APPLIED PHYSICS LETTERS
VOLUME 78, NUMBER 19
7 MAY 2001
Epitaxial growth of „103…-oriented ferroelectric SrBi2 Ta2 O9 thin films on Si„100… Ho Nyung Lee,a) Stephan Senz, Alain Pignolet, and Dietrich Hesse Max-Planck-Institut fu¨r Mikrostrukturphysik, Weinberg 2, D-06120 Halle/Saale, Germany
共Received 10 October 2000; accepted for publication 12 March 2001兲 Non-c-axis-oriented ferroelectric SrBi2 Ta2 O9 共SBT兲 epitaxial thin films with 共103兲 orientation have been grown by pulsed laser deposition on buffered Si共100兲 substrates. For the buffer layers, a heterostructure consisting of MgO共111兲/YSZ共100兲/Si共100兲 was applied to induce the growth of a 共111兲-oriented SrRuO3 共SRO兲 bottom electrode. X-ray diffraction –2 and scans revealed well-defined orientation relationships, viz. SBT共103兲储SRO共111兲储MgO共111兲储YSZ共100兲储Si共100兲; ¯ 1兴储MgO关01 ¯ 1兴储YSZ具001典储Si具001典. The ferroelectric measurements of the SBT关010兴储SRO关01 共103兲-oriented SBT films showed a remanent polarization 共P r 兲 of 5.2 C/cm2 and a coercive field 共E c 兲 of 76 kV/cm for a maximum applied electric field of 440 kV/cm. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1370984兴
Anisotropic properties of bismuth-layered perovskites such as SrBi2 Ta2 O9 共SBT兲 and SrBi2 Nb2 O9 共SBN兲 have been studied by means of epitaxial thin films having different crystallographic orientations grown on single crystalline substrates such as SrTiO3 共STO兲, LaSrAlO4 , and MgO.1–4 SBT or SBN thin films having 共001兲, 共116兲, and 共103兲 orientations have been grown, for example, on 共001兲-, 共011兲-, and 共111兲-oriented STO substrates, respectively, revealing that the three-dimensional epitaxial relationship ¯ 0兴储STO关100兴 is valid for all SBT共001兲储STO共001兲; SBT关11 cases of SBT thin films on STO substrates, irrespective of their orientations.1,2,4,5 The 共103兲-oriented SBT films showed higher remanent polarization and dielectric constant than the 共116兲-oriented SBT films, since epitaxial 共116兲- and 共103兲oriented SBT films have the direction of their spontaneous polarization lying 31° and 56° away from the film plane, respectively. Therefore, the 共103兲-oriented SBT 共or SBN兲 films have a larger vertical polarization component normal to the film plane than the 共116兲-oriented SBT 共or SBN兲 film.1,2 On the other hand, the c-axis-oriented SBT film revealed no ferroelectricity along the 关001兴 axis. The reason for this electrical anisotropy is that the vector of the spontaneous polarization in SBT is directed perpendicularly to the c axis, specifically along the a axis.6,7 Although SBT films with 共116兲 and 共103兲 orientations on STO have been reported, these are still far from practical significance for memory devices, because those complex oxide crystals are not suitable as substrates in microelectronics. For a better compatibility with silicon-based microelectronics, epitaxial SBT films should be grown on silicon substrates. Indeed, our previous reports have already shown that non-c-axis-oriented SBT films with a 共116兲 orientation can be grown on Si共100兲 substrates.8,9 In order to further increase the vertical component of the vector of the spontaneous polarization in SBT films, we investigated the possibility of epitaxial growth of 共103兲-oriented SBT films on buffered Si共100兲 substrates. In this letter, a兲
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we give results of such films deposited by pulsed laser deposition 共PLD兲. All films of SBT, SrRuO3 共SRO兲, MgO, and yttriastabilized ZrO2 共YSZ兲 were deposited in situ by PLD, employing a KrF excimer laser (⫽248 nm兲 operating at a repetition rate of 5 Hz with an energy density of 2–4 J/cm2 . The base pressure in the chamber was 5⫻10⫺8 mbar before raising the substrate temperature. p-Si共100兲 substrates were silver pasted on a stainless steel substrate holder, which was placed parallel to the target at a distance of 6 cm. The target was rotated at about 0.2 Hz during the deposition. The substrate temperature was measured with a K-type thermocouple positioned in the middle of the substrate holder. The epitaxial 50 nm-thick YSZ, 25 nm-thick MgO, 50 nm-thick SRO, and 250 nm-thick SBT thin films were deposited on Si共100兲 substrates at substrate temperatures of 800 °C 共YSZ and MgO兲, 775 °C 共SRO兲, and 740 °C 共SBT兲 in flowing O2 pressures of 1.7⫻10⫺3 mbar 共YSZ and MgO兲, 0.14 mbar 共SRO兲, and 0.4 mbar 共SBT兲, respectively. For the electrical characterization, Pt top electrodes with an area of 1.1⫻10⫺3 cm2 were deposited by rf-sputtering at room temperature. After top electrode deposition, the samples were annealed at 450 °C for 30 min in an oxygen atmosphere to stabilize the contact between SBT and Pt. The heterostructures were structurally characterized by x-ray diffraction 共XRD兲 –2 and scans using a Philips X’Pert MRD four-circle diffractometer. The polarization versus electric field 共P–E兲 hysteresis loops of 共103兲-oriented SBT films were recorded by a TF Analyzer 2000 ferroelectric tester 共AixACCT兲 at 100 Hz. Figure 1共a兲 shows the result of an XRD –2 scan of a SBT/SRO/MgO/YSZ/Si共100兲 structure. The figure displays x-ray spectra for 共103兲-oriented SBT, 共111兲-oriented SRO, 共111兲-oriented MgO, and 共100兲-oriented YSZ films, although there is a small contribution of SRO共110兲 domains and of impurity phases labeled as ( * ), most probably other Sr–Bi–Ta–O compounds, for instance pyrochlore- or fluorite-like phases. It is under study to characterize these impurity phases by transmission electron microscopy. All the Miller indices for SBT and SRO referred to in this work are based on orthorhombic 共a⫽0.5531 nm, b⫽0.5534 nm,
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Appl. Phys. Lett., Vol. 78, No. 19, 7 May 2001
FIG. 1. X-ray diffraction –2 scans 关共a兲 and 共b兲兴 of a SBT/SRO/MgO/ YSZ/Si共100兲 heterostructure. In Fig. 1共b兲, the –2 scan was recorded at ⫽55.1° optimized for the SBT 0010 គ reflection. The peaks of unidentified phases are labeled as (*).
and c⫽2.4984 nm兲 SBT and pseudocubic 共ac ⫽0.3928 nm兲 SRO lattice parameters, respectively.7,10 The orientation relationship between the thin films and the substrate, based on the XRD –2 result, is SBT共103兲储SRO共111兲储MgO共111兲储YSZ共100兲储Si共100兲. The full width at half maximum values of rocking curves of the SBT 4012, SRO 111, MgO 111, and YSZ 200 reflections are 2.2°, 1.9°, 1.5°, and 1.1°, respectively. Another XRD –2 scan optimized for the SBT 0010 reflection, taken at ⫽55.1°, revealed well-defined SBT共103兲 and SRO共111兲 orientations as shown in Fig. 1共b兲 ( ⫽90° corresponds to the substrate surface being parallel to the plane defined by the incident and reflected x-ray beams兲. SBT 00l and SRO h00 peaks are clearly seen revealing that SBT共001兲储SRO共100兲, which means that SBT共103兲储SRO共111兲, because the calculated tilt angles between the corresponding two planes of SBT and SRO are ⬔SBT共103兲:SBT共001兲⫽56.4° and ⬔SRO共111兲:SRO共100兲 ⫽54.7°, respectively 共note: The angle used to record the –2 scan was 55.1°). The unidentified peaks labeled as (*) might be related with the impurity phases mentioned earlier. In order to determine whether the SBT/SRO/MgO/ YSZ/Si heterostructure is epitaxial and to confirm the crystallographic orientation, various scans were conducted. Figure 2 shows scans of 共a兲 Si 111, 共b兲 YSZ 111, 共c兲 MgO 200, 共d兲 SRO 200, and 共e兲 SBT 113 reflections. The reflections were recorded at fixed angles of 39.8° for the SBT film and 54.7° for the other films. As already reported,8,11 a YSZ共100兲 film shows a four-fold growth symmetry on Si共100兲 substrates revealing a cube-on-cube epitaxy relationship as shown in Figs. 2共a兲 and 2共b兲.
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FIG. 2. X-ray diffraction scans of a SBT/MgO/SRO/YSZ/Si共100兲 heterostructure using the 共a兲 Si 111, 共b兲 YSZ 111, 共c兲 MgO 200, 共d兲 SRO 200, and 共e兲 SBT 113 reflections. The fixed angles were 54.7° for Figs. 2共a兲–2共d兲 and 39.8° for Fig. 2共e兲.
Interestingly, the epitaxial orientation relationship between the MgO film and the YSZ film does not follow the MgO共100兲储YSZ共100兲 relationship nor the MgO共110兲储YSZ共100兲 relationship although the latter had been reported before.12 In a scan of the MgO 200 reflection 关Fig. 2共c兲兴 12 peaks were recorded. These peaks correspond to a three-fold symmetry with four different positions of MgO共111兲 domains on the YSZ共100兲 film. The 12 symmetric peaks in the scan of Fig. 2共c兲 provide reliable evidence for the specific in-plane-epitaxial growth pattern of the MgO共111兲 film on the YSZ共100兲 film illustrated in Fig. 3. In Fig. 3, four types of azimuthally rotated domains 共rotated in-plane by 0°, 90°, –90°, and 180° around the normal to the substrate surface兲 are schematically sketched. The four orientation variants result in the 12 scan peaks 关Fig. 2共c兲兴, with a characteristic separation angle of 30° between the neighboring peaks. From these XRD results and the schematic drawing, the epitaxial orientation relationship between MgO共111兲 and YSZ共100兲 films is determined as ¯ 1兴储YSZ具001典, taking into acMgO共111兲储YSZ共100兲; MgO关01 ¯ count that the MgO关01 1兴 direction may be parallel to any of ¯ 0兴, 关001兴, and 关001 ¯ 兴. The the four YSZ directions 关010兴, 关01 ¯ 储 misfit value 共d film-d sub)/d sub along the MgO关 2 11兴 YSZ关010兴 ¯ 11兲MgO and direction was calculated from d film⫽d(2 d sub⫽d(010)YSZ as ⬃0.4%. That along the ¯ 1兴储YSZ关001兴 direction amounts to ⬃15.9%. A simiMgO关01 lar growth trend of MgO共111兲 thin films on YSZ共100兲/ Si共100兲 substrates had been observed before.13,14 From the scan of the SRO 200 reflection at ⫽45° shown in Fig. 2共d兲, an epitaxial growth was revealed for the SRO共111兲 film on the MgO共111兲 film, with the corresponding epitaxial relationship SRO共111兲储MgO共111兲; ¯ 1兴储MgO关01 ¯ 1兴. The SRO film also consists of four SRO关01
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FIG. 4. P–E hysteresis loop of a Pt/SBT共103兲/SRO共111兲 capacitor on a MgO共111兲/YSZ共100兲/Si共100兲 heteroepitaxial substrate. FIG. 3. Schematic drawing of the orientation relationship between the top MgO共111兲 film and the bottom YSZ共100兲 film. The MgO共111兲 plane is rotated in-plane by 共a兲 0°, 共b兲 180°, 共c兲 –90°, and 共d兲 90° around the normal to the substrate surface. The squares represent unit cells of YSZ, seen along the 关100兴 direction. The hatched and nonhatched triangles represent unit cells for MgO, seen along the 关111兴 direction, whereby the hatched and nonhatched planes protrude out of and recede below the 共100兲 YSZ plane, respectively.
azimuthal domains, which are directly ‘‘replicated’’ from the four azimuthal domains of the MgO film. 共The lattice misfit along the 关001兴SRO储关001兴MgO direction is about –6.7%.兲 In Fig. 2共d兲, there is a small contribution originating from a shoulder of the SBT 206 peak, since the fixed angles of 2 ⫽46.18° and ⫽54.7° to record the scan using the SRO 200 reflection are close to the angles 2 and of the SBT 110 reflection in the 共103兲-oriented SBT film, at ⫽53.9°. A triple-domain positioning of the SBT film on each of the four azimuthal domains of the SRO film was revealed for the 共103兲-oriented SBT film on the SRO/MgO/YSZ/Si heterostructure, as shown in Fig. 2共e兲. The 共103兲oriented SBT film growing on these four azimuthal domains of the 共111兲-oriented SRO film turns out to consist of 12 corresponding azimuthal domains 共or multiple twins兲. The epitaxial relationships between the films and the substrate, based on all the XRD results, are SBT共103兲储SRO共111兲储MgO共111兲储YSZ共100兲储Si共100兲; ¯ 1兴储MgO关01 ¯ 1兴储YSZ具001典储Si具001典. SBT关010兴储SRO关01 Figure 4 shows a ferroelectric hysteresis loop revealing a remanent polarization 共P r 兲 of 5.2 C/cm2 and a coercive field 共E c 兲 of 76 kV/cm for a maximum applied electric field of 440 kV/cm. By comparing the P r value with that of a 共116兲-oriented SBT film on a buffered Si共100兲 substrate ( P r ⬍4 C/cm2 ),9 it is concluded that the growth of non-caxis-oriented SBT films, which have the direction of the spontaneous polarization closer to the film normal, is advan-
tageous for the application of these films as a capacitor material to planar-type ferroelectric random access memories. However, the overall value of P r is lower than the reported values for SBT films grown epitaxially on STO substrates1,4 as well as for SBN films on STO.2 This might be a result of a worse crystallinity 共presence of impurity phases兲 and/or the presence of multiple twins in the SBT films on buffered-Si substrates. Work to further improve the film quality is in progress. 1
H. N. Lee, A. Visinoiu, S. Senz, C. Harnagea, A. Pignolet, D. Hesse, and U. Go¨sele, J. Appl. Phys. 88, 6658 共2000兲. 2 J. Lettieri, Y. Jia, M. Urbanik, C. I. Weber, J.-P. Maria, D. G. Schlom, H. Li, R. Ramesh, R. Uecker, and P. Reiche, Appl. Phys. Lett. 73, 2923 共1998兲; J. Lettieri, M. A. Zurbuchen, Y. Jia, D. G. Schlom, S. K. Streiffer, and M. E. Hawley, ibid. 76, 2937 共2000兲. 3 S. E. Moon, T. K. Song, S. B. Back, S.-I. Kwun, J.-G. Yoon, and J. S. Lee, Appl. Phys. Lett. 75, 2827 共1999兲. 4 K. Ishikawa and H. Funakubo, Appl. Phys. Lett. 75, 1970 共1999兲; K. Ishikawa, A. Saiki, and H. Funakubo, Jpn. J. Appl. Phys., Part 1 39, 2102 共2000兲. 5 T. Nagahama, T. Manabe, I. Yamaguchi, T. Kumagai, T. Tsuchiya, and S. Mizuta, Thin Solid Films 353, 52 共1999兲. 6 R. E. Newnham, R. W. Wolfe, and J. F. Dorrian, Mater. Res. Bull. 6, 1029 共1971兲. 7 A. D. Rae, J. G. Thompson, and R. L. Withers, Acta Crystallogr., Sect. B: Struct. Sci. 48, 418 共1992兲. 8 H. N. Lee, S. Senz, A. Visinoiu, A. Pignolet, D. Hesse, and U. Go¨sele, Appl. Phys. A: Mater. Sci. Process. 71, 101 共2000兲. 9 H. N. Lee, S. Senz, N. D. Zakharov, C. Harnagea, A. Pignolet, D. Hesse, and U. Go¨sele, Appl. Phys. Lett. 77, 3260 共2000兲. 10 C. B. Eom, R. J. Cava, R. M. Fleming, J. M. Phillips, R. B. van Dover, J. H. Marshall, J. W. P. Hsu, J. J. Krajewski, and W. F. Peck, Jr., Science 258, 1766 共1992兲. 11 D. K. Fork, D. B. Fenner, G. A. N. Connell, J. M. Phillips, and T. H. Geballe, Appl. Phys. Lett. 57, 1137 共1990兲. 12 X. D. Wu, L. Luo, R. E. Muenchausen, K. N. Springer, and S. Foltyn, Appl. Phys. Lett. 60, 1381 共1992兲. 13 P. A. Stampe and R. J. Kennedy, J. Cryst. Growth 191, 472 共1998兲. 14 C. H. Lei, C. L. Jia, M. Siegert, J. Schubert, Ch. Buchal, and K. Urban, J. Cryst. Growth 204, 137 共1999兲.
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