (Fax: +86-21/6251-3510) 2 Max-Planck-Institut für Mikrostrukturphysik, 06120 Halle/Saale, Germany 3 Department of Electrical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan Received: 23 November 1998/Accepted: 18 September 1999/Published online: 23 February 2000 –  Springer-Verlag 2000

Abstract. Sr0.51 Ba0.48 La0.01 Nb2 O6 (SBLN) thin films were prepared on platinized silicon substrates by pulsed laser deposition (PLD) combined with annealing technique. The preferred orientation, surface morphology, composition, and interfacial properties of the SBLN thin films were characterized by X-ray diffraction, atomic force microscopy, transmission electron microscopy, X-ray energy dispersive spectroscopy, and automatic spreading resistance measurement. The ferroelectric properties were confirmed by P − E hysteresis loops. The frequency variation of the dielectric constant was measured as well. PACS: 77.80; 68.55; 81.60

Ferroelectric niobate single crystals with the tungsten bronze structure are important because of their large pyroelectric coefficient, excellent piezoelectric and electro-optic properties, and their photorefractive sensitivity [1–3]. Recently, the demand for thin-film processing has increased due to the best development integrated-device [4]. For certain applications, epitaxy is not required and a high degree of film orientation is sufficient. This is most obvious for pyroelectric devices which only require the highest possible degree of polarization perpendicular to the film plane [5]. For the strontium barium niobate (SBN) or modified SBN materials, conventional techniques such as gas phase deposition, sputtering, chemical vapor deposition, and laser deposition were used to prepare thin films. Chen et al. fabricated SBN thin films on the single-crystal silicon and fused silica substrates by the sol-gel technique, but the films exhibited random orientation [2, 3]. Although highly oriented SBN thin films on MgO(100) substrates have been synthesized using metal alkoxide solutions, the crystallographic phase of those films was found to be a mixture of the orthorhombic low-temperature phase and the tetragonal tungsten bronze phase [6, 7]. Sakamoto et al. synthesized K(Sr0.75 Ba0.25 )2 Nb5 O15 thin films on MgO(100) and Pt(100)/MgO(100) substrates by the chemical solution deposition method, and observed a (001) preferred orientation of

the films [8]. Gao et al. reported the fabrication and properties of optical waveguiding SBN thin films on MgO-coated Si(111) substrates by a PLD process, but the films exhibit random orientation [9]. In spite of films with preferred orientations and good properties obtained on MgO substrates, it is still a challenge to grow good SBN-type films on silicon substrates in order to make progress in the integration of them into the silicon-based technology. In this paper, textured Pt/Ti bottom electrodes have been fabricated on SiO2 /Si substrates by high-vacuum electron-beam evaporation. (001)-textured SBLN thin films were subsequently prepared on these platinized silicon (Pt/Ti/SiO2 /Si) substrates by PLD, and their excellent ferroelectric and dielectric properties obtained are described here.

1 Experimental procedure The platinized silicon (Pt/Ti/SiO2 /Si) substrates were prepared on wafers of (100)Si, having a 600-nm-thick layer of thermally grown SiO2 , by coating the wafers with 20 nm Ti and 80 nm Pt using an UHV electron beam evaporator (Balzers UMS 500p). Before evaporation the vacuum chamber was initially pumped down to 10−7 Pa and during evaporation the pressure in the chamber fell to 10−6 Pa. The PLD target was prepared by conventional ceramic processing: The stoichiometric SBLN pellet was produced by isostatic pressing and was sintered at 1150 ◦ C. Platinized silicon substrates were mounted onto a heated holder and placed parallel to a target at the distance of 4 cm. Before deposition the chamber was initially pumped down to 5 Pa and a high-purity flow of 8 cm3 min−1 of oxygen was then introduced giving rise to attaining an approximate pressure of 10 Pa. Thickness of the SBLN films is about 1.35 µm, the films were deposited by PLD on the platinized silicon substrates at 650 ◦ C. The repetition frequency and pulse duration of the ArF excimer laser used are 3 Hz and 17 ns, respectively, and the energy per pulse delivered to the target was ≈ 3 J/cm2 . The deposition rate was in the range of 21.6–23.3 nm/min. The as-deposited SBLN

films were immediately annealed at 650 ◦ C for 1 h in an oxygen ambient. The crystallographic structure was investigated by X-ray diffraction (XRD), and the microstructure by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The composition of the films was characterized by X-ray energy dispersive spectroscopy (EDS). The P − E hysteresis loops of the films were evaluated by a modified Sawyer–Tower circuit. The measurement of the dielectric properties was performed using a HP4194A LCR meter. 2 Results and discussion In order to obtain preferred orientation SBLN films and good electric properties, Sr0.51 Ba0.48 La0.01 Nb2 O6 films deposited by PLD have to be annealed at 650 ◦ C during or after deposition. With this annealing step in mind we have first investigated the crystallinity and structure of Pt thin films at 650 ◦ C, annealing the platinized silicon (Pt/Ti/SiO2 /Si) substrates at this temperature. XRD patterns taken after annealing showed that all Pt grains were oriented with their (111) plane nearly parallel to the substrate surface. The Pt surface morphology and fine-scale surface features were revealed by atomic force microscopy and are shown in Fig. 1a. The Pt film has a columnar structure, with part of the grains protruding out of the surface, and with dense column boundaries. A plan view of the film is shown in Fig. 1b. It reveals the individual grains having the same azimuthal orientation, but different heights thus forming a stepped surface. The SBLN target with a Sr/Ba/La molar ratio of 51/48/1 had a tungsten bronze crystal structure, and the XRD pattern showed it was polycrystalline i.e. had no preferred orientation. The preferred orientations of the SBLN films prepared by PLD were found to vary with oxygen pressure, deposition temperature, annealing temperature, and thickness of the films. The oxygen pressure was the most important factor for the preferred orientations [10]. Figure 2 shows that the

Fig. 1. AFM micrographs of the Pt surface morphology

Fig. 2. XRD patterns of SBLN films before (a) and after (b) annealing treatment

preferred orientation of the film deposited at the above mentioned conditions is (001), before (as-deposited at 650 ◦ C) as well as after annealing treatment. The intensities of the (001) peaks are almost equal before and after annealing treatment, but the intensities of (002) peaks after annealing treatment are stronger than before annealing. The results indicate textured growth of grains with the c-axis perpendicular to the substance. From this measurement, a lattice constant of c = 3.93 Å was calculated, which is in good agreement with c = 3.935 Å expected for bulk SBN:61. From Fig. 1, electronbeam evaporated Pt thin films often show strong textured growth, which is dependent on the deposition conditions and the kind of substrate, and it has a cubic structure with Fm3m symmetry. Its lattice constant is 3.922 Å at room temperature. The SBLN film similar to SBN:61 has a tetragonal tungsten bronze phase and its lattice parameters are a = b = 12.449 Å, and c = 3.93 Å [11]. The lattice mismatch is therefore less than 0.20% for the c plane of SBLN grown on three unit cells of Pt. In ionic crystals, lattice match plays an important role in determining the epitaxial orientation of the overlayer, because Pt thin film is expected to result in a good match with the SBLN (001) plane thus greatly affecting the growth of the SBLN film. Meanwhile, according to a simple calculation, the mean free path of the ejected species from the target was 5 cm at 10 Pa, and this is longer than the substrate-to-target distance in our experiment. Therefore, the ejected species from the target did not scatter with ambient oxygen during their flight to the substrate. The kinetic energies of the incident species on the substrate are thus high, and the mobilities of the random walk are also high. Under this condition, the incident species had enough kinetic energy to arrange in the smallest free energy configuration. The surface free energy of crystallographic planes is also an important parameter, which affects the preferred orientation of films. The films will be oriented parallel to the plane that has the smallest surface free energy. As the nuclei are stable and grow further along (001) orientation, it may be that Pt film has a good match with the SBLN (001) plane and a low surface energy of the SBLN (001) plane. AFM observation indicated that the as-deposited SBLN thin film was very flat and had a columnar structure Fig. 3a. By comparison with Pt thin films, grain size and shapes of the SBLN thin films are very similar to those of the Pt thin films. Figure 3b and 3b0 show the surface annealing at 650 ◦ C.

Fig. 3a,b. AFM micrographs of SBLN thin films before (a,a0 ) and after (b,b0 ) annealing treatment

The roughness of the surface has changed from ≈ 65 Å on the as-deposited film to ≈ 55 Å after annealing at 650 ◦ C. This can probably be attributed to a release of compressive stress generated during the deposition process. Because Pt and SBLN film almost have the same grain size and shape, and the SBLN film had a columnar structure, it is probable that the atomic arrangement of a column near the Pt substrate was identical to that near the film surface. This structure is similar to the zone II structure of Thornton’s structure zone model (SZM) [12]. According to the SZM, the film with a zone II structure is described by the term “granular epitaxy” which emphasizes the role of the initial grains on the final film structure [13]. Because the lattice parameter of the Pt film is similar to the unit cell dimensions of SBLN (001), the texturing of the Pt thin film is expected to result in a good match with the SBLN (001) plane thus affecting the growth of the SBLN film. Those AFM results are in agreement with the above XRD patterns. This can be further confirmed that SBLN (001) orientation and growth mode are determined both by the Pt(111) substrate lattice matching by the SBLN (001) low surface energy. Figure 4a shows a cross-sectional XTEM image of the SBLN thin film annealed in oxygen ambient for 1 h. The multilayer structure is clearly observed. The interface between SBLN and the Pt layer is very sharp. The SBLN film is dense and has a columnar structure. The column width did not increase as the growth of the film proceeded, and the film has a microstructure corresponding to the zone II structure of the Thornton zone. This result is in agreement with the AFM results. A HRTEM image of a fragment of the SBLN structure taken along the [1¯ 10] crystallographic direction is shown in Fig. 4b. From this image, the (001) lattice fringes were observed in all the SBLN grains growing on flat Pt(111) planes, indicating (001) preferred orientation parallel to the substrate plane. A lattice constant of c = 4.16 Å was calculated according to formula d(001) = 2cTTB. This result has 6% error to compare with the XRD calculated result. It may be the scale bar has the problem of being too short. EDS spectra were ob-

tained from the SBLN films. EDS analysis indicated that the SBLN film has qualitative approximately the same stoichiometry as the target. Figure 5 shows the spreading resistance of a SBLN film. The multilayer structure is clearly revealed in agreement with the result of XTEM (Fig. 4). Thickness of the SBLN films is 1.35 µm from Fig. 5. Figures 6a and 6b show the ferroelectric hysteresis loops with a voltage of 60 V in peak-to-peak at a frequency of 1 kHz. The coercive field and the remnant polarization of the as-grown films (Fig. 6a) are smaller than for annealed samples (Fig. 6b). Figure 6b shows that E c and Pr are 53 kV/cm and 15.6 µC/cm2 , respectively. The remnant polarization of

Fig. 4a,b. Cross-sectional XTEM image of a SBLN thin film after annealing treatment (a), and a fragment of the ideal SBLN structure taken along the [1¯ 10] crystallographic direction (b)

Fig. 7. The dielectric constant and loss tangent of SBLN thin films before and after annealing treatment

Fig. 5. ASR measurement results of SBLN thin films after annealing treatmen

Fig. 6a,b. P − E hysteresis loops of SBLN films before (a) and after (b) annealing treatment (x axis: 48 kV/cm/div; y axis: 28 µC/cm2 /div)

our SBLN thin films is thus lower than that of SBN single crystals (27 µC/cm2 ) [14]. In comparison with SBN thin films made by sol-gel and chemical solution deposition methods, our PLD-grown SBLN films have a lower E c and higher Pr . These fairly good ferroelectric properties of our SBLN thin films may be related to the textured growth, large columnar grains, and to the dense microstructure. Increasing the volume percentage of the columnar grains with c-oriented SBLN thus obviously gives rise to a higher polarization as the c-axis is the polar axis of the SBLN [15]. The dielectric constant and loss tangent of our SBLN thin films for as-deposited and annealed samples are shown in Fig. 7. The dielectric constant decreases as the frequency increases. The dielectric loss tangent decreases first, and then increases with frequency. The dielectric constant of the annealed sample is larger than that of the as-deposited film. For example, at 1 kHz the dielectric constants are about 360 and 262 before and after annealing, respectively. The dielectric loss tangent is less than 7% at 1 kHz. The results revealed that our SBLN thin films have good ε( f) properties. 3 Conclusion Stoichiometric and well-textured ferroelectric SBLN thin films were grown on platinized silicon (Pt/Ti/SiO2 /Si) sub-

strates by pulsed excimer laser deposition combined with annealing. A columnar structure was found both for Pt films and SBLN films. The column width did not increase as the growth of the film proceeded, and was constant from the substrate across the Pt film to the SBLN film surface. The cross-section TEM images of SBLN films confirmed this result, showing that the SBLN films have a zone II structure in the Thornton zone. The SBLN (001) orientation and growth mode mainly are determined both by the Pt(111) substrate lattice matching and by the SBLN (001) low surface energy. The ferroelectric and dielectric properties of SBLN thin films show that textured SBLN films have good ferroelectric properties in comparison with results reported in the literature films. Acknowledgements. This work is supported by the National Advanced Materials Committee of China (NAMCC), No. 715-002-0110 and National Nature Foundation of China, No. 69738020.

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