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Perovskite lead titanate PLD thin films: study of oxygen incorporation by 18O tracing technique
Materials Chemistry and Physics 59 (1999) 114±119
Perovskite lead titanate PLD thin ®lms: study of oxygen incorporation by 18O tracing technique a
N. Chaouia, E. Millona,*, J.F. Mullera, P. Eckerb, W. Bieckb, H.N. Migeonb
Laboratoire de SpectromeÂtrie de Masse et de Chimie Laser, IPEM, Universite de Metz, 1 boulevard Arago, 57078, Metz, France Centre de Recherche Public, Laboratoire d'Analyse des MateÂriaux, 162A avenue de la FaõÈencerie, L-1511, Luxembourg, Luxembourg
b
Received 7 September 1998; received in revised form 25 January 1999; accepted 15 February 1999
Abstract PbTiO3 (PT) ®lms have been deposited by pulsed laser deposition (PLD) technique on polycrystalline platinum substrate kept at a constant temperature of 5508C in a 30 Pa 18 O atmosphere pressure. The ¯uence and the laser repetition (frequency quadrupled Nd-YAG, 266 nm) have been varied in the range 1.5-3 J cmÿ2 and 2-10 Hz, respectively. The obtained ®lms exhibit a perovskite structure and have been analyzed by dynamic secondary ion mass spectrometry (D-SIMS) in order to determine the concentration pro®les of 16 O (coming from the PbTiO3 target) and 18 O (ambient gas). The effects of different parameters of the PLD process are discussed, such as the cooling step of the ®lm in oxygen atmosphere after deposition, the laser repetition rate and the gas phase reactions. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Laser; Deposition; Ablation; Isotopic tracing; Perovskite; Thin ®lms
1. Introduction Among the various ferroelectrics, lead titanate PbTiO3 (PT) is commonly used because of its interesting ferroelectric and pyroelectric properties. PT thin ®lms can be prepared by the pulsed laser deposition (PLD) process at a high substrate temperature and a rather large oxygen pressure. It is well known that a substrate temperature of about 550±6008C and an ambient oxygen pressure of 20±30 Pa are necessary to obtain the perovskite structure without pyrochlore phase [1,2]. As of now, little information is found in the literature about the precise role of oxygen in the deposition mechanism of PT and related ferroelectric materials such as PZT and PLT. It has been suggested by several authors that such a partial oxygen pressure helps to stabilize lead oxide species, and thus, to protect lead content in the ®lm from volatilization (due to substrate temperature) [3]. However, the respective contributions of each single process to the global deposition process including plasma expansion, deposition, growth and cooling of the ®lm are not well known. The 18 O tracing technique has been widely used to study oxygen diffusion in materials [4,5] as well as thermal or anodic *Corresponding author. Tel.: +33-387-315-854; fax: +33-387-315-851; e-mail: [email protected]
oxidation mechanisms [6,7]. In the ®eld of laser ablation, it constitutes a powerful means to investigate the relative importance of the ambient gas incorporation in the ®lms grown by PLD [8±10]. In this work, the depth pro®le of our perovskite ®lms in oxygen is determined by dynamic secondary ion mass spectrometry (D-SIMS). 2. Experimental The experimental setup used for PLD has been described elsewhere [11]. The target was prepared by cold pressing lead titanium oxide powder (PbTiO3; 99.9% pure) mixed with 10 wt.% excess PbO (99.9% pure) in order to compensate for the decrease in the Pb/Ti ratio of the target after successive laser shots. The obtained pellets of 1.3 cm diameter were annealed at 7008C during a period of 24 h. Degreased platinum sheets (1.2 cm 0.8 cm) held at 5508C were used as substrates. The substrate was chosen in order to perform later electrical measurements. A partial pressure of 30 Pa in 18 O2 (97% pure) was introduced in the ablation cell after the chamber was ¯ushed with Ar and pumped down to 10ÿ4 Pa several times. The optimized experimental laser ablation parameters allowing the formation of perovskite structure thin ®lms are as follows: a laser beam with wavelength at 266 nm Ð
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 0 5 1 - 6
N. Chaoui et al. / Materials Chemistry and Physics 59 (1999) 114±119
115
Table 1 Experimental parameters of PLD experiments Film No. ÿ2
Fluence (J cm ) Repetition rate (Hz) Deposition time (min) Cooling (Pa-gas)a
Reference
1
2
3
4
5
6
7
1.5 10 10 30 16O2
1.5 10 10 30 18O2
1.5 10 10 20 Ar
2 10 10 30 18O2
2.8 10 10 30 18O2
1.5 5 20 30 18O2
1.5 2 50 30 18O2
1.5 10 10 10ÿ4 Ar
All the films have been deposited in a 30 Pa 18 O2 atmosphere except Film 2 which has been grown in a 20 Pa argon atmosphere by using a PbTiO3 target which has been previously subjected to an 18 O2 atmosphere (30 Pa) under laser irradiation. a The cooling rate of the films was 10 8C minÿ1.
frequency quadrupled Nd-YAG; pulse duration: 6 ns. The laser spot diameter on the target was ®xed to 0.1 cm and the target was rotated at 5 rpm. The target±substrate distance was held at 3 cm. Thin ®lm deposition was performed in an ambient 18 O gas in order to distinguish between oxygen coming from the PT target (16 O) and the one coming from the gas phase (18 O). To study the different paths of oxygen incorporation in the ®lm, the laser ¯uence, the repetition rate, and the cooling and the deposition times were varied as summarized in Table 1. The D-SIMS analyses of the obtained ®lms were performed with a modi®ed version of a Cameca IMS-6F instrument [12]. A Cs primary beam of 10 nA with an impact energy of 5.5 keV was focused on a 250 mm 250 mm area. The oxygen isotopes (16 O and 18 O) were
monitored in the positive mode as CsO molecular ions which are closely correlated to the bulk composition of the oxide sample [13]. The D-SIMS pro®les of Cs16 O and Cs18 O versus the erosion time (i.e. depth pro®le) (Fig. 1(a)) lead to the determination of the [18 O] value [14]. 18 O content in the deposited ®lm [18 O] can be de®ned as follows: 18 O n 18 O=n 18 O n 16 O; where n(18 O) and n(16 O) stand for the number of 18 O and 16 O atoms, respectively, in the ®lm. As an example, the [18 O] pro®le of Film 7 is displayed Fig. 1(b). The precision of the isotopic ratio obtained by this method is less than 2%. 3. Results and discussion 3.1. Global approach of oxygen content determination Prior to the deposition in 18 O atmosphere, experiments have been performed in natural 16 O atmosphere under 30 Pa (®lm labeled reference, Table 1). In such conditions, the obtained ®lms display a randomly oriented perovskite structure without pyrochlore phase. Indeed, SEM micrography of the obtained ®lm shows a two-dimension grain structure (Fig. 2(a)), and XRD pattern (Fig. 2(b)) exhibits the peaks corresponding to the perovskite phase1. In contrast to the parameters used for the synthesis of high Tc superconducting ceramics, a high pressure in oxygen (i.e. 3 104 Pa) after the growth step is not required for obtaining the perovskite structure [1,2]. That is why it is not necessary to perform a post growth oxygenation process for perovskite material. The experimental parameters used to prepare ®lm reference (Table 1) constitute our reference conditions for the experiments performed in ambient 18 O2 gas. Such a reference is necessary for specifying the different incorporation paths of oxygen. When the same conditions are carried out in 18 O2 atmosphere (Film 1), the 18 O isotope represents about 46% of the oxygen introduced in the ®lm (Table 2).
Fig. 1. (a) Cs16 O and Cs18 O depth profiles measured with D-SIMS experiments, (b) [18 O ] depth profile of Film 7.
1 CuKa XRD patterns (/2 method) show diffraction peaks correspondÊ ing to the lattice of perovskite PbTiO3 with the following dhkl: 4.07A Ê (1 0 0), 2.81A Ê (1 0 1), 2.76A Ê (1 1 0), 2.27A Ê (1 1 1), 1.98A Ê (0 0 1), 3.88A Ê (2 0 0). (0 0 2), 1.81A
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N. Chaoui et al. / Materials Chemistry and Physics 59 (1999) 114±119
18 Obp represents 18 O atoms introduced in the film during the time interval between two successive pulses, 18 Ogpr represents 18 O atoms which have been incorporated in the film from the gas phase reactions between the plasma and the ambient gas, and 18 Ot is due to 18 O atoms coming directly from the target and is the result of oxygen exchange between the target and the ambient gas during each laser pulse. To study the different steps involved in the PLD process, we start logically with the process occurring at the target, moving to the one occurring in the plasma plume, to ®nish at the process on the ®lm surface. The experimental PLD parameters used are summarized in Table 1.
Fig. 2. (a) SEM micrograph and (b) XRD pattern of PT film obtained in 30 Pa oxygen atmosphere.
3.2. Dependence on specific parameters
3.2.1. Oxygen exchange on the target surface The ®rst contribution of oxygen incorporation in the ®lm can be due to exchange between the PT target and the ambient gas during the PLD process. Just after the laser shot, the impacted area reaches a high temperature leading, therefore, to a molten phase [15], which improves the exchange kinetic of atomic oxygen from the irradiated target towards the gas phase. In our experimental conditions, about 200 laser shots have irradiated the same point on the target. So it is expected that the ablated part will be gradually enriched in 18 O which will be then ejected from the target. To precise this phenomenon, a PT ®lm has been prepared on a non-heated substrate under 20 Pa argon atmosphere by ablation of a target which has already been used for deposition in 18 O2 atmosphere (Film 2, Table 1). Although the experimental conditions used in that case do not lead to a ®lm with a perovskite structure due to the absence of oxygen gas, we can get some information concerning the chemical contribution of 18 O introduced in the ®lm. The obtained enrichment in 18 O is very low (5%, Film 2, Table 2) but a slight increase in the 18 O depth pro®le from the surface towards the interface is observed (Fig. 3). It is important to notice that it is not possible to compare the sputtering time needed to reach the ®lm/substrate interface
The total amount of 18 O measured in the ®lm can be described by an equation which takes into account the contribution of each deposition step for which 18 O could be incorporated in the ®lm: 18 O 18 Oc 18 Obp 18 Ogpr 18 Ot where 18 Oc is the contribution of 18 O atoms which have been introduced in the film during the cooling after the deposition step, Table 2 18 O content in PT films determined by D-SIMS measurements Film No. 18
[ O] (%)
Reference
1
2
3
4
5
6
7
0
46
5
51
58
53
58
38
Fig. 3. [18 O] depth profile of Film 2.
N. Chaoui et al. / Materials Chemistry and Physics 59 (1999) 114±119
to the one allowing the detection of the platinum substrate for Film 7 (Fig. 1). As previously written, the morphological and the structural characteristics are not similar in both cases. The relative enrichment in 18 O for Film 2 towards the substrate (high sputtering time: 600 s) is correlated with the 18 O adsorbed on the target surface at the beginning of the deposition. As and when the ablation occurs, the 18 O content becomes lower on the target, and thus, the ®lm gradually becomes richer in 16 O. Nevertheless, the part of oxygen which is the result of the interactions between the target and the gas phase is, therefore, very low but not negligible. We can also suggest that the liquid droplets ejected from the ablated target could be enriched in 18 O during their transport, and they could then fall back on the target and ®nally be reablated. 3.2.2. Gas phase reaction exchange One of the most important processes occurring during PLD experiments is the interaction between the species of the laser plume and the background gas. We have seen that 54% of oxygen originates directly from the target as 46% of the oxygen incorporated in the ®lm originates from the gas phase (18 O). 16 O may be transferred from the target to the substrate in either atomic or molecular form or in the form of polyatomic clusters as proposed by several authors [8,16]. For instance, laser induced ¯uorescence (LIF) experiments [17] lead to the detection of ionic species in the plasma but this technique fails to characterize large mass compounds (i.e. polyatomic oxide species), and therefore, is limited to the analysis of atomic species or dimers. If cationic ions are ones of the kind of the constituent of the plume, mass spectrometry analyses clearly highlight that the gas phase process consists mainly of neutral species reactions (dissociation, aggregations, exchange reactions) and reactions involving photons, electrons, ions and exited neutrals leading to high mass clusters [16,18]. The transport of oxygen atoms from the target to the substrate occurs by the formation of oxidized species (clusters and neutrals) in the plasma in which oxygen exchange reactions with the ambient gas can happen. The kinetic energy of species emitted from the target should induce the formation of atomic oxygen, and therefore, increase collision and dissociation reactions in the plume. This fact can be discussed by performing PLD experiments with higher laser ¯uence (Films 3 and 4, Table 1). The [18 O] values for the obtained ®lms, Films 3 and 4 (in comparison with Film 1) are given in Table 2 and Fig. 4. A slight increase in the incorporation of 18 O in the ®lm with the laser beam ¯uence is observed. Several mechanisms can be proposed to explain this result: First of all, the increase in the fluence could improve photodissociation of gaseous oxygen in the neighborhood of the ablated target. However, this mechanism may be limited due to the low cross-section; atomic oxygen could also be generated by the dissociation of the ambient gas due to collision reactions
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Fig. 4. [18 O] content vs. the laser fluence.
between energetic electrons, ions and neutrals ejected from the target; one can estimate that a higher amount of molecular oxygen adsorbed on the surface of the film could be dissociated due to the higher kinetic energy and number density of arrival ions (bombardment-induced dissociation process); and finally, higher the energy of the ejected species, higher will be the number density of defects in the film, and consequently, gaseous oxygen incorporation during the cooling step would become more efficient. Reactions of ejected cations with the background gas also have to be discussed. The metal atoms can react with O2 molecules to form a metal oxide. The ability of a metal atom M to react with the background gas molecules can be treated in terms of exothermicities [19]. The reaction exothermicities (D0) corresponding to the formation of MO bond from M and O are given in Table 3. The reaction of Ti with O is highly exothermic and might occur instantaneously. This is not the case with the reaction of Pb with O, which is endothermic and so less favorable because of the low bond energy of PbO. So high collision energy between Pb and O2 is needed for surmounting the energy barrier for this reaction. It can be suggested that the increase in 18 O content in the ®lm with increasing laser ¯uence (kinetic energy of the ejected metal atom) favors the formation of PbO in the plume. The problem of CuO (D0 2.36 eV) formation in Table 3 Binding energies of monoxides and reaction exothermicities for the formation of Ti and Pb monoxides in reaction with O2 M (g)
MO dissociation energya (eV)
D0b (eV)
Ti Pb
6.86 0.05 3.89 0.06
1.75 0.05 ÿ1.22 0.06
a
Values from the Handbook of Chemistry and Physics, 65th edition, 1984±85, CRC Press, Boca Raton, FL. b Values derived from O2 dissociation energy (5.115 0.002 eV).
118
N. Chaoui et al. / Materials Chemistry and Physics 59 (1999) 114±119
laser ablation of `YBaCuO' is analogous to the one of Pb monoxide in PbTiO3 laser ablation. Moreover, our results are in agreement with LIF experiments performed on the Cu O2 reaction for `YBaCuO' ablation mechanism [20] for which CuO content in the plume was found to be very sensitive to the laser ¯uence. According to our results and whatever the mechanism involved in the plasma be, the oxygen exchange reactions between the gas phase and the ablated species are clearly ef®cient and contribute to the process of oxygen incorporation. 3.2.3. Oxygen exchange between two successive laser pulses During the PLD process, the adsorption of gaseous oxygen onto the surface of the ®lm may occur during the time available between two successive laser shots. The repetition rate of the laser pulse and time deposition have been changed simultaneously (Films 5 and 6, Table 1) in order to have the same total number of laser shots comparing to those of Film 1 (10 Hz, 10 min). Oxygen adsorption and diffusion in the ®lm during the time between two laser shots (0.1±0.5 s) is supposed to be signi®cant. The [18 O] values for the obtained ®lms are displayed in Table 2 and Fig. 5. One can note a slight decrease in the incorporation of gaseous oxygen in the ®lm when the repetition rate increases. These results are not in agreement with those observed according to Rutherford backscattering spectroscopy experiments conducted by Gomez San Roman et al. [9,10] for the oxygen transfer during the PLD synthesis of `BiSrCaCuO' thin ®lms. Firstly, the range of laser frequency used by Gomez San Roman et al. [9,10] is shorter (1.7± 5 Hz) than ours (2±10 Hz). Secondly, the grain structure of our ®lms (polycrystal with grain boundaries, Fig. 2(a)) suggests that the area exposed to the ambient gas might be larger than the surface size of heteroepitaxial ®lms [9,10]. This morphological feature facilitates the diffusion processes and allows the supposition that a low incorporation of oxygen may occur even in a short time (0.1±0.5 s).
Fig. 5. [18 O] content vs. the time between two successive laser pulses (the inverse of the laser repetition rate).
Moreover, by assuming the kinetic theory of the gas phase, the amount of particles that reach a surface of 1 cm2 at 5508C under an oxygen pressure of 30 Pa is about 1015. Now, the number of species ejected from the target and that reach the substrate at each pulse is about 1015±1016. This value is in the same range as the previous one. Therefore, one can expect a measurable effect during the time between two successive laser pulses. Nevertheless, the proportion of oxygen incorporated in the ®lm between two successive laser pulses is quite low. This point can be explained by the high activation energy needed for the molecular oxygen adsorption. 3.2.4. Oxygen incorporation during cooling Film 7 was formed under the same conditions as Film 1, excepting cooling which was carried out under vacuum (10ÿ4 Pa). The measured [18 O] value for Film 7 is 38% (Table 2). Upon comparison with the value obtained for Film 1 (46%), it is evident that after the ablation step, the atmosphere of cooling between 5508C and 508C has an in¯uence on the incorporation of oxygen in the ®lm. Indeed, if the difference in oxygen content measured in the two cases be attributed to a loss of oxygen for the sample cooled in vacuum, we should observe the same 18 O/16 O ratio due to the simultaneous loss of 18 O and 16 O. In comparison to superconductor materials which require high pressure to incorporate oxygen in the ®lms, the PT compound is oxidized under low oxygen background (30 Pa) that is ef®cient in introducing oxygen in the ®lm in agreement with thermodynamic data. This phenomenon represents about 8% of the oxygen introduced in the PT ®lms. It can be attributed to a surface gas exchange and diffusion process [21]. The oxygen diffusion mechanisms in perovskite structures involve structural defects in the form of oxygen vacancies and are facilitated by the presence of grain boundaries [22,23]. 4. Conclusions The isotopic tracer technique with 18 O has been used to study the relative importance of the different processes of oxygen incorporation in PT thin ®lms grown by PLD. Firstly, standard experimental PLD conditions leading to the formation of perovskite structure have been determined, which are as follows: a platinum substrate temperature of 5508C, an oxygen pressure of 30 Pa, a laser ¯uence of 1.5 J cm2 at a wavelength of 266 nm, a repetition rate of 10 Hz and a deposition time of 10 min. After varying some of the experimental parameters (cooling, repetition rate, ¯uence), the analyses performed by D-SIMS allowed us to determine 18 O content, and therefore, the contribution of the different incorporation paths of oxygen in the obtained ®lms. For the experimental conditions used, our results suggest that the major part (about 55%) of oxygen incorporated in
N. Chaoui et al. / Materials Chemistry and Physics 59 (1999) 114±119
the ®lm comes directly from the target. The complementary contribution (about 45%) is the result of several phenomena occurring during and after the PLD deposition, with the main contribution being due to exchange reactions (collision, dissociation, aggregation reactions) between the laser plume and the gaseous phase enriched in oxygen. This point explains the fact that it is necessary to perform PLD experiments in oxygen atmosphere to obtain correct stoichiometry in the PT ®lms. The interaction between the ablated target and the gas phase induces oxygen exchange reactions in low proportion. The oxygen of the gas phase can adsorb on the ®lm surface and diffuse between two successive laser pulses. This feature has been related to the grain structure of the ®lm. This contribution represents only a very small part of the oxygen incorporation process. Nevertheless, the cooling of the ®lm after deposition is accountable for 8±10% of the oxygen incorporation by diffusion in the ®lm. Therefore, cooling in oxygen is important for obtaining the appropriate stoichiometry. This is promoted by the high temperature of the substrate. If the different paths of oxygen incorporation have been clari®ed somehow, the understanding of the oxygen adsorption and diffusion mechanisms in the ®lm and its growth processes using the condensation of gaseous species (ionized cluster, neutral molecules) requires further investigations. References [1] T. Imai, M. Okuyama, Y. Hamakawa, Jpn. J. Appl. Phys. 30(9B) (1991) 2163. [2] H. Tabata, O. Murata, T. Kawai, S. Kawai, M. Okuyama, Jpn. J. Appl. Phys. 32 PartI(12A) (1991) 5611.
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[3] H. Tanaka, H. Tabata, T. Kawai, S. Kawai, Jpn. J. Appl. Phys. 33 (1994) L451. [4] I. Sakagushi, H. Haneda, J. Solid State Chem. 124 (1996) 195. [5] R.J. Chater, S. Carter, J.A. Kilner, B.C.H. Steele, in: A. Benninghoven et al. (Eds.), SIMS VIII, Wiley, New York, 1992, 133. [6] E. Rosencher, A. Straboni, S. Rigo, G. Amsel, Appl. Phys. Lett. 34 (1979) 254. [7] J. PerrieÁre, J. Siejka, R.P.H. Chang, J. Appl. Phys. 56 (1984) 2716. [8] M.R. Predtechensky, A.N. Smal', B.A. Kolesov, V.P. Ivanov, Appl. Superconductivity 1 (1993) 2005. [9] R. Gomez-San Roman, R. PeÂrez Cazero, C. MareÂchal, J.P. Enard, J. PerrieÁre, J. Appl. Phys. 80 (1996) 1787. [10] R. Perez Casero, R. Gomez San Roman, C. MareÂchal, J.P. Enard, J. PerrieÁre, Appl. Surface Sci., 96±98 (1996) 697. [11] N. Lobstein, E. Millon, A. Hachimi, J.F. Muller, M. Alnot, J.J. Ehrardt, Appl. Surface Sci. 89 (1995) 307. [12] W. Bieck, B. Rasser, M. Schumacher, P. Sudreau, H.N. Migeon, in: Benninghoven et al. (Eds)., SIMS X, Wiley, New York, 1997. [13] W. Bieck, T. Mootz, F. Saldi, in: G. Gillen et al. (Eds.), SIMS XI, Wiley, New York, 1998. [14] N. Chaoui, E. Millon, J.F. Muller, P. Ecker, W. Bieck, H.N. Migeon, in: Proc. E-MRS'98, Strasbourg, 16±19 June 1998. [15] E. Fogarassy, C.W. White, A. Slaoui, C. Fuchs, P. Siffert, S.J. Pennycook, Appl. Phys. Lett. 53 (1988) 1720. [16] N. Chaoui, E. Millon, J.F. Muller, Chem. Mater. 10(12) (1998) 3888. [17] C.E. Otis, R.W. Dreyfus, Phys. Rev. Lett. 67 (1991) 2102. [18] A. Mele, D. Consalvo, D. Stranges, A. Giardini-Guidoni, R. Teghil, Int. J. Mass Spectrometry Ion Proc. 95 (1990) 359. [19] A. Gupta, J. Appl. Phys. 73 (1993) 7877. [20] C.E. Otis, A. Gupta, B. Braren, Appl. Phys. Lett. 62 (1993) 102. [21] J. Philibert, in: Diffusion et transport de matieÁre dans les solides, les eÂditions de la physique, Paris, 1985. [22] S. Carter, A. Selcuk, R.J. Chater, J. Kayda, J.A. Kilner, B.C.H. Steele, Solid State Ionics 53±56 (1992) 597. [23] J. Mizusaki, Solid State Ionics 52 (1992) 79.
Perovskite lead titanate PLD thin ®lms: study of oxygen incorporation by 18O tracing technique a
N. Chaouia, E. Millona,*, J.F. Mullera, P. Eckerb, W. Bieckb, H.N. Migeonb
Laboratoire de SpectromeÂtrie de Masse et de Chimie Laser, IPEM, Universite de Metz, 1 boulevard Arago, 57078, Metz, France Centre de Recherche Public, Laboratoire d'Analyse des MateÂriaux, 162A avenue de la FaõÈencerie, L-1511, Luxembourg, Luxembourg
b
Received 7 September 1998; received in revised form 25 January 1999; accepted 15 February 1999
Abstract PbTiO3 (PT) ®lms have been deposited by pulsed laser deposition (PLD) technique on polycrystalline platinum substrate kept at a constant temperature of 5508C in a 30 Pa 18 O atmosphere pressure. The ¯uence and the laser repetition (frequency quadrupled Nd-YAG, 266 nm) have been varied in the range 1.5-3 J cmÿ2 and 2-10 Hz, respectively. The obtained ®lms exhibit a perovskite structure and have been analyzed by dynamic secondary ion mass spectrometry (D-SIMS) in order to determine the concentration pro®les of 16 O (coming from the PbTiO3 target) and 18 O (ambient gas). The effects of different parameters of the PLD process are discussed, such as the cooling step of the ®lm in oxygen atmosphere after deposition, the laser repetition rate and the gas phase reactions. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Laser; Deposition; Ablation; Isotopic tracing; Perovskite; Thin ®lms
1. Introduction Among the various ferroelectrics, lead titanate PbTiO3 (PT) is commonly used because of its interesting ferroelectric and pyroelectric properties. PT thin ®lms can be prepared by the pulsed laser deposition (PLD) process at a high substrate temperature and a rather large oxygen pressure. It is well known that a substrate temperature of about 550±6008C and an ambient oxygen pressure of 20±30 Pa are necessary to obtain the perovskite structure without pyrochlore phase [1,2]. As of now, little information is found in the literature about the precise role of oxygen in the deposition mechanism of PT and related ferroelectric materials such as PZT and PLT. It has been suggested by several authors that such a partial oxygen pressure helps to stabilize lead oxide species, and thus, to protect lead content in the ®lm from volatilization (due to substrate temperature) [3]. However, the respective contributions of each single process to the global deposition process including plasma expansion, deposition, growth and cooling of the ®lm are not well known. The 18 O tracing technique has been widely used to study oxygen diffusion in materials [4,5] as well as thermal or anodic *Corresponding author. Tel.: +33-387-315-854; fax: +33-387-315-851; e-mail: [email protected]
oxidation mechanisms [6,7]. In the ®eld of laser ablation, it constitutes a powerful means to investigate the relative importance of the ambient gas incorporation in the ®lms grown by PLD [8±10]. In this work, the depth pro®le of our perovskite ®lms in oxygen is determined by dynamic secondary ion mass spectrometry (D-SIMS). 2. Experimental The experimental setup used for PLD has been described elsewhere [11]. The target was prepared by cold pressing lead titanium oxide powder (PbTiO3; 99.9% pure) mixed with 10 wt.% excess PbO (99.9% pure) in order to compensate for the decrease in the Pb/Ti ratio of the target after successive laser shots. The obtained pellets of 1.3 cm diameter were annealed at 7008C during a period of 24 h. Degreased platinum sheets (1.2 cm 0.8 cm) held at 5508C were used as substrates. The substrate was chosen in order to perform later electrical measurements. A partial pressure of 30 Pa in 18 O2 (97% pure) was introduced in the ablation cell after the chamber was ¯ushed with Ar and pumped down to 10ÿ4 Pa several times. The optimized experimental laser ablation parameters allowing the formation of perovskite structure thin ®lms are as follows: a laser beam with wavelength at 266 nm Ð
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 0 5 1 - 6
N. Chaoui et al. / Materials Chemistry and Physics 59 (1999) 114±119
115
Table 1 Experimental parameters of PLD experiments Film No. ÿ2
Fluence (J cm ) Repetition rate (Hz) Deposition time (min) Cooling (Pa-gas)a
Reference
1
2
3
4
5
6
7
1.5 10 10 30 16O2
1.5 10 10 30 18O2
1.5 10 10 20 Ar
2 10 10 30 18O2
2.8 10 10 30 18O2
1.5 5 20 30 18O2
1.5 2 50 30 18O2
1.5 10 10 10ÿ4 Ar
All the films have been deposited in a 30 Pa 18 O2 atmosphere except Film 2 which has been grown in a 20 Pa argon atmosphere by using a PbTiO3 target which has been previously subjected to an 18 O2 atmosphere (30 Pa) under laser irradiation. a The cooling rate of the films was 10 8C minÿ1.
frequency quadrupled Nd-YAG; pulse duration: 6 ns. The laser spot diameter on the target was ®xed to 0.1 cm and the target was rotated at 5 rpm. The target±substrate distance was held at 3 cm. Thin ®lm deposition was performed in an ambient 18 O gas in order to distinguish between oxygen coming from the PT target (16 O) and the one coming from the gas phase (18 O). To study the different paths of oxygen incorporation in the ®lm, the laser ¯uence, the repetition rate, and the cooling and the deposition times were varied as summarized in Table 1. The D-SIMS analyses of the obtained ®lms were performed with a modi®ed version of a Cameca IMS-6F instrument [12]. A Cs primary beam of 10 nA with an impact energy of 5.5 keV was focused on a 250 mm 250 mm area. The oxygen isotopes (16 O and 18 O) were
monitored in the positive mode as CsO molecular ions which are closely correlated to the bulk composition of the oxide sample [13]. The D-SIMS pro®les of Cs16 O and Cs18 O versus the erosion time (i.e. depth pro®le) (Fig. 1(a)) lead to the determination of the [18 O] value [14]. 18 O content in the deposited ®lm [18 O] can be de®ned as follows: 18 O n 18 O=n 18 O n 16 O; where n(18 O) and n(16 O) stand for the number of 18 O and 16 O atoms, respectively, in the ®lm. As an example, the [18 O] pro®le of Film 7 is displayed Fig. 1(b). The precision of the isotopic ratio obtained by this method is less than 2%. 3. Results and discussion 3.1. Global approach of oxygen content determination Prior to the deposition in 18 O atmosphere, experiments have been performed in natural 16 O atmosphere under 30 Pa (®lm labeled reference, Table 1). In such conditions, the obtained ®lms display a randomly oriented perovskite structure without pyrochlore phase. Indeed, SEM micrography of the obtained ®lm shows a two-dimension grain structure (Fig. 2(a)), and XRD pattern (Fig. 2(b)) exhibits the peaks corresponding to the perovskite phase1. In contrast to the parameters used for the synthesis of high Tc superconducting ceramics, a high pressure in oxygen (i.e. 3 104 Pa) after the growth step is not required for obtaining the perovskite structure [1,2]. That is why it is not necessary to perform a post growth oxygenation process for perovskite material. The experimental parameters used to prepare ®lm reference (Table 1) constitute our reference conditions for the experiments performed in ambient 18 O2 gas. Such a reference is necessary for specifying the different incorporation paths of oxygen. When the same conditions are carried out in 18 O2 atmosphere (Film 1), the 18 O isotope represents about 46% of the oxygen introduced in the ®lm (Table 2).
Fig. 1. (a) Cs16 O and Cs18 O depth profiles measured with D-SIMS experiments, (b) [18 O ] depth profile of Film 7.
1 CuKa XRD patterns (/2 method) show diffraction peaks correspondÊ ing to the lattice of perovskite PbTiO3 with the following dhkl: 4.07A Ê (1 0 0), 2.81A Ê (1 0 1), 2.76A Ê (1 1 0), 2.27A Ê (1 1 1), 1.98A Ê (0 0 1), 3.88A Ê (2 0 0). (0 0 2), 1.81A
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18 Obp represents 18 O atoms introduced in the film during the time interval between two successive pulses, 18 Ogpr represents 18 O atoms which have been incorporated in the film from the gas phase reactions between the plasma and the ambient gas, and 18 Ot is due to 18 O atoms coming directly from the target and is the result of oxygen exchange between the target and the ambient gas during each laser pulse. To study the different steps involved in the PLD process, we start logically with the process occurring at the target, moving to the one occurring in the plasma plume, to ®nish at the process on the ®lm surface. The experimental PLD parameters used are summarized in Table 1.
Fig. 2. (a) SEM micrograph and (b) XRD pattern of PT film obtained in 30 Pa oxygen atmosphere.
3.2. Dependence on specific parameters
3.2.1. Oxygen exchange on the target surface The ®rst contribution of oxygen incorporation in the ®lm can be due to exchange between the PT target and the ambient gas during the PLD process. Just after the laser shot, the impacted area reaches a high temperature leading, therefore, to a molten phase [15], which improves the exchange kinetic of atomic oxygen from the irradiated target towards the gas phase. In our experimental conditions, about 200 laser shots have irradiated the same point on the target. So it is expected that the ablated part will be gradually enriched in 18 O which will be then ejected from the target. To precise this phenomenon, a PT ®lm has been prepared on a non-heated substrate under 20 Pa argon atmosphere by ablation of a target which has already been used for deposition in 18 O2 atmosphere (Film 2, Table 1). Although the experimental conditions used in that case do not lead to a ®lm with a perovskite structure due to the absence of oxygen gas, we can get some information concerning the chemical contribution of 18 O introduced in the ®lm. The obtained enrichment in 18 O is very low (5%, Film 2, Table 2) but a slight increase in the 18 O depth pro®le from the surface towards the interface is observed (Fig. 3). It is important to notice that it is not possible to compare the sputtering time needed to reach the ®lm/substrate interface
The total amount of 18 O measured in the ®lm can be described by an equation which takes into account the contribution of each deposition step for which 18 O could be incorporated in the ®lm: 18 O 18 Oc 18 Obp 18 Ogpr 18 Ot where 18 Oc is the contribution of 18 O atoms which have been introduced in the film during the cooling after the deposition step, Table 2 18 O content in PT films determined by D-SIMS measurements Film No. 18
[ O] (%)
Reference
1
2
3
4
5
6
7
0
46
5
51
58
53
58
38
Fig. 3. [18 O] depth profile of Film 2.
N. Chaoui et al. / Materials Chemistry and Physics 59 (1999) 114±119
to the one allowing the detection of the platinum substrate for Film 7 (Fig. 1). As previously written, the morphological and the structural characteristics are not similar in both cases. The relative enrichment in 18 O for Film 2 towards the substrate (high sputtering time: 600 s) is correlated with the 18 O adsorbed on the target surface at the beginning of the deposition. As and when the ablation occurs, the 18 O content becomes lower on the target, and thus, the ®lm gradually becomes richer in 16 O. Nevertheless, the part of oxygen which is the result of the interactions between the target and the gas phase is, therefore, very low but not negligible. We can also suggest that the liquid droplets ejected from the ablated target could be enriched in 18 O during their transport, and they could then fall back on the target and ®nally be reablated. 3.2.2. Gas phase reaction exchange One of the most important processes occurring during PLD experiments is the interaction between the species of the laser plume and the background gas. We have seen that 54% of oxygen originates directly from the target as 46% of the oxygen incorporated in the ®lm originates from the gas phase (18 O). 16 O may be transferred from the target to the substrate in either atomic or molecular form or in the form of polyatomic clusters as proposed by several authors [8,16]. For instance, laser induced ¯uorescence (LIF) experiments [17] lead to the detection of ionic species in the plasma but this technique fails to characterize large mass compounds (i.e. polyatomic oxide species), and therefore, is limited to the analysis of atomic species or dimers. If cationic ions are ones of the kind of the constituent of the plume, mass spectrometry analyses clearly highlight that the gas phase process consists mainly of neutral species reactions (dissociation, aggregations, exchange reactions) and reactions involving photons, electrons, ions and exited neutrals leading to high mass clusters [16,18]. The transport of oxygen atoms from the target to the substrate occurs by the formation of oxidized species (clusters and neutrals) in the plasma in which oxygen exchange reactions with the ambient gas can happen. The kinetic energy of species emitted from the target should induce the formation of atomic oxygen, and therefore, increase collision and dissociation reactions in the plume. This fact can be discussed by performing PLD experiments with higher laser ¯uence (Films 3 and 4, Table 1). The [18 O] values for the obtained ®lms, Films 3 and 4 (in comparison with Film 1) are given in Table 2 and Fig. 4. A slight increase in the incorporation of 18 O in the ®lm with the laser beam ¯uence is observed. Several mechanisms can be proposed to explain this result: First of all, the increase in the fluence could improve photodissociation of gaseous oxygen in the neighborhood of the ablated target. However, this mechanism may be limited due to the low cross-section; atomic oxygen could also be generated by the dissociation of the ambient gas due to collision reactions
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Fig. 4. [18 O] content vs. the laser fluence.
between energetic electrons, ions and neutrals ejected from the target; one can estimate that a higher amount of molecular oxygen adsorbed on the surface of the film could be dissociated due to the higher kinetic energy and number density of arrival ions (bombardment-induced dissociation process); and finally, higher the energy of the ejected species, higher will be the number density of defects in the film, and consequently, gaseous oxygen incorporation during the cooling step would become more efficient. Reactions of ejected cations with the background gas also have to be discussed. The metal atoms can react with O2 molecules to form a metal oxide. The ability of a metal atom M to react with the background gas molecules can be treated in terms of exothermicities [19]. The reaction exothermicities (D0) corresponding to the formation of MO bond from M and O are given in Table 3. The reaction of Ti with O is highly exothermic and might occur instantaneously. This is not the case with the reaction of Pb with O, which is endothermic and so less favorable because of the low bond energy of PbO. So high collision energy between Pb and O2 is needed for surmounting the energy barrier for this reaction. It can be suggested that the increase in 18 O content in the ®lm with increasing laser ¯uence (kinetic energy of the ejected metal atom) favors the formation of PbO in the plume. The problem of CuO (D0 2.36 eV) formation in Table 3 Binding energies of monoxides and reaction exothermicities for the formation of Ti and Pb monoxides in reaction with O2 M (g)
MO dissociation energya (eV)
D0b (eV)
Ti Pb
6.86 0.05 3.89 0.06
1.75 0.05 ÿ1.22 0.06
a
Values from the Handbook of Chemistry and Physics, 65th edition, 1984±85, CRC Press, Boca Raton, FL. b Values derived from O2 dissociation energy (5.115 0.002 eV).
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laser ablation of `YBaCuO' is analogous to the one of Pb monoxide in PbTiO3 laser ablation. Moreover, our results are in agreement with LIF experiments performed on the Cu O2 reaction for `YBaCuO' ablation mechanism [20] for which CuO content in the plume was found to be very sensitive to the laser ¯uence. According to our results and whatever the mechanism involved in the plasma be, the oxygen exchange reactions between the gas phase and the ablated species are clearly ef®cient and contribute to the process of oxygen incorporation. 3.2.3. Oxygen exchange between two successive laser pulses During the PLD process, the adsorption of gaseous oxygen onto the surface of the ®lm may occur during the time available between two successive laser shots. The repetition rate of the laser pulse and time deposition have been changed simultaneously (Films 5 and 6, Table 1) in order to have the same total number of laser shots comparing to those of Film 1 (10 Hz, 10 min). Oxygen adsorption and diffusion in the ®lm during the time between two laser shots (0.1±0.5 s) is supposed to be signi®cant. The [18 O] values for the obtained ®lms are displayed in Table 2 and Fig. 5. One can note a slight decrease in the incorporation of gaseous oxygen in the ®lm when the repetition rate increases. These results are not in agreement with those observed according to Rutherford backscattering spectroscopy experiments conducted by Gomez San Roman et al. [9,10] for the oxygen transfer during the PLD synthesis of `BiSrCaCuO' thin ®lms. Firstly, the range of laser frequency used by Gomez San Roman et al. [9,10] is shorter (1.7± 5 Hz) than ours (2±10 Hz). Secondly, the grain structure of our ®lms (polycrystal with grain boundaries, Fig. 2(a)) suggests that the area exposed to the ambient gas might be larger than the surface size of heteroepitaxial ®lms [9,10]. This morphological feature facilitates the diffusion processes and allows the supposition that a low incorporation of oxygen may occur even in a short time (0.1±0.5 s).
Fig. 5. [18 O] content vs. the time between two successive laser pulses (the inverse of the laser repetition rate).
Moreover, by assuming the kinetic theory of the gas phase, the amount of particles that reach a surface of 1 cm2 at 5508C under an oxygen pressure of 30 Pa is about 1015. Now, the number of species ejected from the target and that reach the substrate at each pulse is about 1015±1016. This value is in the same range as the previous one. Therefore, one can expect a measurable effect during the time between two successive laser pulses. Nevertheless, the proportion of oxygen incorporated in the ®lm between two successive laser pulses is quite low. This point can be explained by the high activation energy needed for the molecular oxygen adsorption. 3.2.4. Oxygen incorporation during cooling Film 7 was formed under the same conditions as Film 1, excepting cooling which was carried out under vacuum (10ÿ4 Pa). The measured [18 O] value for Film 7 is 38% (Table 2). Upon comparison with the value obtained for Film 1 (46%), it is evident that after the ablation step, the atmosphere of cooling between 5508C and 508C has an in¯uence on the incorporation of oxygen in the ®lm. Indeed, if the difference in oxygen content measured in the two cases be attributed to a loss of oxygen for the sample cooled in vacuum, we should observe the same 18 O/16 O ratio due to the simultaneous loss of 18 O and 16 O. In comparison to superconductor materials which require high pressure to incorporate oxygen in the ®lms, the PT compound is oxidized under low oxygen background (30 Pa) that is ef®cient in introducing oxygen in the ®lm in agreement with thermodynamic data. This phenomenon represents about 8% of the oxygen introduced in the PT ®lms. It can be attributed to a surface gas exchange and diffusion process [21]. The oxygen diffusion mechanisms in perovskite structures involve structural defects in the form of oxygen vacancies and are facilitated by the presence of grain boundaries [22,23]. 4. Conclusions The isotopic tracer technique with 18 O has been used to study the relative importance of the different processes of oxygen incorporation in PT thin ®lms grown by PLD. Firstly, standard experimental PLD conditions leading to the formation of perovskite structure have been determined, which are as follows: a platinum substrate temperature of 5508C, an oxygen pressure of 30 Pa, a laser ¯uence of 1.5 J cm2 at a wavelength of 266 nm, a repetition rate of 10 Hz and a deposition time of 10 min. After varying some of the experimental parameters (cooling, repetition rate, ¯uence), the analyses performed by D-SIMS allowed us to determine 18 O content, and therefore, the contribution of the different incorporation paths of oxygen in the obtained ®lms. For the experimental conditions used, our results suggest that the major part (about 55%) of oxygen incorporated in
N. Chaoui et al. / Materials Chemistry and Physics 59 (1999) 114±119
the ®lm comes directly from the target. The complementary contribution (about 45%) is the result of several phenomena occurring during and after the PLD deposition, with the main contribution being due to exchange reactions (collision, dissociation, aggregation reactions) between the laser plume and the gaseous phase enriched in oxygen. This point explains the fact that it is necessary to perform PLD experiments in oxygen atmosphere to obtain correct stoichiometry in the PT ®lms. The interaction between the ablated target and the gas phase induces oxygen exchange reactions in low proportion. The oxygen of the gas phase can adsorb on the ®lm surface and diffuse between two successive laser pulses. This feature has been related to the grain structure of the ®lm. This contribution represents only a very small part of the oxygen incorporation process. Nevertheless, the cooling of the ®lm after deposition is accountable for 8±10% of the oxygen incorporation by diffusion in the ®lm. Therefore, cooling in oxygen is important for obtaining the appropriate stoichiometry. This is promoted by the high temperature of the substrate. If the different paths of oxygen incorporation have been clari®ed somehow, the understanding of the oxygen adsorption and diffusion mechanisms in the ®lm and its growth processes using the condensation of gaseous species (ionized cluster, neutral molecules) requires further investigations. References [1] T. Imai, M. Okuyama, Y. Hamakawa, Jpn. J. Appl. Phys. 30(9B) (1991) 2163. [2] H. Tabata, O. Murata, T. Kawai, S. Kawai, M. Okuyama, Jpn. J. Appl. Phys. 32 PartI(12A) (1991) 5611.
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[3] H. Tanaka, H. Tabata, T. Kawai, S. Kawai, Jpn. J. Appl. Phys. 33 (1994) L451. [4] I. Sakagushi, H. Haneda, J. Solid State Chem. 124 (1996) 195. [5] R.J. Chater, S. Carter, J.A. Kilner, B.C.H. Steele, in: A. Benninghoven et al. (Eds.), SIMS VIII, Wiley, New York, 1992, 133. [6] E. Rosencher, A. Straboni, S. Rigo, G. Amsel, Appl. Phys. Lett. 34 (1979) 254. [7] J. PerrieÁre, J. Siejka, R.P.H. Chang, J. Appl. Phys. 56 (1984) 2716. [8] M.R. Predtechensky, A.N. Smal', B.A. Kolesov, V.P. Ivanov, Appl. Superconductivity 1 (1993) 2005. [9] R. Gomez-San Roman, R. PeÂrez Cazero, C. MareÂchal, J.P. Enard, J. PerrieÁre, J. Appl. Phys. 80 (1996) 1787. [10] R. Perez Casero, R. Gomez San Roman, C. MareÂchal, J.P. Enard, J. PerrieÁre, Appl. Surface Sci., 96±98 (1996) 697. [11] N. Lobstein, E. Millon, A. Hachimi, J.F. Muller, M. Alnot, J.J. Ehrardt, Appl. Surface Sci. 89 (1995) 307. [12] W. Bieck, B. Rasser, M. Schumacher, P. Sudreau, H.N. Migeon, in: Benninghoven et al. (Eds)., SIMS X, Wiley, New York, 1997. [13] W. Bieck, T. Mootz, F. Saldi, in: G. Gillen et al. (Eds.), SIMS XI, Wiley, New York, 1998. [14] N. Chaoui, E. Millon, J.F. Muller, P. Ecker, W. Bieck, H.N. Migeon, in: Proc. E-MRS'98, Strasbourg, 16±19 June 1998. [15] E. Fogarassy, C.W. White, A. Slaoui, C. Fuchs, P. Siffert, S.J. Pennycook, Appl. Phys. Lett. 53 (1988) 1720. [16] N. Chaoui, E. Millon, J.F. Muller, Chem. Mater. 10(12) (1998) 3888. [17] C.E. Otis, R.W. Dreyfus, Phys. Rev. Lett. 67 (1991) 2102. [18] A. Mele, D. Consalvo, D. Stranges, A. Giardini-Guidoni, R. Teghil, Int. J. Mass Spectrometry Ion Proc. 95 (1990) 359. [19] A. Gupta, J. Appl. Phys. 73 (1993) 7877. [20] C.E. Otis, A. Gupta, B. Braren, Appl. Phys. Lett. 62 (1993) 102. [21] J. Philibert, in: Diffusion et transport de matieÁre dans les solides, les eÂditions de la physique, Paris, 1985. [22] S. Carter, A. Selcuk, R.J. Chater, J. Kayda, J.A. Kilner, B.C.H. Steele, Solid State Ionics 53±56 (1992) 597. [23] J. Mizusaki, Solid State Ionics 52 (1992) 79.