Applied Surface Science 252 (2006) 8756–8759 www.elsevier.com/locate/apsusc
Texture control of Pb(Zr, Ti)O3 thin films with different post-annealing processes S.W. Jiang *, Q.Y. Zhang, W. Huang, B. Jiang, Y. Zhang, Y.R. Li School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, China Received 27 June 2005; received in revised form 8 December 2005; accepted 12 December 2005 Available online 24 January 2006
Abstract The non-crystalline Pb(Zr, Ti)O3 thin films sputtered on Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrates at room temperature were crystallized by conventional furnace annealing (CFA) and rapid thermal annealing (RTA), respectively. It was found that the RTA process favored the (1 1 1)preferred orientation in lead zirconate titanate (PZT) thin films while the CFA process favored the (1 0 0)-preferred orientation. The origin of the different orientation selection might be due to the different epitaxial nucleation mechanism. The long heating duration would lead to the aggregation of Pb and the formation of PbO(1 0 0) on film surface; therefore, the nucleation at the PbO(1 0 0)/PZT interface on film surface might lead to the (1 0 0)-preferred orientation. However, the nucleation at the PZT/Pt(1 1 1) electrode interface by RTA process would result in the formation of (1 1 1)-preferred orientation. The RTA-derived (1 1 1)-preferentially oriented PZT thin films exhibited a high remnant polarization of 35 mC/cm2. # 2005 Elsevier B.V. All rights reserved. PACS: 81.10.Aj; 77.55.+f; 64.70.Kb; 68.55.Jk Keywords: Lead zirconate titanate; Post-annealing; Film orientation; Ferroelectricity
1. Introduction Lead zirconate titanate Pb(Zr, Ti)O3 (PZT) thin films have shown excellent ferroelectric characteristics for non-volatile ferroelectric random access memories (NVFeRAM) . For applications, the presence of preferred orientations in PZT thin films is a key factor that determines the final ferroelectric properties of thin films. For example, (1 1 1)-oriented PZT thin films are usually regarded to be desirable for NVFeRAM application due to the large remnant polarization. However, the PZT thin films are typically prepared at relatively low temperatures, such as by sol–gel method, then post-annealed at higher temperatures to form well-crystallized ones. Therefore, it is usually difficult to control the formation of preferred orientation in PZT thin films.
* Corresponding author. Tel.: +86 28 83202140; fax: +86 28 83202569. E-mail address: [email protected]
(S.W. Jiang). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.12.112
Much effort on preparing well-oriented PZT thin films has been focused on the effect of substrates [2,3], buffer layers between film and substrate [4,5], annealing temperature , annealing atmosphere , Zr/Ti atomic ratio , etc. In recent years, post-annealing treatments using the rapid thermal annealing (RTA) technique, instead of conventional furnace annealing (CFA) technique, have been popularly adapted to crystallize PZT thin films because this method offered a low thermal budget process, and therefore led to reduction in interface diffusion and minimization of Pb loss. Araujo and Eiras  reported that the PZT films crystallized by the RTA method showed higher remnant polarization and lower coercive field than those by CFA method. However, to our knowledge, few researches on film orientation selection and its influence on the ferroelectric properties resulting from different postannealing techniques have been reported. In this paper, non-crystalline PZT thin films prepared on Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrates by sputtering at low temperatures are post-annealed by means of rapid thermal annealing and conventional furnace annealing, respectively.
S.W. Jiang et al. / Applied Surface Science 252 (2006) 8756–8759
The different preferred orientations in the post-treated PZT thin films are observed. The PZT thin films using RTA process present (1 1 1)-preferred orientation, whereas the PZT thin films using CFA process show (1 0 0)-preferred orientation. The origin of the orientation selection in PZT thin films has been discussed. 2. Experiments Non-crystalline PZT thin films with the thickness of 200 nm were deposited onto Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrates at room temperature in an rf-magnetron sputtering system (ULVAC MPS-5000-FCI). Both the Ti (50 nm) and Pt (150 nm) films were prepared on thermally oxidized Si(1 0 0) substrates prior to PZT deposition. The as-deposited PZT thin films were annealed at temperatures from 500 to 700 8C in O2 gas atmosphere by two different processes of RTA and CFA. The RTA process with a heating rate of 50 8C/s and short annealing duration of 1 min was carried out in an infraredheated rapid heating system (ULVAC-RIKO RTP-6), while the CFA process with a slow heating rate of 20 8C/min and long annealing duration of 10 min was carried out in a conventional tube furnace. In order to lower the loss of Pb, PZT thin films was sealed in an Al2O3 crucible during CFA process. The crystal structure and orientation of the PZT thin films were analyzed using X-ray diffraction (XRD, BEDE D1). The surface morphology of the PZT thin films was observed using atom force microscopy (AFM, Seiko SPA-300HV). In order to investigate the ferroelectric properties of the prepared PZT thin films, top Pt electrodes of 0.3 mm diameter were deposited by vacuum evaporation. The ferroelectric polarization versus electric field was measured at a frequency of 1 kHz using a ferroelectric tester (Radiant Precision LC2000). 3. Results and discussion The XRD patterns for CFA-annealed PZT thin films at different temperatures are shown in Fig. 1, from which we can see the evolution of preferred orientations from random orientation to (1 0 0)-preferred orientation in PZT thin films. When the annealing temperature was below 530 8C, the random-oriented perovskite PZT thin films with (1 0 0), (1 1 0) and (1 1 1) components were obtained, and there also existed traces of pyrochlore phase. When annealing temperature was increased to 560 8C, the intensity of (1 0 0) and (2 0 0) peaks increased while that of (1 1 1) and (1 1 0) peaks decreased, indicating that (1 0 0)-preferred orientation became dominant. In the meantime, pyrochlore phase entirely disappeared. At 600 8C, almost pure (1 0 0)-preferred orientation in PZT thin films was observed. Fig. 2 shows the XRD patterns of RTA-annealed PZT thin films at various temperatures. Due to short annealing duration, the annealing temperature for RTA process was little higher than that for CFA process. No peaks of perovskite phase except pyrochlore phase were observed at 550 8C. At the annealing temperatures up to 600 8C and higher, the perovskite structure appeared and the PZT thin films showed (1 1 1)-preferred orientation. When the annealing temperature was increased to
Fig. 1. The XRD patterns of PZT thin films post-annealed at 510, 530, 560 and 600 8C by CFA process.
700 8C, the intensity of (1 1 1) perovskite PZT peak was greatly increased. This result indicated that the (1 1 1) orientation selection in PZT thin films post-annealed by RTA process is quite different from the (1 0 0) orientation by CFA process. Thus, it can be seen that the film orientation in PZT thin films is very sensitive to the heating rates of post-annealing processes. For the crystallization during post-annealing, the nuclei may form at the film/substrate interface, film surface and the defects in films. In general, random nucleation at the film surface and the defects would lead to the random orientation in thin films , while epitaxial nucleation at the lattice-matched film/ substrate interface would have a tendency to form the preferentially oriented thin films. All nucleation processes thermodynamically and kinetically compete with others. The interface between film and Pt(1 1 1) electrode is considered to play an important role in the evolution of perovskite formation and orientation. The (1 1 1)-oriented Pt electrode is favored for the preparation of (1 1 1)-preferred PZT thin films, for the
Fig. 2. The XRD patterns of PZT thin films post-annealed at 550, 600, 650 and 700 8C by RTA process.
S.W. Jiang et al. / Applied Surface Science 252 (2006) 8756–8759
PZT(1 1 1) is liable to nucleate epitaxially at the interface between film and Pt(1 1 1) electrode. However, the (1 1 1)preferred PZT thin films are not always obtained. This can be confirmed by our above CFA-annealed results and others [11,12]. We conjecture that CFA and RTA processes have led to the different nucleation mechanisms due to their different heating rates, thus the different film orientation in the PZT thin films. The surface morphologies of the PZT thin films revealed by the AFM are shown in Fig. 3. In the case of CFA process, the film surface was characterized by the large grains aggregates dotted by white particles (Fig. 3a). Energy dispersive spectroscopy
(EDS) analysis showed that these white particles are Zr, Tidepleted and Pb-enriched phases. We deduced that these Pbenriched phases could be dominantly PbO. Pb in the inner films evaporated out during the long heating duration of CFA process and consequently aggregated on the film surface in sealed crucible. Es-Souni et al.  had observed similar aggregation of Pb-enriched phases on film surface for La-doped PZT thin films. We speculate that due to the existence of these PbO particles, the nucleation on the film surface would be dominant mechanism with PbO particles as nucleation seeds during the crystallization. The PbO seeds took (1 0 0)-preferred orientation because the (1 0 0) planes had a lower surface energy, and the misfit at the PbO(1 0 0)/PZT interface is 2.5% along [1 0 0] axis, which is acceptable for epitaxial nucleation. Gong et al.  had reported the PbO buffer layer to be efficient seeding layer for the formation of (1 0 0)-oriented PZT thin films. On the other hand, the (1 0 0) plane has the lowest surface energy among (1 0 0), (1 1 0) and (1 1 1) planes for PZT , indicating that the (1 0 0)-preferred nucleation at the PbO(1 0 0)/PZT interface might be thermodynamically favorable. We therefore infer that (1 0 0)-preferred nucleation at the PbO(1 0 0)/PZT interface on the film surface has led to the formation of the (1 0 0)-preferred PZT thin films by CFA process. In contrast, for the PZT thin films post-annealed by RTA process, the dense flat surface with no PbO particles was observed (Fig. 3b). The evaporation of Pb component in PZT thin films had been effectively suppressed owing to the short heating duration. In this case the epitaxial nucleation at the PZT/Pt(1 1 1) interface would be the dominant mechanism. Certainly this would lead to the (1 1 1)-preferred orientation in PZT thin films. The hysteresis loops of the PZT films with (1 0 0)- and (1 1 1)-preferred orientations obtained by means of RTA and CFA, respectively, are shown in Fig. 4, from which the orientation dependence on ferroelectric properties was demonstrated. In the case of the (1 1 1)-preferred PZT thin films annealed by RTA at 650 8C, good ferroelectricity with a remnant polarization 2Pr of 35 mC/cm2 was observed. As to the (1 0 0)-preferred PZT thin films annealed by CFA at 600 8C, the
Fig. 3. The AFM micrographs of the surface morphology for the: (a) (1 0 0)oriented PZT thin films and (b) (1 1 1)-oriented PZT thin films.
Fig. 4. The ferroelectric hysteresis loops of the PZT thin films with different preferred orientations.
S.W. Jiang et al. / Applied Surface Science 252 (2006) 8756–8759
remnant polarization 2Pr was only 15 mC/cm2. This result indicated that RTA-treated PZT thin films exhibited higher remnant polarization than the CFA-treated. Accordingly, posttreatment by RTA process is an effective way to produce the (1 1 1)-preferred PZT thin films with a large remnant polarization desirable for NvFeRAM usage. 4. Conclusion In conclusion, the preferred orientation and ferroelectric properties of PZT thin films crystallized by CFA and RTA processes have been investigated, respectively. It was found that the RTA process favored the (1 1 1)-preferred orientation in PZT thin films while the CFA process favored the (1 0 0)preferred orientation. The different orientation selection by CFA and RTA processes might be originated from their different nucleation mechanism. The nucleation at the PbO(1 0 0)/PZT interface on film surface by CFA process might lead to the (1 0 0)-preferred orientation, while nucleation at the PZT/Pt(1 1 1) electrode interface by RTA process would result in the formation of (1 1 1)-preferred orientation. The RTA-derived (1 1 1)-preferentially oriented PZT thin films exhibited a high remnant polarization of 35 mC/cm2.
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