Metal-Organic Chemical Vapor Deposition of Ferroelectric

Metal–organic chemical vapor deposition of Bi2Mn4O10 films on SrTiO3 〈100〉. Graziella Malandrino , Zaira Lipani , Roberta G. Toro , Maria E. Fra...
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Chem. Mater. 2006, 18, 1016-1022

Metal-Organic Chemical Vapor Deposition of Ferroelectric SrBi2Ta2O9 Films from a Fluorine-Containing Precursor System G. G. Condorelli,*,† M. Favazza,† C. Bedoya,† A. Baeri,† G. Anastasi,† R. Lo Nigro,‡ N. Menou,§ C. Muller,§ J. G. Lisoni,| D. Wouters,| and I. L. Fragala`† Dipartimento di Scienze Chimiche, UniVersita` di Catania, Via Andrea Doria 6, 95125 Catania, Italy, IMM-CNR, Stradale Primosole 50, 95121 Catania, Italy, L2MP, Laboratoire Mate´ riaux et Microe´ lectronique de ProVence, UMR CNRS 6137, UniVersite´ du Sud Toulon Var, BP 20132, F-83957 La Garde Cedex, France, and IMEC, InteruniVersity MicroElectronics Center, Kapeldreef 75, B-3001 LeuVen, Belgium ReceiVed May 31, 2005. ReVised Manuscript ReceiVed NoVember 21, 2005

MOCVD fabrication of ferroelectric SrBi2Ta2O9 (SBT) films using Sr(hfac)2‚tetraglyme, Bi(C6H5)3, and Ta(OC2H5)5 precursors is reported. The SBT phase has been reproducibly obtained adopting a twostep procedure, namely, the deposition of a fluorine-containing Sr-Bi-Ta-O(F) matrix followed by the annealing step at 800 °C. The multicomponent deposition process was optimized by tuning the overall process to individual kinetics associated with each singled-out precursor, to film morphologies and microstructures, and finally to film properties. Particular attention has also been focused on the annealing process of the deposited Sr-Bi-Ta-O(F) matrix aimed at an efficient crystallization step of the SBT phase as well as on an efficient elimination of fluorine phases associated with decomposition of the fluorinated Sr precursors.

Introduction SrBi2Ta2O9 (SBT) ferroelectric films have been extensively investigated for new, nonvolatile ferroelectrics memory applications (NVFeRAM) due to their superior endurance.1-2 SBT films have been prepared by several techniques including sol-gel,3 metal-organic deposition (MOD),4 pulsed laser ablation,5 and metal-organic chemical vapor deposition (MOCVD).6-11 Among them, the MOCVD technique is most well-suited for the semiconductor industry, especially due to the higher step coverage compared to other deposition techniques.12-14 Nevertheless, the choice of a good set of * Corresponding author. Tel.: ++39-95-7385069. Fax: ++39-95-580138. E-mail: [email protected]. † Universita ` di Catania. ‡ IMM-CNR. § Universite ´ du Sud Toulon Var. | Interuniversity MicroElectronics Center.

(1) Araujo, C. A.; Cuchiaro, J. D.; McMillan, L. D.; Scott, M. C.; Scott, J. F. Nature 2002, 374, 627. (2) Funakubo, H. Top. Appl. Phys. 2004, 93, 95. (3) Boyle, T. J.; Bucheit, C. D.; Rodriguez, M. A.; Al-Shareef, H. N.; Hernandez, B. A.; Scott, B.; Ziller, J. W. J. Mater. Res. 1996, 11, 9. (4) Desu, S. B.; Li, T. K. Mater. Sci. Eng. B 1995, 32, 562. (5) Desu, S. B.; Vijay, D. P. Mater. Sci. Eng. B 1995, 32, 75. (6) Li, T.; Zhu, Y.; Desu, S. B.; Peng, C. H.; Nagata M. Appl. Phys. Lett. 1996, 68, 5. (7) Nukaga, N.; Ono, H.; Shida, T.; Machida, H.; Suzuki, T.; Funakubo, H. Integr. Ferroelectr. 2002, 45, 215. (8) Shin, D. S.; Hoi, H. S.; Kim, Y. T.; Choi, I. H. J. Cryst. Growth 2000, 209, 1009. (9) Nukaga, N.; Funakubo, H. Jpn. J. Appl. Phys. 1999, 38, 5428. (10) Isobe, C.; Ami, T.; Hironaka, K.; Watanabe, K.; Sugiyama, M.; Nagel, N.; Katori, K.; Ikeda, Y.; Gutleben, C. D.; Tanaka, M.; Yamoto, H.; Yagi, H. Integr. Ferroelectr. 1997, 14, 95. (11) Isobe, C.; Ami, T.; Hironaka, K.; Hishikawa, S. AdV. Mater. Opt. Electron. 2000, 10, 183. (12) Ramesh, R.; Aggarwal, S.; Auciello, O. Mater. Sci. Eng. R-Rep. 2001, 32. (13) Zambrano, R. Mater. Sci. Semicond. Process. 2002, 5, 305. (14) Jones, A. C.; Chalker, P. R. J. Phys. D: Appl. Phys. 2003, 36, 80.

metal-organic source precursors is still an open question and still represents the major challenge. In this context, any industrial application requires a careful and reproducible control of the properties of films and, in turn, the full understanding of parameters affecting the film growth. In this perspective, both kinetics and the deposition mechanisms of precursors have critical roles as far as the industrial scaling up is concerned. To date, the most used strontium precursors consist of fluorine-free Sr β-diketonates often adducted with neutral ligands (polyethers and polyamines). Several studies have also been reported for the kinetics of multicomponent SBT depositions using a fluorine-free precursor system such as Bi(CH3)3-Sr[Ta(OC2H5)6]2,15 Bi(C6H5)3-Sr[Ta(OC2H5)5]2,16 and Sr(tmhd)2-Bi(C6H5)3-Ta(OC2H5)517 (tmhd ) tetramethylheptandione) systems. However, fluorine-free Sr β-diketonates present low thermal stabilities that preclude easy and clean sublimation/evaporation processes and also cause significant problems during storage and chemical manipulation.18,19 It is known that the introduction of fluorinecontaining moieties in metal β-diketonates improves precursor volatility.20 In particular, Sr hexafluoroacetylacetonates (15) Funakubo, H.; Nukaga, N.; Ishikawa, K.; Kokubun, H.; Machida, H.; Shinozaki, K.; Mizutani, N. Ferroelectrics 1999, 232, 123. (16) Jimbo, T.; Sano, H.; Takahasmi, Y.; Funakubo, H.; Tokumitsu, E.; Ishiwara, H. Jpn. J. Appl. Phys. 1999, 38, 6456. (17) Zhu, Y.; Desu, S. B., Li, T.; Ramanathan, S.; Nagata, M. J. Mater. Res. 1997, 12, 3. (18) Weiss, F.; Lindner, J.; Senateur, J. P.; Dubourdieu, C.; Galindo, V.; Audier, M.; Abrutis, A.; Rosina, M.; Frohlich, K.; Haessler, W.; Oswald, S.; Santiso, A. F. Surf. Coat. Technol. 2000, 133-134, 191. (19) Gardiner, R.; Brown, D. W.; Kirlin, P. S.; Rheingold, A. L. Chem. Mater. 1991, 3, 10539. (20) Schulz, D. L.; Marks, T. J. In CVD of Nonmetals; Rees, W. S., Jr., Ed.; VCH: Weinheim, Germany, 1996; p 71.

10.1021/cm051151+ CCC: $33.50 © 2006 American Chemical Society Published on Web 01/28/2006

MOCVD of Ferroelectric SrBi2Ta2O9 Films

adducted with neutral ligands such as polyethers have been shown21,22 to possess better properties in terms of stability during the sublimation and transport processes. Despite these advantages, the use of fluorine-containing precursors for SBT deposition has been poorly investigated.23 Some preliminary studies have been reported on the use of the Sr(hfac)2‚ tetraglyme (H-hfac)1,1,1,5,5,5-hexafluoroacetylacetone), Ta(OC2H5)5, and Bi(C6H5)3 precursor system for the formation of SBT phases,22b but no electrical tests have been performed and little information on deposition kinetics associated with the multicomponent precursor kinetics are available. In this context, the present paper reports on a fully comprehensive MOCVD exploitation of the multiple-source (Sr(hfac)2‚ tetraglyme, Ta(OC2H5)5, and Bi(C6H5)3) precursor system. The capabilities of the present precursor system have been evaluated in terms of morphological, structural, and electrical properties of deposited film. Attention has been paid to both elimination of fluorine phases as well as the study of kinetics and mechanisms of multicomponent deposition. The latter point is of relevance since the knowledge of the precursor chemistry during the MOCVD process is a key point for the process optimization itself and, hence, for the evaluation of the combination of adopted precursor. Experimental Section Sr(hfac)2‚tetraglyme was synthesized from Sr(OH)2_8H2O (STREM chemicals Inc.) and Hhfac and tetraglyme (Aldrich) according to the procedure previously reported.21,22,24 Bismuth triphenyl (Bi(C6H5)3) (Merck, 99+%) is a moist-stable white powder (mp 78 °C). The vapor pressure, estimated with a bubbler apparatus, is around 97.5 mTorr at 80 °C.14 Tantalum pentaethoxide (Ta(OC2H5)5) (Aldrich, 99+%) is a colorless liquid very sensitive to hydrolysis. The vapor pressure estimated is around 750 Torr at 160 °C and its thermal decomposition occurs at 175 °C. The decomposition byproduct has an extremely low vapor pressure and is moisture-unstable. Mono- and multicomponent depositions were performed in a MOCVD horizontal, cold wall reactor. A detailed description of the experimental apparatus for single deposition has been reported elsewhere.25 Briefly, the MOCVD reactor, made of an 80 cm length quartz tube (i.d. ) 8 cm), consists of contiguous sections for precursor sublimation (three subsections), gas mixing, and film deposition (Figure 1). Each section was independently heated within a (2 °C error margin, using computer-controlled hardware. The partial pressure of each precursor was varied by tuning the sublimation temperature and calculated from the total quantity of the sublimed precursor determined by weight loss measurements according to the transpiration process reported by Temple and Reisman.26 Films were deposited on Pt/TiO2/PSG/Si technological substrates consisting of TiO2 and Pt, deposited by a conventional sputtering (21) Condorelli, G. G.; Baeri, A.; Fragala`, I. L. Chem. Mater. 2002, 14, 4307. (22) (a) Bedoya, C.; Condorelli, G. G.; Di Mauro, A.; Anastasi, G.; Fragala`, I. L.; Lisoni J.; Wouters, D. Mater. Sci. Eng. B 2005, 118, 264. (b) Condorelli, G. G.; Baeri, A.; Anastasi, G.; Fragala`, I. L. Mater. Sci. Semicond. Process. 2003, 5, 167. (23) Seong, N. J.; Yoon, S. G.; Lee, S. S. Appl. Phys. Lett. 1997, 71, 81. (24) Malandrino, G.; Castelli, F.; Fragala I. L. Inorg. Chim. Acta 1994, 224, 203. (25) Condorelli, G. G.; Gennaro, S.; Fragala`, I. L. Chem. Vap. Deposition 2000, 6, 185. (26) Temple, D.; Reisman, A. J. Electrochem. Soc. 1989, 11, 136.

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Figure 1. Schema of the multicomponent MOCVD reactor.

technique on p-type Si(100) with 250 nm phosphorus-doped SiO2 (PSG). TiO2 was used as an adhesion layer for the metal films. The multilayer substrates were placed on the susceptor (Advanced Ceramics Boralectric Heating Element) and heated between 300 and 650 °C in a total pressure range of 6-10 Torr. Note that due to the presence of the heated susceptor, the temperature of the reactor wall ranged from 200 to 450 °C depending on the susceptor temperature. This effect can account for the precursor depletion on the reactor wall reported in the following sections. Typical MOCVD conditions adopted prepurified Ar (99.999%) as carrier gas (100 sccm) and O2 (99.999%) as reaction gas (500 sccm). The typical deposition time was 30 min. The Sr-Bi-Ta-O(F) matrixes obtained with the MOCVD processes were annealed in the 600800 °C range (PTOT ) 1 atm) for 1 h under oxygen flow (500 sccm). Grazing incident X-ray diffraction (GIXRD) measurements were performed with a Bruker AXS D5005 X-ray diffractometer equipped with a copper anode operated at 40 kV and 30 mA, Soller slits, Go¨ebel mirror, and an attachment for thin film measurements. Measurements were performed with a detector scan from 10° < 2θ < 60°. The angle between the X-ray source and the sample surface was fixed at 0.5°. Surface morphologies and film compositions were analyzed by scanning electron microscopy (SEM), performed with a LEO 1400 microscope equipped with energy dispersive X-ray (EDX) microanalysis. Film thicknesses (d) were estimated from attenuation of pertinent EDX intensities, namely, the ratio of total Sr KR (ISr), Bi and Ta MR (IBi and ITa) intensities relative to that of Pt MR (ISub) features. The related calibration curve ((ISr + IBi + ITa)/ISub vs d) was determined using standard samples with thicknesses directly measured by SEM cross sections. Growth rates (GA; A ) SrF2, Bi2O3, and TaOx) of each component of the Sr-Bi-Ta matrix were determined using the total thickness and the atomic fractions (XA) determined from EDX measurements (GA (nm/h) ) XA‚d (nm)/t (h) where t is the total deposition time). X-ray photoelectron spectroscopy (XPS) measurements were performed with a PHI 5600 Multi Technique System equipped with an Al standard X-ray source operating at 14 kV and a hemispherical analyzer. The electron takeoff (θ) was 45°. Depth profiles were obtained by alternating sputter etching rastered over a 3 × 3 mm2 area (with a 4 kV argon ion gun) and XPS analysis. Binding energy (B.E.) was calibrated by reference to the C1s signal of the adventitious carbon (285.0 eV).27,28 Topographic characterizations were performed by AFM measurements using a Solver P47 NT-MDT Co in semicontact mode. Images were obtained adopting a Si tip with a resonance frequency of 200 kHz. The noise level before and after each measurement was 0.01 nm. Ferroelectric characterizations were carried out using the aixACCT TF Analyzer 2000 system. Dynamic Hysteresis Measurements (DHM) were performed using triangular waveform with an amplitude of 10 V and a frequency of 100 Hz. (27) Teterin, Y. A.; Sosulnikiv, M. I. Physica C 1993, 212, 306. (28) Swift, P. Surf. Interface Anal. 1982, 4, 47.

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Figure 2. Dependence of growth rates vs reciprocal deposition temperature of SrF2, Bi2O3, and TaOx during the multicomponent deposition process. Inset shows the temperature dependence during single-source deposition processes.

Results and Discussion Temperature Dependence of Growth Rate. Independent studies on the MOCVD behaviors of single precursors represent, of course, the necessary steps to understand the mutual influences among Sr, Bi, and Ta precursors in the SBT phase fabrication. Our previous works have elucidated individual deposition processes from Sr(hfac)2‚tetraglyme, Bi(C6H5)3, and Ta(OC2H5)5 sources.21,22,29,30 Thus, depositions from Sr(hfac)2‚tetraglyme and Bi(C6H5)3 lead to the formation of polycrystalline SrF2 and Bi2O3, respectively. By contrast, films deposited from Ta(OC2H5)5 are amorphous TaOx oxide. XPS depth profiles have shown in all cases no significant carbon contaminations. Deposition of SBT precursor matrixes were made using the Sr(hfac)2‚tetraglyme, Bi(C6H5)3, and Ta(OC2H5)5 sources in a multicomponent reactor. Relevant results were compared with those obtained for the deposition of each single component. Figure 2 shows the log plots of temperature dependencies of growth rates of SrF2, Bi2O3, and TaOx at constant precursor partial pressures for the multicomponent deposition process. The temperature dependence of growth rates for the deposition of each single component is shown in the inset. The SrF2 deposition rate remains almost constant over the range 350-650 °C, thus indicating a mass-transport ratelimited regime. The same behavior is similarly observed in the case of the single-component deposition.21 The deposition rate of Bi2O3 becomes significant only above 350 °C. Beyond this value, a linear dependence is observed, thus indicating that deposition occurs under a reaction rate-limited regime in the investigated range, similarly to the behavior of the single-component deposition. The apparent activation energy (EBi) is lower (70 ( 5 kJ/mol) than that obtained in the single-component deposition process (100 ( 10 kJ/ mol).29 The TaOx growth rate is almost constant in the 350-450 °C temperature range. Above this temperature, it decreases slowly, probably due to precursor depletion either on the wall or in the gas phase. In the single-component deposition mode, the growth rate steadily increases with temperature, even though the low activation energy (35 ( 5 kJ/mol) does (29) Bedoya, C.; Condorelli, G. G.; Anastasi, G.; Baeri, A.; Scerra, F.; Fragala, I. L.; Lisoni, J. G.; Wouters, D. Chem. Mater. 2004, 16, 3176. (30) Condorelli, G. G.; Baeri, A.; Fragala`, I. L.; Lauretta, V.; Smerlo, G. Mater. Sci. Semicond. Process. 2003, 5, 135.

Figure 3. Typical XRD patterns in grazing incidence of films deposited on a Pt electrode at (a) 450 °C and (b) 500 °C.

not allow a clear distinction between mass transport and reaction rate-limited regimes. The comparison of kinetic plots in Figure 2, both in the single- and multicomponent processes, clearly indicates that deposition temperatures in the 450-500 °C range are the most suited for the deposition of Sr-Bi-Ta-O(F) matrixes with the correct stoichiometry. In this temperature range Sr, Bi, and Ta growth rates are comparable and, moreover, in these conditions Sr and Ta deposition rates can be easily controlled since both are almost insensitive to the deposition temperature. The control of the Bi film concentration remains, however, much more critical since the Bi deposition rate depends on the deposition temperature. GIXRD patterns and SEM micrographs of films deposited at (a) 450 °C and (b) 500 °C are shown in Figures 3 and 4, respectively. Combined data provide evidence that films deposited at 450 °C are homogeneous and amorphous while films deposited at 500 °C consist of polycrystalline SrF2 and Bi2O3 and amorphous TaOx. The latter films are not homogeneous and consist of Bi-excess grains of about 200-300 nm size. XPS depth profiles do not show carbon contamination along all the film bulk in this temperature range. Partial Pressure Dependence of Growth Rate. Effects of partial pressure on the growth rate were compared for both mono- and multicomponent MOCVD processes at 450 °C. SrF2 deposition rates either using the single-component Sr(hfac)2‚tetraglyme or the multisource processes in the 0-2 mTorr range at 450 °C are reported in Figure 5. Growth occurs under a mass transport rate-limited regime and the deposition rate linearly depends on partial pressure (PSr) for both single- and multicomponent processes. Note that in the last case the deposition rate is not affected by the partial

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Figure 5. Dependence of SrF2 growth rate on Sr partial pressure PSr (a) for single-component SrF2 deposition and (b) for multicomponent SrF2 depositions (PBi and PTa in the 1-3 and 0.3-4 mTorr ranges).

Figure 4. Typical SEM micrographs of films deposited on a Pt electrode at (a) 450 °C and (b) 500 °C.

pressure of the other precursors in all the investigated ranges (1-4 and 0.3-4 mTorr for PBi and PTa, respectively). The dependence of the Bi2O3 deposition rate upon the Bi partial pressure (PBi) in the 0-4 mTorr range has been investigated for single-component and multicomponent deposition processes at 450 °C. In the first case, the rate increases with PBi (Figure 6a). Moreover, the dependence is not linear and shows the shape of a typical Langmuir adsorption isotherm.29 Therefore, the rate-determining step of Bi2O3 growth upon PBi involves a surface reaction. In the case of the multicomponent deposition, the Bi2O3 growth rate critically depends on the SrF2 and TaOx growth rates. In fact, almost scattered data have been obtained if growth rates of companions elements were not controlled. By contrast, a stable and reproducible dependence of the Bi2O3 growth rate is obtained (Figure 6b) once constant TaOx and SrF2 growth rates (100 (10 and 30(10 nm/h, respectively) are maintained. In this case, the growth rate at 450 °C steadily increases until an almost constant value is reached above 1 mTorr. This behavior is similar to that observed for the single-component system.29 Parent data for TaOx growth rate adopting (a) the single Ta(OC2H5)5 source and (b) multicomponent sources over the range 0.3-4 mTorr at 450 °C are displayed in Figure 7. In the case of the single-source deposition process (Figure 7a),

Figure 6. Dependence of Bi2O3 growth rate on PBi adopting (a) singlecomponent and (b) multicomponent processes at constant TaOx (100 (10 nm/h) and SrF2 (30 ( 10 nm/h) growth rates (PTa ) 1.5-2.0 mTorr and PSr ) 0.1-0.3 mTorr).

the rate increases up to a constant value above 2.0 mTorr. The presence of a saturation value suggests that deposition is limited by the reaction rate at high partial pressure (PTa > 2 mTorr) and that, in particular, a surface reaction is involved in the rate-limiting step.

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Figure 7. Dependence of TaOx growth rate on PTa: (a) single-component and (b) multicomponent deposition processes (PSr < 1 mTorr).

For the multicomponent system, the growth rates show similar dependencies vs PTa. Nevertheless, low Bi partial pressures (PBi ) 1.2 ( 0.2 mTorr) induce faster Ta deposition rates compared to higher Bi partial pressure (PBi ) 2.5 ( 0.3 mTorr). By contrast, the Sr partial pressure (PSr < 1 mTorr) does not affect significantly the TaOx growth rate. Interactions between Precursors. Present data on the multicomponent kinetics provide evidence of interactions between precursors that strongly affect the kinetics of single components. In particular, there is evidence of either interactions in the gas phase or surface reactions between precursors during the multicomponent deposition. Figure 8 shows that the SrF2 growth rate does not depend on the partial pressure of both (a) Bi(C6H5)3 and (b) Ta(OC2H5)5. Therefore, the SrF2 deposition process remains unaffected either in mono- or in multicomponent MOCVD reactors. Figures 9a and 9b show the dependence of the growth rate of bismuth oxide upon PTa and TaOx growth rate, respectively (PBi ) 1.2 ( 0.3 mTorr and PSr ) 0.2 ( 0.1 mTorr). The Bi2O3 growth rates increase in the 0.1-2 mTorr PTa range. In this pressure range, it linearly depends on the growth rate of TaOx (Figure 9b). Ta partial pressures higher than 2 mTorr cause a marked falloff (Figure 9a). Even more interesting, Bi2O3 growth rate decreases with the increase of the SrF2 mole fraction in the films (Figure 9c), almost independently from PBi, PSr, and PTa (1-3, 0.13, and 0.1-4 mTorr, respectively). In addition, multicomponent deposition processes limited only to Sr and Bi precursors (hence, without the Ta source) bring about the formation of pure SrF2 without any trace of Bi2O3 (as shown from EDX measurements). These observations provide evidence that the Bi precursor does not find suitable

Condorelli et al.

Figure 8. Dependence of SrF2 growth rate upon (a) PBi (PSr ) 0.1-0.3 mTorr and PTa ) 1-2 mTorr) and (b) PTa (PSr ) 0.1-0.3 mTorr and PBi ) 1-1.5mTorr).

adsorption sites on SrF2 in the absence of TaOx sites. On the other hand, TaOx deposition creates actives sites suitable for Bi(C6H5)3 adsorption, even though there is competition between both Bi(C6H5)3 and Ta(OC2H5)5 adsorption for the same active sites as can be inferred from the dependence of Bi2O3 growth rate on PTa. Deposition Pathways. Present kinetic data suggest that the deposition process of Bi and Ta precursors involves a heterogeneous process, similar to results found for singlecomponent depositions.21,29,30 In particular, the singlecomponent process of Bi(C6H5)3 at 450 °C mainly involves a heterogeneous process which leads to the dissociation of phenyl groups and the formation of Bi2O3.29 The singlecomponent deposition of the Ta(OC2H5)5 precursor points to a heterogeneous process leading to the dissociation of ethanol.30 For both precursors, the multicomponent process points to a similar heterogeneous process involving similar reaction byproducts (phenyl groups and ethanol). It has already been shown21,22 that the decomposition process of Sr(hfac)2‚tetraglyme occurs in accordance with two pathways. The first is relevant above 300 °C and involves the heterogeneous dissociation of the tetraglyme ancillary ligand, followed by the decomposition of adsorbed Sr(hfac)2. The second pathway becomes significant above 400 °C in a hot wall reactor and leads to the homogeneous formation of CF2O and CF3C(O)HCdCdO byproducts. Note, however, that in the presently investigated temperature range, the observed SrF2 growth rate does not depend on the reaction pathways since deposition occurs under a mass transport limited regime.

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Figure 10. Typical (a) GIXRD pattern and (b) SEM micrographs of SBT films deposited on Pt at 450 °C and annealed at 800 °C under O2 flow.

Figure 9. Dependence of Bi2O3 growth rate upon (a) PTa and (b) TaOx growth rate (PBi ) 1.2 ( 0.3 mTorr and PSr ) 0.2 ( 0.1 mTorr); (c) Bi2O3 growth rate vs SrF2 mole fraction in the films (PBi ) 1-3 mTorr, PSr ) 0.1-3 mTorr and PTa ) 0.1-4 mTorr).

Chart 1. Proposed Decomposition Pathways for SBT Ferroelectric Film from Sr(hfac)2‚tetraglyme, Bi(C6H5)3, and Ta(OC2H5)5 Precursors

A schematic sketch of the deposition pathways occurring during the multicomponent SBT deposition is reported in Chart 1. The model involves decomposition of the Sr precursor to SrF2 independently of any reactive site (SrF2, Bi2O3, TaOx,

or Pt) involved in the process. SrF2, however, does not provide suitable adsorption sites for the Bi(C6H5)3 precursor. By contrast, Bi(C6H5)3 undergoes a heterogeneous deposition process (to Bi2O3) only over oxide surfaces, namely, TaOx and Bi2O3. Finally, the Ta(OC2H5)5 precursor can adsorb on both fluoride and oxide surfaces and, upon decomposition, forms TaOx sites required for the Bi(C6H5)3 adsorption. On oxide active sites, there is, therefore, competition between Bi(C6H5)3 and Ta(OC2H5)5. Effect of Postdeposition Annealing. Postdeposition annealing of Sr-Bi-Ta deposited films at 450 °C was performed in the temperature range 600-800 °C under O2 flow. Thermal treatments at 650 and 700 °C cause the formation of both SBT and nonstoichiometric fluorite phases.31,32 By contrast, annealing at 800 °C under O2 flow leads to random oriented polycrystalline SBT films without cracks, in accordance with the GIXRD pattern and SEM micrograph (Figure 10). Figure 11 shows AFM images of films (a) as-deposited at 450 °C (roughness ∼ 4 ( 1 nm) and (b) annealed at 800 °C (roughness ∼ 13 ( 1 nm). The size of grains lies between 250 and 350 nm in both cases (Figure 11) with an overall thickness around 160 nm evaluated by EDX. The XPS depth profiles and EDX data have shown that the thermal treatment at 800 °C induces a complete elimination of any fluorine content (sensitivity < 0.1%). Ferroelectric Measurements. Ferroelectric properties of SBT films fabricated on a Pt electrode at 450 °C and annealed at 800 °C under O2 flow were measured using a triangular waveform with amplitude of 10 V and a frequency of 100 Hz. The Pt top electrode was sputtered through a (31) Liu, J.; Zhang, S.; Yang, C.; Dai, L. J. Am. Ceram. Soc. 2005, 88, 85. (32) Osaka, T.; Sakakibara, A.; Seki, T.; Ono, S. Jpn. J. Appl. Phys., Part 1 1998, 37, 597.

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800 °C and obtained either by MOD33 or by liquid-delivery MOCVD.34 Nevertheless, despite the large Pr value, the P-E loop evidences a strong relaxation of the polarization measured 1 s after voltage switch-off. This characteristic can be related to a not fully controlled microstructure of the film which may also explain the drastically larger coercive field Ec (110 kV/cm) as compared to the values (30-60 kV/cm) generally reported in the literature.35 In conclusion, the hysteresis loop is in agreement with the ferroelectric response of a pure SBT phase without any fluorine contaminant and intermediate nonstoichiometric fluorite phase. Conclusions

Figure 11. AFM images of films (thickness 160 nm) deposited on Pt (a) at 450 °C and then (b) annealed at 800 °C under O2 flow.

Figure 12. Hysteresis loop measured on a SBT film (thickness 110 nm) deposited on Pt at 450 °C and then annealed at 800 °C under O2 flow.

shadow mask. The P-E hysteresis loop (Figure 12) points out the ferroelectric character of the SBT film with a remnant polarization 2Pr ) 18.7 µC‚cm-2 and a coercive field 2Ec ) 220 kV/cm. The remnant and saturation polarizations (2Psat ) 50 µC‚cm-2 at 500 kV/cm) are totally comparable with those usually measured on SBT thin films annealed at 750-

The present MOCVD process adopting Sr(hfac)2‚tetraglyme, Ta(OC2H5)5, and Bi(C6H5)3 precursors has proven well-suited for deposition of ferroelectric SBT films. Kinetics of singleand multicomponent systems were contrasted in order to optimize the multicomponent deposition process. Kinetic data indicates that temperatures around 450 °C are the most suited for the multicomponent deposition with the adopted precursor system. Moreover, it has been observed that the SrF2 growth rate does not depend on the partial pressure of other precursors. By contrast, Bi and Ta growth rates are almost linearly dependent on each other. This kinetic behavior can be explained in terms of a heterogeneous model requiring TaOx sites for activation of Bi(C6H5)3 decomposition which, by contrast, is inhibited by SrF2. Crystallized SBT films were obtained from multielement matrixes with a thermal treatment under O2 at 800 °C. These conditions promote a complete elimination of any fluorine contaminants. XRD data point to pure polycrystalline SBT phase without any intermediate fluorite phase left. Hysteresis loop measurements point to ferroelectric properties of film and agree well with a total removal of contaminants and of nonstoichiometric fluorite phase. Finally, present data have demonstrated that pure ferroelectric SBT films can be grown with a simple MOCVD process adopting the Sr(hfac)2‚ tetraglyme, fluorine-containing precursor which is very stable for storage and manipulation. The process, therefore, represents a promising and attractive method for fabrication of nonvolatile memories. Acknowledgment. Authors gratefully thank the European Commission (IST-2000-30153-FLEUR contract) and the MIUR (FISR 2003 research programs) for financial support. CM051151+ (33) Moert, M.; Schindler, G.; Mikolajick, T.; Nagel, N.; Hartner, W.; Dehm, C.; Kohlstedt, H.; Waser, R. Appl. Surf. Sci. 2005, 249, 23. (34) Shin, W. C.; Choi, K. J.; Yoon, S. G. Thin Solid Films 2002, 409, 133. (35) Viapana, M.; Schwitters, M.; Wouters, D. J.; Maes, H. E.; Van der Biest, O. Mater. Sci. Eng. B 2005, 118, 34.