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Oriented PbZrxTi1−xO3 thin films obtained at low substrate temperature by pulsed laser deposition
Thin Solid Films 311 Ž1997. 171–176
Oriented PbZr xTi 1yxO 3 thin films obtained at low substrate temperature by pulsed laser deposition P. Verardi a b
a,)
, M. Dinescu b, F. Craciun a , V. Sandu
b
CNR Istituto di Acustica ‘O.M. Corbino’, Õia Cassia 1216, Rome I-00189, Italy Institute of Atomic Physics, Bucarest-Magurele, POB-MG 16, R76900, Romania Received 2 April 1997; accepted 20 August 1997
Abstract One step deposition of oriented thin films of PbZr xTi 1yxO 3 onto Ž100.- and Ž111.-Si substrates by pulsed laser ablation at low substrate temperatures Ž3758C. is reported. X-ray diffraction analysis showed that the films grew with preferential Ž111. orientation on both substrates but energy dispersive spectroscopy revealed different compositions despite identical targets and substrate temperatures. Direct piezoelectric measurements showed good piezoelectric properties of the films, obtained in the absence of any subsequent poling. q 1997 Elsevier Science S.A. Keywords: Laser ablation; Piezoelectric effect; Plasma processing and deposition
1. Introduction Enormous current interest in ferroelectric thin films is due to their multiple potential applications in electronic devices, which use the high dielectric, piezoelectric and electro-optic properties of ferroelectric materials. Examples are non-volatile memory elements, high value capacitors, infrared devices, SAW-delay lines, sensors and actuators, electro-optic displays, optical switches, ferroelectric field-effect transistors, etc. w1–8x. In addition their potential use in micromechanical devices is under increasing study w9x. Lead zirconate-titanate ŽPZT. is a prime candidate for these applications due to its high bulk piezoelectric properties Že.g., longitudinal piezoelectric coefficient d 33 ( 300 pCrN.; the preparation as thin films favours its direct integration in semiconductor technologies w10x. Consequently, the deposition of PZT thin films on Si substrates started to develop in order to take advantage of Si micromachining technology. Using PZT films on Si substrates, various Si monolithic devices such as ultrasonic sensors, atomic force microscope ŽAFM. sensors and non-volatile memories have been reported w1,8,11x. This process of implementation of ferroelectric thin films in monolithic devices require high quality thin films deposited on semi) Corresponding author. Tel.: q39 6 30365765; fax: q39 6 30365341; e-mail: [email protected]
0040-6090r97r$17.00 q 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 6 9 3 - 7
conductors or metals. Previous work has been generally devoted to sol–gel deposition w12x and sputtering w10,13x methods, as well as to metal-organic chemical-vapor-deposition ŽMOCVD. w14–17x. However, problems of stoichiometry control and post-processing at high temperatures Žlarger than 6008C. of amorphously deposited materials w10,18x are common. A one-step deposition technique based on pulsed laser ablation has been developed in order to avoid post-processing. Pulsed laser deposition ŽPLD. is a unique and relatively simple technique which allows the transfer of complicated material compositions from bulk to thin films or multi-layer structures w19x. However, PZT films deposition requires temperatures of 550–7008C in oxygen ambient w20x. It has been observed w6,13x that important factors in the synthesis and crystallisation of PZT thin films are accurate control of the oxidation kinetics and of the lead content of the films. The latter can vary widely due to the tendency of Pb and PbO to evaporate at high temperatures. Moreover, atom diffusion at the interface substrate-film can occur at high temperatures, leading to non-stoichiometric PZT compositions and poor quality interface Žlow conductivity electrodes, etc... While the perovskite crystal structure can be usually obtained at high substrate temperatures, in most cases, film processing at lower temperatures is desirable. Indeed, low temperature deposition could, in principle, avoid reaction and diffusion between the film and the substrate as well as film fractur-
P. Verardi et al.r Thin Solid Films 311 (1997) 171–176
172
ing induced by thermal stress at the interface. Our aim was to obtain well-crystallised and oriented thin films at lowest possible temperature of the substrate. It has been shown that with PLD different thin films with tailored characteristics can be formed by varying the target composition, the ambient atmosphere and the substrate temperature w20x. With this method we have prepared oriented ferroelectric PZT thin films, by using a Q-switched Nd-YAG laser ablation technique, at a relatively low substrate temperature, of about 3758C. Most applications require PZT to be deposited on conducting layers, therefore we carried out the deposition on Au-coated Si substrates. The obtained films have been ferroelectric PbŽZr xTi 1yx .O 3 grown on Ž100.- and Ž111.-Si substrates of thickness 500 m m cov˚ thickness. ered with a Au layer of approximately 1000 A
2. Experimental The experimental setup Žtypical for a PLD experiment. is presented in Fig. 1. The beam of a Q-switched Nd-YAG laser Ž400 mJrpulse at 1064 nm and pulsewidth 10 ns. operating at 10 Hz was directed in a vacuum chamber and focused on a commercial PZT5-type target. During the deposition the target was rotated in order to avoid the crater formation. The incident laser fluence was set at approximately 25 Jrcm2 . The ablated material was deposited on a substrate fixed approximately 4 cm away from the target. Two types of substrates, Si Ž111. and Si Ž100., have been used in the experiments; few layers of Cr Žin order to improve Au adhesion. and Au electrodes of ˚ thickness, with Ž111. orientation, approximately 1000 A have been previously deposited in an evaporation chamber on these substrates, after adequate cleaning. The gold films have been continuous, without droplets and preserved this
aspect even after PZT deposition. The substrate was mechanically attached to a heater and the temperature was monitored with a chromelalumel thermocouple embedded in the heating block. The measured deposition temperature Ž; 3758C. represents therefore the block temperature and not the substrate surface temperature which is slightly lower. During the deposition an oxygen ambient pressure of 150 mTorr has been maintained. The oxygen has been introduced in the vacuum chamber previously evacuated up to a residual pressure of 2 = 10y6 Torr. After the deposition the films were cooled down to 508C in oxygen atmosphere at the same pressure as during the deposition. These parameters have been chosen by taking into account the factors which influence the formation of PZT films; the oxygen partial pressure, the lattice matching between the film and the substrate, the substrate temperature and the distance target-substrate. The oxygen pressure is important for maintaining the Pb content of the film. Indeed it has been shown that usually the Pb content of the deposited film decreases dramatically at low oxygen pressures w20x. Therefore the composition of the film changes from the stoichiometric ratio PbrŽTi, Zr. s 1r1. For ambient O 2 pressures above 100 mTorr, Pb, Ti and Zr atoms are well oxidized as in the PZT compound. Consequently the PZT film can be formed without stoichiometric change under O 2 pressures greater than 100 mTorr. In our case this parameter was set at about 150 mTorr. Regarding the substrate temperature, it has been observed that if the growth occurs on a non-heated substrate, the structure of the film is usually amorphous w20x; crystalline films are obtained either by post-annealing, or in a one-step process on a heated substrate. Moreover, detailed analysis of X-ray diffraction patterns w6,13x have shown that the crystallographic phases in the film are strongly dependent on the substrate temperature. Different authors observed that at low temperatures Ž350–5008C. the pyrochlore phase dominated the film structure w13x. Also, previously reported XRD analysis of PZT films deposited on Pt-coated Si at substrate temperatures lower than 5008C in 300 mTorr of oxygen indicated that the films were primarily pyrochlore w20x. A similar result was obtained for the deposition of PZT on Pt-coated GaAs w20x. However, at slightly higher temperatures Ž6008C., the perovskite phase was stabilized. In our case, the temperature of deposition has been set to 3758C and, as it will be shown in the following, films deposited at this low substrate temperature are perovskite and strongly oriented. 3. Results and discussion
Fig. 1. PLD experimental setup.
Energy dispersive spectroscopy ŽEDS. and X-ray diffraction were used to determine the composition and the crystal structure of the deposited films, which have been compared with those of the original target. Fig. 2 presents the u-2 u X-ray diffraction pattern of the original target before laser irradiation Župper curve.. Dur-
P. Verardi et al.r Thin Solid Films 311 (1997) 171–176
ing the ablation, the target itself is modified by the action of the high-fluence laser beam; these random modifications could be responsible for variations in thin film compositions and properties. For comparison, the same target was examined after irradiation and the pattern is represented by the lower curve in Fig. 2. It can be observed that indeed a pyrochlore phase is present in a great quantity in the sample after irradiation for 35 min. This is probably due to a subsequent recrystallisation of the target lead-deficient material after irradiation. Fig. 3 presents the X-ray diffraction pattern of a PZT thin film deposited on Si Ž111.rAu Župper curve. and Si Ž100.rAu Žlower curve.. The deposition conditions have been the same: laser fluence 25 Jrcm2 , pulsewidth 10 ns, laser frequency 10 Hz, the distance target-substrate 4 cm, deposition time 35 min and substrate temperature 3758C. EDS analysis show that in the first case the sample composition ratio PbrZrrTi is 44r28r28 while in the second is 50r30r20. This can be due to phase separation with different element content which takes place on a small scale, as the typical analyzed area by EDS was very small. From the diffraction spectra, it can be observed that perovskite compositions of both films are well oriented: all films exhibited a preferential alignment of the Ž111. planes parallel to the film surface. Indeed, while many peaks are present on the pattern of the target powder ŽFig. 2., which is a typical pattern of polycrystalline PZT with tetragonal perovskite structure, almost only Ž111. peak is observed on the X-ray patterns of the PZT thin films, therefore these
173
films are highly Ž111. oriented. In the case of PZT deposited on Si Ž100.rAu, the small peak at approximately 448 attributed to PZT Ž200. together with the baselinedrift around 308 attributed to some small amorphouse phase indicate a lower degree of orientation than in the case PZTrSi Ž111.. In both patterns, due to the high absorption in PZT films, gold peak is missing. Film orientation is determined by factors like the epitaxial mismatch between the film and the substrate, the substrate temperature and the film thickness. It has been demonstrated that, under epitaxial strain, a single crystal ferroelectric thin film has an equilibrium structure with a periodic array of domain walls w21x. In ferroelectric thin films, the formation of a periodic domain pattern prevents the extension of the interfacial strain field, thus minimizing the total energy of the structure. If the film crystalline structure changes from a high-symmetry phase to a lower symmetry phase during cooling from the growth temperature, as it is our case ŽCurie temperature Tc ( 3008C., the bulk epitaxial strain can be relieved by a domain formation w15,22x. For the oriented growth, it has been observed by Tabata et al. w23x that the lattice matching at the interface between films and substrates is of great importance; better-matched are the lattice constants of the film and substrates, lower temperatures for the oriented growth are required; these results have been obtained for the deposition of PbTiO 3 on Ž100. SrTiO 3 substrates, but we believe that this is also the explanation for our samples formation at low substrate temperature.
Fig. 2. X-ray diffraction patterns from PZT target before Župper curve. and after Žlower curve. laser irradiation.
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P. Verardi et al.r Thin Solid Films 311 (1997) 171–176
Fig. 3. X-ray diffraction patterns from PZT film samples deposited on Si Ž111.rAu Župper curve. and Si Ž100.rAu Žlower curve.. Due to the high absorption in PZT films the Au peak is missing.
The surface morphology of PZT films was observed by scanning electron microscopy ŽSEM.. On different substrates it has been found that the films have a microstructure with a very fine grain size, uniform and without porosity; this can be observed, for example, in Fig. 4a. which shows the SEM micrograph for the PZT film deposited on Si Ž111.rAu. Instead, the topographic image on the same surface at a higher magnification ŽFig. 4b. reveals a complex structure formed by irregular grains of diameter 1–2 m m, made up at their turn by grains of smaller size Ž0.2 m m.. A similar structure is observed on Si Ž100.rAurPZT sample ŽFig. 5.. Regarding the film dimensions, the surface of the film obtained with uniform properties was roughly 1 cm2 and the thickness, as determined by profilometry, was approximately 3.5 m m. The piezoelectric properties of PZT films have been measured directly by a special probe ŽIt must be mentioned that generally the measurements of piezoelectric properties previously reported by other authors we.g., Ref. w10xx are not direct, but use effective properties obtained by considering compliance values of thin films equal to those of bulk materials, which is not generally proved.. Our method consists of the following: a voltage amplitude is applied by a pulse generator on an acoustic transducer which converts the electric signal into an acoustic one. The acoustic signal is transmitted through a probe into the test film which converts it into an electric pulse measured by an oscilloscope. The proportional constant between the applied elec-
tric voltage and the measured electric voltage generated by the piezoelectric thin film is related to the piezoelectric constant d 33 . To obtain the absolute value of d 33 , the measurement is done comparatively with a known material like quartz or lithium niobate. It must be mentioned that direct measurements are possible only if Ža. the thickness of dielectric substrate is not very large Žabout 100 m m.; Žb. the resistivity of semiconductor substrate is low enough Žabout 10 V .; and Žc. there is an additional metal film under the film to be tested. The measurements done by using this method on our samples revealed that they were all piezoelectric without poling, and the direction of polarization was the same for all the samples. Similar results have been reported by Takayama and Tomita w24x on epitaxial PZT thin films grown by sputtering on MgO single crystals and on Ž100. Pt. They obtained tetragonal Ž001. oriented and rhombohedral Ž111. oriented PZT films which were polarized even without a poling treatment and had good ferroelectric properties. They concluded that the existence of pre-aligned polarization depends upon the substrate but they could not suggest exact causes of the phenomenon. Piezoelectricity for unpolled PZT films has also been reported by Etzold et al. w25x and Surowiak et al. w26x for sputtered heteroepitaxial PbTiO 3 films on Ž100. MgO. Lefki and Dormans obtained also piezoelectric as-deposited PZT films by MOCVD w17x. They attributed the origin of ferroelectric domain orientation to the internal stresses created during the deposition and subsequent cooling of the sample which
P. Verardi et al.r Thin Solid Films 311 (1997) 171–176
175
can force an alignment of the domains. We believe that this could also be an explanation for the orientation of domains in our samples. The measured d 33 piezoelectric constant values varied slightly between different measurement points on the surface of our samples approximately around 35–40 pCrN. This value represents only approximately 20% of the PZT bulk constant but it is adequate for many thin film applications. This fact together with the
Fig. 5. SEM image of PZTrAurSi Ž100. film sample. The dotted segment corresponds to 6 m m.
low temperature one step deposition makes the obtained thin films useful in many integrated piezoelectric devices.
4. Conclusions In summary, oriented PZT films with good piezoelectric properties were deposited on Si Ž100.rAu and Si Ž111.rAu substrates at extremely low substrate temperatures of 3758C with a Nd-YAG laser ablation technique. This temperature is low enough to directly deposit the PZT on useful semiconducting devices. We believe that our present results could be useful in making ferroelectric thin films for the purpose of practical applications.
Acknowledgements The authors wish to acknowledge M. Acciarini for technical help. One of us ŽM.D.. gratefully acknowledges support from the Italian Ministry of Foreign Affairs.
References
Fig. 4. Ža. SEM image of PZTrAurSi Ž111. film sample. The dotted segment corresponds to 30 m m. Žb. Same as in Ža.. The dotted segment corresponds to 6 m m.
w1x R. Takayama, Y. Tomita, K. Ijima, I. Ueda, J. Appl. Phys. 61 Ž1987. 411. w2x J.F. Scott, C.A. Araujo, Science 246 Ž1989. 1400. w3x S.K. Dey, R. Zuleeg, Ferroelectrics 108 Ž1990. 37. w4x G.H. Haertling, J. Vac. Sci. Technol. A9 Ž1991. 414. w5x J.J. Lee, C.L. Thio, S.B. Desu, J. Appl. Phys. 78 Ž1995. 5073.
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w6x K. Sreenivas, M. Sayer, D.J. Baar, M. Nishioka, Appl. Phys. Lett. 52 Ž1988. 709. w7x H. Adachi, T. Mitsuyu, O. Yamazaki, K. Wasa, J. Appl. Phys. 60 Ž1986. 736. w8x M.H. Francombe, S. Krishnaswany, J. Vac. Sci. Technol. A8 Ž1990. 1382. w9x T. Maeder, P. Muralt, L. Sagalowicz, I. Reaney, M. Kohli, A. Kholkin, N. Setter, Appl. Phys. Lett. 68 Ž1996. 776. w10x S. Watanabe, T. Fujiu, T. Fujii, Appl. Phys. Lett. 66 Ž1995. 1481. w11x T. Fujii, S. Watanabe, M. Suzuki, T. Fujiu, J. Vac. Sci. Technol. B13 Ž1995. 1119. w12x Y.L. Tu, S.J. Milne, J. Mater. Sci. 30 Ž1995. 2507. w13x K. Sreenivas, M. Sayer, J. Appl. Phys. 64 Ž1988. 1484. w14x B.S. Kwak, E.P. Boyd, A. Erbil, Appl. Phys. Lett. 53 Ž1988. 1702. w15x B.S. Kwak, A. Erbil, J.D. Budai, M.F. Chisholm, L.A. Boatner, B.J. Wilkens, Phys. Rev. B 49 Ž1994. 14865. w16x M. de Keijser, G.J.M. Dormans, P.J. van Veldhoven, P.K. Larsen, Integrated Ferroelectrics 3 Ž1993. 131.
w17x K. Lefki, G.J.M. Dormans, J. Appl. Phys. 76 Ž1994. 1764. w18x K. Iijima, I. Ueda, K. Kujimiya, Jpn. J. Appl. Phys. 30 Ž9B. Ž1991. 2149. w19x D.B. Chrisey, G.K. Huber ŽEds.., Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. w20x J.S. Horwitz, K.S. Grabowski, D.B. Grabowski, R.E. Leuchtner, Appl. Phys. Lett. 59 Ž1991. 1565. w21x B.S. Kwak, A. Erbil, B.J. Wilkens, J.D. Budai, M.F. Chisholm, L.A. Boatner, Phys. Rev. Lett. 68 Ž1992. 3733. w22x R. Bruisma, A. Zangwill, J. Phys. ŽParis. 47 Ž1986. 2055. w23x H. Tabata, T. Kawai, S. Kawai, O. Murata, J. Fujioka, S. Minakata, Appl. Phys. Lett. 59 Ž1991. 2354. w24x R. Takayama, Y. Tomita, J. Appl. Phys. 65 Ž1989. 1666. w25x K.F. Etzold, R.A. Roy, K.L. Saenger, J.J. Cuomo, IEEE, New York, Proceedings of the IEEE Ultrasonics Symposium, Honolulu, 1990, p. 747. w26x Z. Surowiak, V.M. Mukhortov, V.P. Dudkevich, Ferroelectrics 139 Ž1993. 1.
Oriented PbZr xTi 1yxO 3 thin films obtained at low substrate temperature by pulsed laser deposition P. Verardi a b
a,)
, M. Dinescu b, F. Craciun a , V. Sandu
b
CNR Istituto di Acustica ‘O.M. Corbino’, Õia Cassia 1216, Rome I-00189, Italy Institute of Atomic Physics, Bucarest-Magurele, POB-MG 16, R76900, Romania Received 2 April 1997; accepted 20 August 1997
Abstract One step deposition of oriented thin films of PbZr xTi 1yxO 3 onto Ž100.- and Ž111.-Si substrates by pulsed laser ablation at low substrate temperatures Ž3758C. is reported. X-ray diffraction analysis showed that the films grew with preferential Ž111. orientation on both substrates but energy dispersive spectroscopy revealed different compositions despite identical targets and substrate temperatures. Direct piezoelectric measurements showed good piezoelectric properties of the films, obtained in the absence of any subsequent poling. q 1997 Elsevier Science S.A. Keywords: Laser ablation; Piezoelectric effect; Plasma processing and deposition
1. Introduction Enormous current interest in ferroelectric thin films is due to their multiple potential applications in electronic devices, which use the high dielectric, piezoelectric and electro-optic properties of ferroelectric materials. Examples are non-volatile memory elements, high value capacitors, infrared devices, SAW-delay lines, sensors and actuators, electro-optic displays, optical switches, ferroelectric field-effect transistors, etc. w1–8x. In addition their potential use in micromechanical devices is under increasing study w9x. Lead zirconate-titanate ŽPZT. is a prime candidate for these applications due to its high bulk piezoelectric properties Že.g., longitudinal piezoelectric coefficient d 33 ( 300 pCrN.; the preparation as thin films favours its direct integration in semiconductor technologies w10x. Consequently, the deposition of PZT thin films on Si substrates started to develop in order to take advantage of Si micromachining technology. Using PZT films on Si substrates, various Si monolithic devices such as ultrasonic sensors, atomic force microscope ŽAFM. sensors and non-volatile memories have been reported w1,8,11x. This process of implementation of ferroelectric thin films in monolithic devices require high quality thin films deposited on semi) Corresponding author. Tel.: q39 6 30365765; fax: q39 6 30365341; e-mail: [email protected]
0040-6090r97r$17.00 q 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 6 9 3 - 7
conductors or metals. Previous work has been generally devoted to sol–gel deposition w12x and sputtering w10,13x methods, as well as to metal-organic chemical-vapor-deposition ŽMOCVD. w14–17x. However, problems of stoichiometry control and post-processing at high temperatures Žlarger than 6008C. of amorphously deposited materials w10,18x are common. A one-step deposition technique based on pulsed laser ablation has been developed in order to avoid post-processing. Pulsed laser deposition ŽPLD. is a unique and relatively simple technique which allows the transfer of complicated material compositions from bulk to thin films or multi-layer structures w19x. However, PZT films deposition requires temperatures of 550–7008C in oxygen ambient w20x. It has been observed w6,13x that important factors in the synthesis and crystallisation of PZT thin films are accurate control of the oxidation kinetics and of the lead content of the films. The latter can vary widely due to the tendency of Pb and PbO to evaporate at high temperatures. Moreover, atom diffusion at the interface substrate-film can occur at high temperatures, leading to non-stoichiometric PZT compositions and poor quality interface Žlow conductivity electrodes, etc... While the perovskite crystal structure can be usually obtained at high substrate temperatures, in most cases, film processing at lower temperatures is desirable. Indeed, low temperature deposition could, in principle, avoid reaction and diffusion between the film and the substrate as well as film fractur-
P. Verardi et al.r Thin Solid Films 311 (1997) 171–176
172
ing induced by thermal stress at the interface. Our aim was to obtain well-crystallised and oriented thin films at lowest possible temperature of the substrate. It has been shown that with PLD different thin films with tailored characteristics can be formed by varying the target composition, the ambient atmosphere and the substrate temperature w20x. With this method we have prepared oriented ferroelectric PZT thin films, by using a Q-switched Nd-YAG laser ablation technique, at a relatively low substrate temperature, of about 3758C. Most applications require PZT to be deposited on conducting layers, therefore we carried out the deposition on Au-coated Si substrates. The obtained films have been ferroelectric PbŽZr xTi 1yx .O 3 grown on Ž100.- and Ž111.-Si substrates of thickness 500 m m cov˚ thickness. ered with a Au layer of approximately 1000 A
2. Experimental The experimental setup Žtypical for a PLD experiment. is presented in Fig. 1. The beam of a Q-switched Nd-YAG laser Ž400 mJrpulse at 1064 nm and pulsewidth 10 ns. operating at 10 Hz was directed in a vacuum chamber and focused on a commercial PZT5-type target. During the deposition the target was rotated in order to avoid the crater formation. The incident laser fluence was set at approximately 25 Jrcm2 . The ablated material was deposited on a substrate fixed approximately 4 cm away from the target. Two types of substrates, Si Ž111. and Si Ž100., have been used in the experiments; few layers of Cr Žin order to improve Au adhesion. and Au electrodes of ˚ thickness, with Ž111. orientation, approximately 1000 A have been previously deposited in an evaporation chamber on these substrates, after adequate cleaning. The gold films have been continuous, without droplets and preserved this
aspect even after PZT deposition. The substrate was mechanically attached to a heater and the temperature was monitored with a chromelalumel thermocouple embedded in the heating block. The measured deposition temperature Ž; 3758C. represents therefore the block temperature and not the substrate surface temperature which is slightly lower. During the deposition an oxygen ambient pressure of 150 mTorr has been maintained. The oxygen has been introduced in the vacuum chamber previously evacuated up to a residual pressure of 2 = 10y6 Torr. After the deposition the films were cooled down to 508C in oxygen atmosphere at the same pressure as during the deposition. These parameters have been chosen by taking into account the factors which influence the formation of PZT films; the oxygen partial pressure, the lattice matching between the film and the substrate, the substrate temperature and the distance target-substrate. The oxygen pressure is important for maintaining the Pb content of the film. Indeed it has been shown that usually the Pb content of the deposited film decreases dramatically at low oxygen pressures w20x. Therefore the composition of the film changes from the stoichiometric ratio PbrŽTi, Zr. s 1r1. For ambient O 2 pressures above 100 mTorr, Pb, Ti and Zr atoms are well oxidized as in the PZT compound. Consequently the PZT film can be formed without stoichiometric change under O 2 pressures greater than 100 mTorr. In our case this parameter was set at about 150 mTorr. Regarding the substrate temperature, it has been observed that if the growth occurs on a non-heated substrate, the structure of the film is usually amorphous w20x; crystalline films are obtained either by post-annealing, or in a one-step process on a heated substrate. Moreover, detailed analysis of X-ray diffraction patterns w6,13x have shown that the crystallographic phases in the film are strongly dependent on the substrate temperature. Different authors observed that at low temperatures Ž350–5008C. the pyrochlore phase dominated the film structure w13x. Also, previously reported XRD analysis of PZT films deposited on Pt-coated Si at substrate temperatures lower than 5008C in 300 mTorr of oxygen indicated that the films were primarily pyrochlore w20x. A similar result was obtained for the deposition of PZT on Pt-coated GaAs w20x. However, at slightly higher temperatures Ž6008C., the perovskite phase was stabilized. In our case, the temperature of deposition has been set to 3758C and, as it will be shown in the following, films deposited at this low substrate temperature are perovskite and strongly oriented. 3. Results and discussion
Fig. 1. PLD experimental setup.
Energy dispersive spectroscopy ŽEDS. and X-ray diffraction were used to determine the composition and the crystal structure of the deposited films, which have been compared with those of the original target. Fig. 2 presents the u-2 u X-ray diffraction pattern of the original target before laser irradiation Župper curve.. Dur-
P. Verardi et al.r Thin Solid Films 311 (1997) 171–176
ing the ablation, the target itself is modified by the action of the high-fluence laser beam; these random modifications could be responsible for variations in thin film compositions and properties. For comparison, the same target was examined after irradiation and the pattern is represented by the lower curve in Fig. 2. It can be observed that indeed a pyrochlore phase is present in a great quantity in the sample after irradiation for 35 min. This is probably due to a subsequent recrystallisation of the target lead-deficient material after irradiation. Fig. 3 presents the X-ray diffraction pattern of a PZT thin film deposited on Si Ž111.rAu Župper curve. and Si Ž100.rAu Žlower curve.. The deposition conditions have been the same: laser fluence 25 Jrcm2 , pulsewidth 10 ns, laser frequency 10 Hz, the distance target-substrate 4 cm, deposition time 35 min and substrate temperature 3758C. EDS analysis show that in the first case the sample composition ratio PbrZrrTi is 44r28r28 while in the second is 50r30r20. This can be due to phase separation with different element content which takes place on a small scale, as the typical analyzed area by EDS was very small. From the diffraction spectra, it can be observed that perovskite compositions of both films are well oriented: all films exhibited a preferential alignment of the Ž111. planes parallel to the film surface. Indeed, while many peaks are present on the pattern of the target powder ŽFig. 2., which is a typical pattern of polycrystalline PZT with tetragonal perovskite structure, almost only Ž111. peak is observed on the X-ray patterns of the PZT thin films, therefore these
173
films are highly Ž111. oriented. In the case of PZT deposited on Si Ž100.rAu, the small peak at approximately 448 attributed to PZT Ž200. together with the baselinedrift around 308 attributed to some small amorphouse phase indicate a lower degree of orientation than in the case PZTrSi Ž111.. In both patterns, due to the high absorption in PZT films, gold peak is missing. Film orientation is determined by factors like the epitaxial mismatch between the film and the substrate, the substrate temperature and the film thickness. It has been demonstrated that, under epitaxial strain, a single crystal ferroelectric thin film has an equilibrium structure with a periodic array of domain walls w21x. In ferroelectric thin films, the formation of a periodic domain pattern prevents the extension of the interfacial strain field, thus minimizing the total energy of the structure. If the film crystalline structure changes from a high-symmetry phase to a lower symmetry phase during cooling from the growth temperature, as it is our case ŽCurie temperature Tc ( 3008C., the bulk epitaxial strain can be relieved by a domain formation w15,22x. For the oriented growth, it has been observed by Tabata et al. w23x that the lattice matching at the interface between films and substrates is of great importance; better-matched are the lattice constants of the film and substrates, lower temperatures for the oriented growth are required; these results have been obtained for the deposition of PbTiO 3 on Ž100. SrTiO 3 substrates, but we believe that this is also the explanation for our samples formation at low substrate temperature.
Fig. 2. X-ray diffraction patterns from PZT target before Župper curve. and after Žlower curve. laser irradiation.
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P. Verardi et al.r Thin Solid Films 311 (1997) 171–176
Fig. 3. X-ray diffraction patterns from PZT film samples deposited on Si Ž111.rAu Župper curve. and Si Ž100.rAu Žlower curve.. Due to the high absorption in PZT films the Au peak is missing.
The surface morphology of PZT films was observed by scanning electron microscopy ŽSEM.. On different substrates it has been found that the films have a microstructure with a very fine grain size, uniform and without porosity; this can be observed, for example, in Fig. 4a. which shows the SEM micrograph for the PZT film deposited on Si Ž111.rAu. Instead, the topographic image on the same surface at a higher magnification ŽFig. 4b. reveals a complex structure formed by irregular grains of diameter 1–2 m m, made up at their turn by grains of smaller size Ž0.2 m m.. A similar structure is observed on Si Ž100.rAurPZT sample ŽFig. 5.. Regarding the film dimensions, the surface of the film obtained with uniform properties was roughly 1 cm2 and the thickness, as determined by profilometry, was approximately 3.5 m m. The piezoelectric properties of PZT films have been measured directly by a special probe ŽIt must be mentioned that generally the measurements of piezoelectric properties previously reported by other authors we.g., Ref. w10xx are not direct, but use effective properties obtained by considering compliance values of thin films equal to those of bulk materials, which is not generally proved.. Our method consists of the following: a voltage amplitude is applied by a pulse generator on an acoustic transducer which converts the electric signal into an acoustic one. The acoustic signal is transmitted through a probe into the test film which converts it into an electric pulse measured by an oscilloscope. The proportional constant between the applied elec-
tric voltage and the measured electric voltage generated by the piezoelectric thin film is related to the piezoelectric constant d 33 . To obtain the absolute value of d 33 , the measurement is done comparatively with a known material like quartz or lithium niobate. It must be mentioned that direct measurements are possible only if Ža. the thickness of dielectric substrate is not very large Žabout 100 m m.; Žb. the resistivity of semiconductor substrate is low enough Žabout 10 V .; and Žc. there is an additional metal film under the film to be tested. The measurements done by using this method on our samples revealed that they were all piezoelectric without poling, and the direction of polarization was the same for all the samples. Similar results have been reported by Takayama and Tomita w24x on epitaxial PZT thin films grown by sputtering on MgO single crystals and on Ž100. Pt. They obtained tetragonal Ž001. oriented and rhombohedral Ž111. oriented PZT films which were polarized even without a poling treatment and had good ferroelectric properties. They concluded that the existence of pre-aligned polarization depends upon the substrate but they could not suggest exact causes of the phenomenon. Piezoelectricity for unpolled PZT films has also been reported by Etzold et al. w25x and Surowiak et al. w26x for sputtered heteroepitaxial PbTiO 3 films on Ž100. MgO. Lefki and Dormans obtained also piezoelectric as-deposited PZT films by MOCVD w17x. They attributed the origin of ferroelectric domain orientation to the internal stresses created during the deposition and subsequent cooling of the sample which
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can force an alignment of the domains. We believe that this could also be an explanation for the orientation of domains in our samples. The measured d 33 piezoelectric constant values varied slightly between different measurement points on the surface of our samples approximately around 35–40 pCrN. This value represents only approximately 20% of the PZT bulk constant but it is adequate for many thin film applications. This fact together with the
Fig. 5. SEM image of PZTrAurSi Ž100. film sample. The dotted segment corresponds to 6 m m.
low temperature one step deposition makes the obtained thin films useful in many integrated piezoelectric devices.
4. Conclusions In summary, oriented PZT films with good piezoelectric properties were deposited on Si Ž100.rAu and Si Ž111.rAu substrates at extremely low substrate temperatures of 3758C with a Nd-YAG laser ablation technique. This temperature is low enough to directly deposit the PZT on useful semiconducting devices. We believe that our present results could be useful in making ferroelectric thin films for the purpose of practical applications.
Acknowledgements The authors wish to acknowledge M. Acciarini for technical help. One of us ŽM.D.. gratefully acknowledges support from the Italian Ministry of Foreign Affairs.
References
Fig. 4. Ža. SEM image of PZTrAurSi Ž111. film sample. The dotted segment corresponds to 30 m m. Žb. Same as in Ža.. The dotted segment corresponds to 6 m m.
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