Structural characteristics of RF-sputtered BaTiO3 thin films

Thin Solid Films 389 Ž2001. 43᎐50

Structural characteristics of RF-sputtered BaTiO 3 thin films Liliana Predaa , Laurent Courselle b , Bernard Despax b,U , J. Bandet c , Adelina Ianculescu d a

Department of Mathematics and Physics, Uni¨ ersity ‘POLITEHNICA’ Bucharest, 313 Splaiul Independentei, 77206 Bucharest, Romania Laboratoire de Genie Electrique (LGET-UMR 5003 CNRS) Uni¨ ersite´ PAUL SABATIER, 118 Route de Narbonne, F31062 Toulouse, France c Laboratoire de Physique des Solides (LPST-ESA UMR 5477) Uni¨ ersite´ PAUL SABATIER, 118 Route de Narbonne, F31062 Toulouse, France d Department of Materials Science & Engineering, Uni¨ ersity ‘POLITEHNICA’ Bucharest, 1 Polizu Str. P.O. Box 12-134, Bucharest, Romania b

Received 21 April 2000; received in revised form 19 January 2001; accepted 25 February 2001

Abstract In this work some structural characteristics of the thin films deposited by a radio frequency-magnetron sputtering technique from a hot pressed BaTiO 3 ceramic target were studied. The BarTi ratio was measured by means of X-ray fluorescence in order to determine the real chemical composition of the films. The results showed that this ratio varied along the radial axis of the discharge. Besides, the evolution in phase composition of the annealed films as a function of the thermal treatment conditions Žtemperature and plateau. as well as the structural characteristics Žunit cell parameter and crystallite size. were investigated by X-ray diffraction ŽXRD.. Infrared ŽIR. and Raman spectroscopies were used in order to obtain more details about the distorted structure of such fine-grained thin films. In the case of these films, Raman spectroscopy carried out from 99 to 473 K did not emphasize steep, distinct transitions between the different polycrystalline BaTiO 3 forms when compared with the bulk BaTiO 3. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Infrared spectroscopy; Raman scattering; Sputtering

1. Introduction Properties of BaTiO 3 thin films have received much attention in recent years due to their application in microelectronics and integrated optics technologies. Various methods like hydrothermal synthesis w1x, spincoated processing w2x, laser deposition w3x or sputtering w4,5x are in use for the fabrication of BaTiO 3 thin films. Compared with bulk materials, thin films still exhibit attractive dielectric properties such as high dielectric

U

Corresponding author. Tel.: q33-5-61-55-67-97; fax: q33-5-6155-64-52. E-mail address: [email protected] ŽB. Despax..

constant and find applications in miniaturized devices from the micron to the nano scale. However, the structure and the properties of thin-layer ferroelectrics are well known to exhibit a number of deviations from those of bulk ceramic or single crystals, for example the crystal size effects in thin films w1,5x. In an attempt to understand the variation in electrical properties of annealed BaTiO 3 thin films deposited by sputtering at a low temperature, a structural analysis was carried out. We found that the composition of deposited material is very important as it determines its ultimate properties. The realization of films with high dielectric constants close to those of bulk materials is conditioned by the physico-chemical properties of the films depending largely on the prevailing conditions

0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 0 8 8 5 - 9

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L. Preda et al. r Thin Solid Films 389 (2001) 43᎐50

Table 1 Elaboration conditions Deposition conditions Target Target size Substrate Target᎐substrate distance Sputtering gas Sputtering gas pressure RF power Substrate temperature Deposition rate Substrate position along the radial axis Annealing conditions Temperature Plateau Atmosphere

BaTiO3 powder 10 cm diameter Palladium foil 4 cm Argon Ž100%. 10 mtorr 200 W 300⬚C 4᎐18 nmrmin 0᎐6 cm

600᎐1000⬚C 5᎐35 h Air

under which the layers are elaborated. Indeed, both the crystal size and the deviation from the stoichiometry of the films have a marked influence on their dielectric behavior. A number of similar studies were undertaken for the phase characterization in the barium᎐titanium oxide system w6,7x. 2. Experimental techniques 2.1. Sample preparation The thin films were deposited by radio frequencymagnetron sputtering from a hot pressed BaTiO 3 ceramic target. The sputtering was carried out in pure argon maintained at a pressure of 10 mtorr. The substrate holder was heated to a temperature of 300⬚C. Palladium foils were used as a substrate. Under the above conditions, the deposition rate was higher in the central part of the discharge Ž18 nm miny1 . and decreased gradually to the periphery Ž4 nm miny1 . depending on the substrate position in the deposition chamber ŽTable 1.. The as-grown films were amorphous in most cases and they showed a non-stoichiometry in the composition. Post deposition annealing of the samples was carried out in air at different temperatures in order to obtain polycrystalline films with different phase compositions.

a Perkin-Elmer 1760 X Fourier Transformed Infrared ŽFTIR. spectrometer to observe the variations in chemical bond densities. For specular measurements palladium foils were used as substrates. Palladium is not transparent to infrared ŽIR. radiation. As a result, it is impossible to make use of the classical IR transmission. However, because of the metallic character of the substrate, which behaves like a mirror for the IR radiation, it is possible to use the specular reflection technique. For a thickness of the deposited film with a value close to the wavelength of the incident IR radiation, this technique is also well-known for ‘double absorption’ involving specific phenomena dependent on the angle of incidence. In order to interpret the phenomena, it is necessary to understand more precisely how the light interacts with oriented crystals. Indeed because of the character of electromagnetic radiation, in conventional transmission spectroscopy at normal incidence only transverse optical ŽTO. modes Žparallel to the surface . interact with the electric field associated with radiation. Thus at a normal incidence, only the TO modes are detected in an oriented network. The plane of incidence is formed by the vector of incidence Ž k . and the parallel component of electric field Ž E 5 . whereas the perpendicular component Ž E H . is perpendicular to the plane of incidence and parallel to the substrate surface. A radiation at an oblique incidence allows us to obtain the parallel component of the electric field out of the sample surface. This excites the longitudinal optical phonons. With the specular measurement, the incident radiation is reflected once on the surface of the deposited film and twice on the substrate ᎐film interface. In order to reckon the real absorption spectrum, background data on palladium foil is subtracted from the initial spectrum. The propagation in the film is then accompanied by an absorption at characteristic wavelengths of the deposited materials. Raman analysis was carried out with a Dilor X-Y spectrometer equipped with a charge coupled device multichannel detection system. The film thickness was determined by the TENCOR surface profilometer and had values ranging between 1 and 3 ␮m. 3. Results and discussion 3.1. X-Ray diffraction

2.2. Composition measurements and structural in¨ estigations of the thin films The BarTi ratio was measured by means of X-ray fluorescence using a Tracor X-ray Spectrace 5000 for composition characterization of the films. X-Ray diffraction measurements ŽXRD. were performed by a Seifert XRD 3000TT diffractometer for the identification and characterization of the crystallized phases in the film. Infrared analyses were performed by means of

Barium titanate is normally crystallized in a tetragonal phase at ambient temperature. Numerous authors have already indicated the interpretation difficulties related to small grain sized materials. In general, the difference between a tetragonal phase and a cubic phase is confirmed by the separation of diffraction peaks Ž002. and Ž200.. Several works reported the existence of a critical grain size at which it becomes impossible to clearly distinguish a crystalline form of BaTiO 3

L. Preda et al. r Thin Solid Films 389 (2001) 43᎐50

from other forms. Moreover, the disappearance of ferroelectricity phenomena for grain sizes ranging between 0.09 and 0.2 ␮m as a function of processing route and parameters was also pointed out w1,8x. However, in the case of fine-grained thin films it was concluded that the room temperature pseudocubic structure preceded the stable room temperature tetragonal form characteristic of coarse-grained bulk BaTiO 3 . According to some authors w9,10x, the so-called pseudocubic phase occurs when the grain size decreases sufficiently so that the unit cell becomes less and less tetragonal as a result of the internal strains. They proposed a shell᎐core grain model in which an individual grain consists of a cubic shell and a tetragonal grain interior. This tetragonal core becomes smaller and smaller with the grain size decrease and at the critical grain size it vanishes. In other words, the ferroelectricity disappears. However, this grain size effect is not the only reason for which a clear distinction between the different BaTiO 3 phases is difficult to make. The internal stress in the finegrained materials also causes a modification of the orthorhombic-tetragonal transformation temperature, which is shifted to approximately 25⬚C w11,12x. As a result, for this kind of material, it is preferable to discuss in terms of a change of phase transition temperature between the two different BaTiO 3 configurations due to a slight modification of the barrier energy. In our case the as-grown films exhibit an amorphous structure evidenced by the lack of diffraction peaks. After annealing the films become crystalline, their phase composition and the degree of crystallinity being strongly influenced by both chemical composition ŽBarTi ratio. and annealing parameters Žannealing temperature and plateau. w12᎐14x. The X-ray diffraction patterns of the films deposited on palladium substrates and annealed at 650⬚C are presented in Fig. 1a᎐c for various BarTi ratios. The curve Žc. arises from a stoichiometric sample ŽBarTi s 1.. As one can ascertain, the layers do not present any preferential orientation and the peak positions are in accordance with those of a bulk material w13,14x. Given the small average grain size, estimated by the diffraction peak width at half maximum, the crystalline phase is supposed to be pseudocubic. As soon as the BarTi ratio decreases, the diffraction peak intensity, characteristic of BaTiO 3 , decreases and a new peak-group begins to appear. When the quantity of titanium present in the layer increases, the layers develop a second chemical phase during their annealing twined to that of BaTiO 3 and identified as BaTi 2 O5 . As such, the excess of titanium leads to titanium-rich second phase, BaTi 2 O5 ŽFig. 1a,b.. The X-ray diffraction patterns of stoichiometric layers at different annealing temperatures are presented in Fig. 2. The significant increase of crystal size with the annealing temperature is emphasized by a distinct

45

Fig. 1. X-Ray diffraction patterns of thin films for different compositions after annealings at 650⬚C with a plateau of 10 h. Ža. BarTi s 0.52, Žb. BarTi s 0.75, Žc. BarTi s 1.

decrease of full peak-width at half maximum ŽFWHM.. Comparing with the BaTiO 3 spectrum obtained on powders, we observe a change in the film orientation at the lowest annealing temperatures. Indeed, the relative peak amplitudes are no longer the same as those observed on a bulk material. An orientation of Ž110. plane, predominant above 600⬚C is noticed. This observation is in accordance with the results of Nagamoto w15x. This tendency is due to the higher atom occupation density of the Ž110. plane among the crystal planes with low indices. The average diffracting crystal size has been evaluated from Laue᎐Scherrer formula, by observing the FWHM ŽFig. 3.. The films crystallize after annealing above a temperature of 600⬚C and present grain size of ˚ Above 650⬚C, the average crystal approximately 130 A. size increases more or less linearly with the rise in annealing temperature and reaches up to values near ˚ at 1000⬚C. These values are well below the 500 A critical grain size to obtain a well determined crystalline phase and make the phase characterization difficult. The lattice parameter evolution as a function of crystal size is presented in Fig. 4. It decreases linearly

Fig. 2. X-Ray diffraction patterns of BaTiO 3 stoichiometric film at different annealing temperatures, Ža. 650⬚C ᎏ 10 h, Žb. 900⬚C ᎏ 10 h, Žc. 1000⬚C ᎏ 10 h.

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L. Preda et al. r Thin Solid Films 389 (2001) 43᎐50

Fig. 3. Average crystal size as a function of annealing temperature.

as soon as the grain size increases and tends to stabilize ˚ Žannealing ) 900⬚C. at a value of itself beyond 300 A lattice parameter close to the one obtained for the bulk ˚ for the film and 3.994 A ˚ for the material Ž4.009 A powders.. Similar observations were reported by Lee et al. w16x who obtained a stabilization at annealing temperatures above 900⬚C. As a consequence to the temperature treatment, a decrease in layer thickness as well as formation of microcracks on the surface were observed w16,17x. They were attributed to a densification of the film during its crystallization indicating a densification of the crystalline structure. Therefore, the atoms approach each other contributing to a lattice parameter decrease w17x, but have a tendency to exert an extensive stress on the layer inducing microcracks. 3.2. Infrared spectroscopy by specular measurements Fig. 5 shows the infrared spectra obtained by specular measurements on non-stoichiometric samples after different annealings. We note the presence of BaTiO 3 Ž475 cmy1 and 717 cmy1 . as well as BaTi 2 O5 formation as already shown by XRD. This latter formation tends to become more pronounced as the annealing temperature increases. Small quantities of BaO Ž1380 cmy1 , 1090 cmy1 and 1020 cmy1 . and TiO 2 Ž458 cmy1 . are also present. The BaO peak identification was

Fig. 4. Evolution of the lattice parameter of BaTiO 3 thin films as a function of the average size of crystals.

Fig. 5. IR spectra obtained by specular measurement of under stoichiometric films with different compositions after annealing at 10 h. Ža. BarTi s 0.75 at 650⬚C, Žb. BarTi s 0.65 at 650⬚C, Žc. BarTi s 0.59 at 1000⬚C, Žd. BarTi s 0.52 at 900⬚C. BT: BaTiO 3 .

carried out by comparison with an IR spectrum of BaO powder. Nevertheless, it should be remarked that the peaks located at approximately 1400 cmy1 are attributed by some authors w18x to a vibration mode of COy 3 , due to the presence of BaCO 3 in their samples. Knowing the strong reactivity of BaO, we can assume a weak and superficial BaCO 3 formation in our samples issued from the reaction between BaO and CO 2 contained in ambient air. The TiO 2 presence was identified from literature data w7,19,20x. Now Fig. 6 represents the structural evolution of non-stoichiometric samples of identical BarTi ratios for different annealing plateaus. Comparing a, b and c spectra successively, a more important formation of BaTi 2 O5 after an annealing of 35 h at 650⬚C Žspectrum b., underlining the influence of annealing duration can be noticed. However, the annealing temperature plays an important role in BaTi 2 O5 formation. For an annealing of 35 h at 600⬚C Žcurve c., we observe a better formation of BaTiO 3 Ž479 cmy1 . and an attenuation in BaTi 2 O5 . A significant decrease of BaO indicated by the peak reduction at 1426 cmy1 in b and c spectra can be remarked upon.

Fig. 6. IR spectra obtained by specular measurement of understoichiometric films ŽBarTi s 0.65. after different annealings Ža. 650⬚C ᎏ 10 h, Žb. 650⬚C ᎏ 35 h, Žc. 600⬚C ᎏ 35 h.

L. Preda et al. r Thin Solid Films 389 (2001) 43᎐50

These IR measurements show that the phase formation is influenced besides the BarTi ratio by both annealing temperature and plateau. The higher the annealing temperature and longer the duration, the more important will be the formation of BaTi 2 O5 . This result agrees with the findings of Ritter et al. w6x indicating the appearance of BaTi 2 O5 formation at approximately 650⬚C. In addition, the presence of BaO and TiO 2 peaks and their diminution with higher and longer annealing provide proof to the solid state reaction which occurs to form BaTiO 3 or BaTi 2 O5 . As a result of BaTiO 3 formation, the non-stoichiometry in the residual amorphous matrix increases ŽBarTi ratio decreases. and forces BaTi 2 O5 crystallization. Thus, one can assume as being more probable the clause that BaTi 2 O5 crystallizes independently and competes with BaTiO 3 crystallization rather than being a reaction product of BaTiO 3 with TiO 2 . This assumption is supported by the differential increase of both BaTiO 3 and BaTi 2 O5 phases in a non-stoichiometric thin film when the annealing temperature rises above 650⬚C. This means that the BaTi 2 O5 structure is not developed by the BaTiO 3 phase consumption in a potential BaTiO 3 q TiO 2 ª BaTi 2 O5 reaction. Fig. 7 shows the spectra obtained on stoichiometric layers. The formation of the BaTi 2 O5 phase has completely disappeared as indicated by XRD. The spectrum Ža. obtained on an unannealed layer presents broad peaks and seems difficult to interpret. It is nevertheless possible to detect the presence of BaO Ž1445 cmy1 , 1395 cmy1 and 1010 cmy1 . and TiO 2 Ž450 cmy1 .. After an annealing at 600⬚C Žspectrum b., a significant decrease of BaO peaks occur and two principal peaks appear at 487 cmy1 and 720 cm ᎐ 1, respectively. They are better defined as annealing temperature increases and have a tendency to shift slightly towards smaller wavelengths. According to the literature w20᎐22x, the Ti᎐O vibration of BaTiO 3 is characterized by a transversal optical

Fig. 7. IR spectra obtained by specular measurement for stoichiometric films: Ža. as deposited, Žb. annealing at 600⬚C ᎏ 10 h, Žc. annealing 650⬚C ᎏ 10 h, Žd. annealing 900⬚C ᎏ 5 h, Že. annealing 1000⬚C ᎏ 7 h.

47

Fig. 8. IR specular measurements of BaTiO 3 film for various angles of incidence.

mode indicated at 487 cmy1 attributed to the stretching mode of TiO6 octahedron, as well as by a longitudinal optical mode of A 1 symmetry at 720 cmy1 . It is difficult to emphasize the longitudinal optical ŽLO. mode by normal incidence transmission measurements. However, the bigger the incident angles gets, the more intense the longitudinal optical modes become, whereas the transverse optical ŽTO. modes decrease with the increase of the incident angle. In our case, the use of a specular reflection method allows us to easily obtain a well-defined peak corresponding to LO mode at 720 cmy1 . Busca w20x, Perry w23x and Last w21x reported the presence of BaTiO 3 longitudinal mode, both in orthorhombic and tetragonal forms. Thus, the only way to separate the two phases would be to take into account the presence or absence of a 180-cmy1 vibration mode, which characterizes the lattice deformation. In order to provide this information on the elucidation of the crystalline structure of thin BaTiO 3 films, Raman spectroscopy was used. Fig. 8 represents the infrared spectra obtained at variable incident angles from normal. These measurements permit us to identify the vibration modes of the observed spectrum lines, the LO modes getting more intense as the angle of incidence gets higher, just the opposite of TO modes. A detailed explanation of the principle behind this phenomenon is found in the literature w24x. There appears to be important changes depending on the incident angle considered. Actually, when the angle of incidence increases, the parallel component of electric field leaves the surface plane of the layer deactivating a part of the vibration mode located at 487 cmy1 which is assigned to a TO mode, whereas the peak located at 720 cmy1 which is assigned to a LO vibration mode increases. Notice that these results confirm a preferential orientation of polycrystalline growth due to annealing as already observed in XRD. Moreover, a weak peak at 458 cmy1 attributed to TiO 2 , practically invisible at the lowest angles of incidence, becomes much more distinct and activated with the angle of incidence increasing according to a LO mode. So, we can conclude that traces of

L. Preda et al. r Thin Solid Films 389 (2001) 43᎐50

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3.3. Raman spectroscopy Fig. 10a,b shows the Raman spectra carried out on the sputtering target Žtetragonal structure . and on a stoichiometric BaTiO 3 thin film, respectively. As regards the spectrum obtained on target powder, we clearly observed four peaks at the following frequencies: very broad at 264 cmy1 , and very narrow at 305, 519 and 718 cmy1 . The wide peak at 264 cmy1 is attributed by Perry et al. w23x to a LO mode, they also ascribe the band at approximately 718 cmy1 to be associated with the highest frequency LO mode of A 1 symmetry. The sharp peak located at 305 cmy1 and assigned to the B 1 mode is particularly useful due to its inalterable position with crystal orientation w25,26x. Numerous authors w20,27x identify the peaks at 305 and 718 cmy1 as being characteristic of tetragonal BaTiO 3 . The peak observed at 518 cmy1 is assigned to a TO vibration, in accordance with the works of Asiaie et al. and Busca et al. w1,20x. The same peaks are observed on a stoichiometric BaTiO 3 thin film. A comparison with the target powder spectrum shows an amplitude decrease of the last peaks mentioned. More particularly, the peak at 264 cmy1 is strongly affected, the amplitude decrease being accompanied by an obvious flattening and displacement towards lower frequencies. This effect could be explained in terms of the grain size, clearly smaller in the film than in the ceramic target. As the grain sizes are too small, the lattice stretching observed along the w100x direction, characteristic of a tetragonal phase, will be extremely attenuated so that it is more precise to talk about a pseudocubic phase, which also resulted from the diffraction data. The appearance of two supplementary peaks was pointed out: a peak located at 652 cmy1 attributed to the palladium substrate from the Raman spectrum obtained from a virgin substrate and a small peak at 185 cmy1 . This latter result is in good agreement with the one obtained by several authors w20,23x. They noticed the peak located at approximately 180 cmy1 , as well as a broadening of the 270-cmy1 peak, when measurements were carried out at ambient temperature on samples whose grain sizes were small enough to favor the pseudocubic form. The work of Perry and Hall w23x also detected the presence of the vibration at

Fig. 9. IR specular measurement of stoichiometric films at an angle of incidence of 40⬚ for various thicknesses.

TiO 2 are still present in the layer. The evolution of these spectra with thickness is shown in Fig. 9. We remark an attenuation of the TO vibration mode located at 487 cmy1 when the thickness decreases. This phenomenon is due to the perpendicular field cancellation in the immediate neighborhood of the metallic surface. In fact, there exists a small conical zone nearby the metal due to the IR beam width where incoming and reflected perpendicular components of the electric field of light are superimposed to each other and, therefore, the total perpendicular component of electric field associated with light is nil w24x. Hence, the ŽTO. excitation modes will be much weaker as the layer thins. It has been demonstrated by Luxon w22x that the perovskites grain size modifies the width and the frequency of the absorption peaks. The peaks have a tendency to get narrow and to shift towards lower frequencies as the crystallite size increases. Table 2 gives frequencies and widths of the two principal peaks observed on the spectra for different grain sizes obtained. The LO mode shifts towards lower frequencies Ž18 cmy1 . and the ŽTO. mode is displaced approximately 7 cmy1 . Hence the crystallite size remains an important factor to be accounted for in the analysis of infrared spectra, above all when comparing measurements on samples whose crystallinity may seem to differ. For thin films locally involving ordered small domains of high permittivity, the form and the size of these domains can explain the differences remarked with respect to the bulk material or powders whose grain sizes are in general much higher.

Table 2 Frequencies and widths of the two principal peaks as a function of annealing conditions Thermal treatment

LO mode ᎏ 720 cmy1 y1

Position Žcm 600⬚Cr5 h 600⬚Cr10 h 650⬚Cr10 h 900⬚Cr5 h 1000⬚Cr7 h

726 725 722 718 708

.

TO mode ᎏ 490 cmy1 y1

FWHM Žcm 54 54 41 47 45

.

Position Žcmy1 .

FWHM Žcmy1 .

493 490 490 488 486

58 57 54 30 28

L. Preda et al. r Thin Solid Films 389 (2001) 43᎐50

approximately 180 cmy1 . Moreover, they found that this peak differentiated the Raman spectrum of the tetragonal BaTiO 3 from that of the orthorhombic one. The same tetragonal-pseudocubic distortion of the crystalline lattice was reported by Arlt et al. w11x for a critical grain size of approximately 0.2᎐0.3 ␮m of fine-grained BaTiO 3 powder. At the same time, their XRD measurements revealed the presence of the diffraction pattern characteristic of the orthorhombic BaTiO 3 . The smaller the grain size, the more important will be the stress and the internal strains acting upon the crystalline lattice in order to distort it into an orthorhombic form. Therefore, one can talk in terms of a metastable crystal structure moving towards stabilization in an orthorhombic phase at ambient temperature w11,12x. Bearing in mind that in our case the grain sizes are of the order of 20᎐40 nm Žthus, significantly smaller than the critical size mentioned. the strains imposed by the grain size on the lattice are sufficiently high to assert their orthorhombic nature. Indeed, in the case of the tetragonal phase characteristic of the target Žin other words: when the crystal size is sufficiently large in order to avoid a lattice disturbance ., the vibration mode located at 183 cmy1 will be masked by the interference of the broad 260-cmy1 peak. When the

Fig. 10. Ža. Raman spectrum of the sputtering target material Žin the frame, a zoom of the 180᎐310 cmy1 region.; Žb. Raman spectrum of stoichiometric BaTiO 3 thin film.

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Fig. 11. Raman spectra of BaTiO 3 thin film performed at different analysis temperatures.

grain size decreases and the structure moves towards a pseudocubic phase, the interference attenuates and the overlapping of the two neighboring peaks disappears. Consequently, the flattening of the 260-cmy1 peak makes easier the observation of the mode located at 180᎐190 cmy1 . In order to confirm these results, it is necessary to analyze the temperature response of the structure. Fig. 11 represents the Raman spectrum evolution within the 99᎐473 K temperature range. At temperatures below y95⬚C, one can notice the progressive appearance of two new peaks at 163 cmy1 and 488 cmy1 due to the rhombohedral phase formation. A rise in peak amplitudes, located at 180 and 264 cmy1 , with the temperature decrease can also be remarked upon. These results are in good agreement with the work of Perry and Hall who investigated the evolution of Raman spectra with temperature on BaTiO 3 powders w23x. In our case a supplementary peak at 448 cmy1 is also detected in the low temperature range. We identified it as being specific to TiO 2 traces and we noticed its disappearance after an annealing under ultra-vacuum at 600⬚C. At ambient temperature, the structure is orthorhombic as we said earlier. An increase in temperature attenuates the peak at 180 cmy1 and its complete disappearance occurs at approximately 60⬚C, characterizing a tetragonal phase. The peak at 305 cmy1 seems to have disappeared at 200⬚C which signifies a cubic phase. However, we did not find in our Raman temperature study of BaTiO 3 film, a steep transition between phases as shown in case of powder materials Žcrystal size ) 0.2 ␮m. w23x. Comparing with the bulk BaTiO 3 , Raman spectroscopy of these thin films leads us to assume a specific behavior with a diffuse ferroelectric transition Žcharacteristic, also, of the very fine-grained BaTiO 3 ceramics.. This slight ferroelectricity is naturally attributed to the small size of crystallites or, more precisely, to the small size of ferroelectricity domains.

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L. Preda et al. r Thin Solid Films 389 (2001) 43᎐50

4. Conclusions In comparison with the listed literature data, this work emphasizes the influence of BarTi ratio of the as-grown films on the phase composition of the same films crystallized by annealing. The obtaining of BaTi 2 O5 as a secondary crystalline phase in the nonstoichiometric films under annealing at high temperature implies important microstructural changes with some possible consequences on the electrical behavior of the films. BaTi 2 O5 favors a decrease of the film porosity by filling in the voids between the columnar BaTiO 3 crystals. In turn, this could lead to improvement in some electrical properties Žsuch as decrease in the leakage current values. of these films although other properties might be affected Ždielectric constant.. The correlation between phase composition᎐microstructure ᎐electrical properties will be the objective of a future work. By varying composition one can optimize the BaTiO 3rBaTi 2 O5 phase ratio in the crystallized films in order to obtain the desired dielectric properties for the application under consideration. The evolution of BarTi towards the unit value caused the disappearance of the secondary BaTi 2 O5 phase so that the film becomes single phase, BaTiO 3 with more or less defects being identified as an unique phase. The IR specular measurements at oblique incidence of annealed stoichiometric films clearly reveal the presence of a LO vibration mode at 720 cmy1 . This peak cannot be seen in a conventional transmittance measurement where the electric field associated with the light activates the vibration mode parallel to the surface. Raman spectroscopy confirms the presence of the 720-cmy1 peak. At ambient temperature the vibration mode at positive 180 cmy1 detected by Raman spectroscopy characterizes the existence of an orthorhombic phase. An increase in temperature attenuates the peak at 180 cmy1 and its complete disappearance occurs at approximately 60⬚C, characterizing a tetragonal phase. When lowering the temperature, two new peaks appear at 163 cmy1 and 488 cmy1 near y95⬚C characterizing the appearance of a rhombohedral phase. Temperature study by Raman spectroscopy of a BaTiO 3 film does not show any steep transition between phases. Raman

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