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Some peculiarities at preparation of Bi4Ti3O12 films for bolometric applications
Applied Surface Science xxx (xxxx) xxx–xxx
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Some peculiarities at preparation of Bi4Ti3O12 films for bolometric applications ⁎
Š. Chromik , M. Španková, M. Talacko, E. Dobročka, T. Lalinský Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravská cesta 9, 841 04 Bratislava, Slovakia
A R T I C LE I N FO
A B S T R A C T
Keywords: BTO films Dielectric films LSMO film Surface morphology
We discuss properties of Bi4Ti3O12 (BTO) films in BTO/CeO2/YSZ multilayered buffer system, on silicon and on silicon-on-insulator (SOI) substrates, employed for the preparation of epitaxial La0.67Sr0.33MnO3 (LSMO) films. We try to find a correlation between the parameters of film preparation and structural properties and morphology of the BTO films with the goal to eliminate defects. We also point out that many morphological defects observed on the final LSMO films prepared for bolometers originate in the bottom BTO layer. We have found out that the actual substrate temperature is a very important parameter during the process of BTO film deposition since deviations of only few degrees from the proper temperature can cause the growth of mixture crystal orientation.
1. Introduction Due to their high Curie temperature (TC = 370 K), perovskite manganite La0.67Sr0.33MnO3 (LSMO) films are good candidates for room temperature sensor applications. Such films can be used for fabricating uncooled bolometer made on circular SOI membranes [1]. Bi4Ti3O12 (BTO) films deposited on silicon-on-insulator (SOI) substrates in BTO/ CeO2/YSZ multilayered buffer system are used for preparation of epitaxial La0.67Sr0.33MnO3 films [2]. In spite of the fact that we have successfully prepared some model structures of uncooled bolometer able to detect THz- radiation about 1.4THz [3], some problems with obtaining reproducible smooth surface morphology of LSMO films still exist. Nevertheless, BTO based samples remain best candidates for applications demanding LSMO films with high TC and good structural quality associated with good lattice matching between materials [4,5]. There exist several papers describing preparation and structural properties of BTO films, discussing defects in the orientation of LSMO film, and appearance of aggregates or disturbed areas [4,6,7]. However, the mechanism of such defects formation has not been sufficiently resolved. Watanabe et al. [7] pointed out that it is indeed difficult to reliably predict the crystalline orientation of LSMO films grown on BTO. The BTO crystal structure is always very different from that of the underlying materials. Typically, for (1 0 0)/(1 0 0)-oriented growth of BTO, long range lattice matching with an underlying crystal is necessary. Watanabe et al, observed an intermediate region between (1 0 0)/ (0 1 0) and (1 1 8) orientations appearing at the deposition temperature, where both orientations coexisted. Consequently, a systematic study of
⁎
the crystalline orientation of BTO is essential for understanding and predicting the growth of BTO films on YSZ buffered Si substrate. This study provides basic data for achieving the preferably-oriented films on Si substrates, providing films with uniform cell and smooth morphology, suitable for bolometer and other potential ferroic applications. We try to find a correlation between the parameters of the film preparation and structural properties or morphology of the BTO with the goal to eliminate the observed defects. We also point out that many morphological defects observed on the final LSMO films prepared for the bolometers originate in the bottom BTO layer. 2. Experimental The films were grown in-situ growth using MBE/PLD-2000 deposition system equipped with excimer 248 nm laser Compex 102. The top single crystalline Si layer and buried oxide layer in SOI wafers employed in the experiments had thickness of 1 µm and 2 µm, respectively. First, 200 nm thick YSZ layer was deposited by PLD, on SOI substrate in vacuum background 10-5 Pa at temperature 790 °C, with the deposition rate of about 0.043 nm/s. The KrF excimer laser was operated at 3 Hz pulse rate. The energy density was 6 J/cm2 and the spot size on the target 2 mm2. The target-substrate distance was 80 mm. SOI and Si substrates (12 × 12 mm2) were radiantly heated from the back. During the deposition in oxygen, the partial pressure of oxygen was kept at 2.7 × 10−3 Pa for a short time (80 pulses) in the first stage of the deposition to reduce the native amorphous SiO2 layer. Then the working oxygen pressure was increased to 9.3 × 10−3Pa. The next in-situ
Corresponding author. E-mail address: [email protected] (Š. Chromik).
https://doi.org/10.1016/j.apsusc.2018.06.059 Received 9 March 2018; Received in revised form 4 June 2018; Accepted 8 June 2018 0169-4332/ © 2018 Published by Elsevier B.V.
Please cite this article as: Chromik, Š., Applied Surface Science (2018), https://doi.org/10.1016/j.apsusc.2018.06.059
Applied Surface Science xxx (xxxx) xxx–xxx
Š. Chromik et al.
deposited 10 nm thick CeO2 buffer layer (deposition rate 0.1 nm/s) and 30 nm thick BTO film (deposition rate 0.045 nm/s) were grown at the substrate temperature 710 °C and at partial oxygen pressure of 35 and 50 Pa, respectively, at the repetition frequency of pulses 5 Hz. The energy density was 5 J/cm2. Finally, 30–70 nm thick LSMO film was deposited (deposition rate about 0.1 nm/s) at the substrate temperature 730 °C and partial pressure of oxygen 50 Pa. The energy density was 6 J/cm2 and the repetition frequency 10 Hz. Immediately after the LSMO deposition, oxygen was sequentially introduced up to the pressure of 5.5 × 104Pa with the simultaneous cooling of the substrate to room temperature at the rate of 20 °C/min. The crystallographic orientation in the direction perpendicular to the film surface was determined by recording X-ray diffraction (XRD) patterns in θ-2θ configuration (Bruker D8 DISCOVER diffractometer). The degree of the preferred orientation was deduced from the full width at the half maximum (FWHM) of the rocking curve. Surface morphology of the films was investigated using Scanning electron microscope (SEM JEOL JSM 7600F).
Fig. 2. Rocking curves of YSZ films prepared on different SOI substrates in the same vacuum cycle do not show the same FWHM.
spite of the fact that FWHM has a tendency to decrease [7] with increasing thickness of YSZ film, the best way to control it is to concentrated on one type of substrate only. Another important conclusion is that the top LSMO film only reproduces the defects created during YSZ + CeO2 + BTO preparation. SEM analysis confirmed that surface morphology of BTO and top LSMO film (Fig. 3a,b) is identical. This is a clear evidence that the blocks originate in BTO films, while the top LSMO films do not play any role in this process. This observation supports the results of Perna et al. [4]. Their TEM analysis showed the presence of defects in the form of vertical grain boundary planes running across the entire thickness of LSMO, and originating at steps located at the bottom BTO/LSMO interface. Therefore, we have focused our attention mainly on the preparation and properties of the BTO + CeO2 + YSZ structure. Watanabe et al. [7] have prepared BTO films on various types of single-crystal oxide substrates by metal-organic chemical vapor deposition. They found out that BTO films tend to grow with a wide variety of crystal orientations,
3. Results and discussion Different types of morphology observed in LSMO films are shown in Fig. 1. Fig. 1a,b and c show different density of blocks outgrowing from the film. Fig. 1d shows smooth surface of the film without any outgrowths. To better understand the origin of the observed defects we have divided the fabrication process into the following separate steps: (1) YSZ deposition, (2) YSZ + CeO2 + BTO, (3) YSZ + CeO2 + BTO + LSMO. The detailed study of the first step (YSZ) and statistics of about 100 experiments has shown that a necessary (but not sufficient) condition for obtaining smooth LSMO film is a narrow rocking curve of the YSZ films (FWHM ≤ 1°). The treatment to reduce the native amorphous SiO2 layer, described in experimental part of this paper, is not valid in generally. YSZ films grown on different SOI substrates and those prepared in one vacuum cycle did not show the same FWHM (Fig. 2). In
Fig. 1. Different surface morphologies of LSMO films grown on the same substrate (polymorphism) are often observed. 2
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Fig. 3. The morphology of BTO + CeO2 + YSZ surface (a) and the final LSMO + BTO + CeO2 + YSZ multilayer structure (b) are identical. LSMO film was ex-situ deposited.
substrates with a total thickness above 300 µm. We asked ourselves about the reason of such a behavior, when the rocking curves of all substrates were comparable. We decided to examine the influence of substrate temperature because its value was not very exactly controlled due to radiation heating. Although we realize that other parameters, like the thickness of the BTO films and other buffer layers, energy of pulses etc., are also important factors, nevertheless, it was possible to keep them reproducibly constant in the experiments. We know that any impulse like imperfection of the YSZ layer, stress induced in the film can start a not optimal growth. Fig. 6 shows local area with the presence of different crystals. This remains in agreement with results of Watanabe et al. [8], who observed regions between the (1 0 0)/(0 1 0) and (1 1 8) orientations where both the (1 0 0)/(0 1 0) and (1 1 8) orientations of BTO coexisted. Such features were observed in all films deposited. We found out that the substrate temperature is indeed a very sensitive key parameter during the deposition and cooling of the BTO film to room temperature. We use Oxygen-Resistant SiC heater with rotating substrate without any contact silver paste. The advantage of such a heater is the resulting thickness homogeneity and no contamination of the backside of samples (for example residual of the silver paste). However, the disadvantage is that the thermocouple is located near the heater and its calibration by a pyrometer for not transparent sample at a pressure of 133 Pa, delivered by the manufacturer the device is not accurate enough. The actual temperature of the substrate placed in the hole of the Inconel holder and the holder itself were measured through a viewport by the pyrometer. The difference between the temperatures is about 10 °C (at the holder temperature 700 °C) due to different thermal capacity of the Inconel holder and employed Si substrate. Such a small difference has a dramatic effect on the surface morphology. We want to stress that the careful temperature control is necessary during all processes of film preparation, including the cooling steps. The morphology of two samples prepared subsequently - (one can expect the same properties of optical path and consequently the same energy of pulse) where all parameters of deposition process were the same except the temperature where we started to increase oxygen pressure during cooling process – exhibits significant difference. When we started to introduce the oxygen at higher temperature 630 °C, we observe sequential changes of the morphology (Fig. 7) with increasing distance from the edge of the sample (12 × 12 mm2) placed in contact with colder Inconel holder (symmetrically to all edges). In the case when we introduce the oxygen at lower temperature 620 °C, we expect less change (increase) of substrate temperature (due to introducing higher oxygen pressure into the chamber) and the morphology is much better. In this case smooth morphology area, like in Fig. 7a, spreads to the whole surface of the sample. We believe that this is the main reason why thick SOI and Si substrates of about 300 µm were more successful at the same level of radiation heating than the thinner ones (due to their
Fig. 4. XRD patterns of LSMO/BTO/CeO2/YSZ films (smooth LSMO- red line, LSMO with rotated blocks – black line, blue *, ♦, ∇ correspond to 111 BTO, 028 BTO, and 208 BTO diffraction maxima). In the case of rotated blocks LSMO diffraction peaks are missing. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
because of the large volume of the unit cell and anisotropic crystalline structure of the BTO [8]. Indeed, it is possible to observe different surface morphologies of BTO for more or less the same preparation parameters. In most cases, one observes outgrowing blocks perpendicular to the surface. XRD pattern of LSMO/BTO/CeO2/YSZ/Si shown in Fig. 4 confirms the presence of YSZ and BTO phases in the case of a smooth surface, as well as in the case of a surface with outgrowths (blocks). However, we register missing LSMO diffraction peaks in the case of rough surface morphology. In addition to 00l BTO diffraction line, maxima corresponding to crystallites with arbitrary orientations were detected in diffraction patterns of LSMO film with rough surface, e.g. lines 111, 028, and 208. On the other hand, YSZ diffraction peaks are not changed. This evidences that the YSZ film grown under outgrowths- rotated BTO blocks is still compact and homogenous. Due to very small thickness of CeO2 film (≤10 nm) we were not able to register diffraction maxima belonging to this material. The above conclusions are supported by φ-scans taken from the films. Fig. 5a,b show epitaxial cube on the cube growth of YSZ and BTO films. The orientation relations within the interface plane of the layers with smooth surface morphology can be described as Si [1 0 0]||YSZ [1 0 0]||BTO [1 0 0]||LSMO[1 1 0]. The 117 peaks of BTO coincide with 101 peaks of LSMO. However, for a layer with rotated BTO blocks we register wider peaks of the BTO maxima of lower intensity and no LSMO peaks, indicating poorer quality of the not continuous layer. Our statistics shows that the best smooth final LSMO films were obtained on SOI or Si (1 0 0) substrates with a thickness of about 300 µm. We conclude therefore, that the best way is to use SOI 3
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Fig. 5. φ-scans taken from the LSMO film with rotated blocks (a) with a smooth surface (b). LSMO film with rotated BTO blocks is of poorer quality (wider peaks of the BTO maxima of lower intensity and no LSMO peaks).
bigger thermal capacity). The correction of the temperature during preparation and cooling process in most cases resulted in a smooth morphology but for some types of substrates it did not. Therefore, there is still an open question about the influence of the substrate itself. We conclude that choosing of optimal parameters of the substrate, especially exactly controlling the substrate temperature, along with optimizing thickness of Si or SOI substrates, and deformation (bending) of the substrate after preparation of the multilayer structure [1], enables one to fabricate BTO films with smooth surface morphology. 4. Conclusion Bi4Ti3O12 films were epitaxially grown on YSZ/CeO2 buffered Si (1 0 0) and SOI substrates by PLD deposition. We have shown that rough surface morphology of the top LSMO film originates in the underlying BTO film. XRD analyses confirmed the expected composition and the fact that the YSZ film grown under BTO outgrowths (rotated
Fig. 6. The detail of the LSMO film surface with different crystallites in underlying BTO films.
Fig. 7. The morphology of YSZ + CeO2 + BTO surface at the edge of the sample (a), more to center (1.5 mm from the edge of the sample) (b) and in the middle of the sample (c). The substrate temperature was 730 °C when oxygen pressure was increased. 4
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BTO blocks) is still compact and homogeneous. However, beside 00l diffraction of BTO, maxima corresponding to crystallites with arbitrary orientations were detected in diffraction patterns of LSMO film with rough surfaces. SEM analyses showed frequently a different form of crystal orientation of BTO and consequently rough surfaces. We found out that the actual temperature of the substrate during all processes of BTO film preparation is very important and that a temperature difference only few degrees can start the growth of mixture crystal orientation despite having other parameters of processes optimized. The obtained results reveal the origin of rough LSMO film surface and enable more reliable fabrication of BTO films on YSZ buffered substrate. More detailed study of the substrate influence on BTO film surface morphology is necessary in future.
[2]
[3] [4]
[5] [6]
Acknowledgment [7]
This work was supported by the Slovak Research and Development Agency, APVV-14-0613, APVV-16-0315 and Slovak Grant Agency for Science, VEGA 2/0117/18.
[8]
References [1] T. Lalinský, J. Dzuba, G. Vanko, V. Kutiš, J. Paulech, G. Gálik, M. Držík, Š. Chromik,
5
P. Lobotka, Thermo-mechanical analysis of uncooled La0.67Sr0.33MnO3 microbolometer made on circular SOI membrane, Sens. Actuat. A 265 (2017) 321–328, http://dx.doi.org/10.1016/j.sna.2017.08.024. Š. Chromik, V. Štrbik, E. Dobročka, T. Roch, A. Rosová, M. Španková, T. Lalinský, G. Vanko, P. Lobotka, M. Ralbovský, P. Choleva, LSMO thin films with high metalinsulator transition temperature on buffered SOI substrates for uncooled microbolometers, Appl. Surf. Sci. 312 (2014) 30–33, http://dx.doi.org/10.1016/j.apsusc. 2014.05.051. Final workshop of EMRP New07 project on THz Security, 29/May/2015, Davos, Switzerland. P. Perna, L. Mechin, M.P. Chauvat, P. Ruterana, Ch. Simon, U. Scotti di Uccio, High Curie temperature for La0.7Sr0.3MnO3 thin films deposited on CeO2/YSZ-based buffered silicon substrates, J. Phys. Condens. Matter 21 (2009) 306005, http://dx.doi. org/10.1088/0953-8984/21/30/306005. J.-H. Kim, S.I. Khartsev, A.M. Grishin, Epitaxial colossal magnetoresistive La0.67(Sr, Ca)0.33MnO3 films on Si, Appl. Phys. Lett. 82 (2003) 4295–4297. L. Méchin, P. Perna, C. Barone, J.-M. Routoure, Ch. Simon, La0.7Sr0.3MnO3 thin films on Bi4Ti3O12/CeO2/yttria-stabilised-zirconia buffered Si(0 0 1) substrates: electrical, magnetic and 1/f noise properties, Mater. Sci. Eng. B 144 (2007) 73–77, http://dx. doi.org/10.1016/j.mseb.2007.07.067. T. Watanabe, H. Funakubo, Epitaxial growth map for Bi4Ti3O12 films: a determining factor for crystal orientation, Japan. J. Appl. Phys. 44 (2005) 1337–1343, http://dx. doi.org/10.1143/JJAP.44.1337. T. Watanabe, K. Saito, H. Funakubo, Orientation of Bi4Ti3O12-based ferroelectric thin films prepared on various kinds of substrates by metalorganic chemical vapor deposition, J. Cryst. Growth 235 (2002) 389–393.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Some peculiarities at preparation of Bi4Ti3O12 films for bolometric applications ⁎
Š. Chromik , M. Španková, M. Talacko, E. Dobročka, T. Lalinský Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravská cesta 9, 841 04 Bratislava, Slovakia
A R T I C LE I N FO
A B S T R A C T
Keywords: BTO films Dielectric films LSMO film Surface morphology
We discuss properties of Bi4Ti3O12 (BTO) films in BTO/CeO2/YSZ multilayered buffer system, on silicon and on silicon-on-insulator (SOI) substrates, employed for the preparation of epitaxial La0.67Sr0.33MnO3 (LSMO) films. We try to find a correlation between the parameters of film preparation and structural properties and morphology of the BTO films with the goal to eliminate defects. We also point out that many morphological defects observed on the final LSMO films prepared for bolometers originate in the bottom BTO layer. We have found out that the actual substrate temperature is a very important parameter during the process of BTO film deposition since deviations of only few degrees from the proper temperature can cause the growth of mixture crystal orientation.
1. Introduction Due to their high Curie temperature (TC = 370 K), perovskite manganite La0.67Sr0.33MnO3 (LSMO) films are good candidates for room temperature sensor applications. Such films can be used for fabricating uncooled bolometer made on circular SOI membranes [1]. Bi4Ti3O12 (BTO) films deposited on silicon-on-insulator (SOI) substrates in BTO/ CeO2/YSZ multilayered buffer system are used for preparation of epitaxial La0.67Sr0.33MnO3 films [2]. In spite of the fact that we have successfully prepared some model structures of uncooled bolometer able to detect THz- radiation about 1.4THz [3], some problems with obtaining reproducible smooth surface morphology of LSMO films still exist. Nevertheless, BTO based samples remain best candidates for applications demanding LSMO films with high TC and good structural quality associated with good lattice matching between materials [4,5]. There exist several papers describing preparation and structural properties of BTO films, discussing defects in the orientation of LSMO film, and appearance of aggregates or disturbed areas [4,6,7]. However, the mechanism of such defects formation has not been sufficiently resolved. Watanabe et al. [7] pointed out that it is indeed difficult to reliably predict the crystalline orientation of LSMO films grown on BTO. The BTO crystal structure is always very different from that of the underlying materials. Typically, for (1 0 0)/(1 0 0)-oriented growth of BTO, long range lattice matching with an underlying crystal is necessary. Watanabe et al, observed an intermediate region between (1 0 0)/ (0 1 0) and (1 1 8) orientations appearing at the deposition temperature, where both orientations coexisted. Consequently, a systematic study of
⁎
the crystalline orientation of BTO is essential for understanding and predicting the growth of BTO films on YSZ buffered Si substrate. This study provides basic data for achieving the preferably-oriented films on Si substrates, providing films with uniform cell and smooth morphology, suitable for bolometer and other potential ferroic applications. We try to find a correlation between the parameters of the film preparation and structural properties or morphology of the BTO with the goal to eliminate the observed defects. We also point out that many morphological defects observed on the final LSMO films prepared for the bolometers originate in the bottom BTO layer. 2. Experimental The films were grown in-situ growth using MBE/PLD-2000 deposition system equipped with excimer 248 nm laser Compex 102. The top single crystalline Si layer and buried oxide layer in SOI wafers employed in the experiments had thickness of 1 µm and 2 µm, respectively. First, 200 nm thick YSZ layer was deposited by PLD, on SOI substrate in vacuum background 10-5 Pa at temperature 790 °C, with the deposition rate of about 0.043 nm/s. The KrF excimer laser was operated at 3 Hz pulse rate. The energy density was 6 J/cm2 and the spot size on the target 2 mm2. The target-substrate distance was 80 mm. SOI and Si substrates (12 × 12 mm2) were radiantly heated from the back. During the deposition in oxygen, the partial pressure of oxygen was kept at 2.7 × 10−3 Pa for a short time (80 pulses) in the first stage of the deposition to reduce the native amorphous SiO2 layer. Then the working oxygen pressure was increased to 9.3 × 10−3Pa. The next in-situ
Corresponding author. E-mail address: [email protected] (Š. Chromik).
https://doi.org/10.1016/j.apsusc.2018.06.059 Received 9 March 2018; Received in revised form 4 June 2018; Accepted 8 June 2018 0169-4332/ © 2018 Published by Elsevier B.V.
Please cite this article as: Chromik, Š., Applied Surface Science (2018), https://doi.org/10.1016/j.apsusc.2018.06.059
Applied Surface Science xxx (xxxx) xxx–xxx
Š. Chromik et al.
deposited 10 nm thick CeO2 buffer layer (deposition rate 0.1 nm/s) and 30 nm thick BTO film (deposition rate 0.045 nm/s) were grown at the substrate temperature 710 °C and at partial oxygen pressure of 35 and 50 Pa, respectively, at the repetition frequency of pulses 5 Hz. The energy density was 5 J/cm2. Finally, 30–70 nm thick LSMO film was deposited (deposition rate about 0.1 nm/s) at the substrate temperature 730 °C and partial pressure of oxygen 50 Pa. The energy density was 6 J/cm2 and the repetition frequency 10 Hz. Immediately after the LSMO deposition, oxygen was sequentially introduced up to the pressure of 5.5 × 104Pa with the simultaneous cooling of the substrate to room temperature at the rate of 20 °C/min. The crystallographic orientation in the direction perpendicular to the film surface was determined by recording X-ray diffraction (XRD) patterns in θ-2θ configuration (Bruker D8 DISCOVER diffractometer). The degree of the preferred orientation was deduced from the full width at the half maximum (FWHM) of the rocking curve. Surface morphology of the films was investigated using Scanning electron microscope (SEM JEOL JSM 7600F).
Fig. 2. Rocking curves of YSZ films prepared on different SOI substrates in the same vacuum cycle do not show the same FWHM.
spite of the fact that FWHM has a tendency to decrease [7] with increasing thickness of YSZ film, the best way to control it is to concentrated on one type of substrate only. Another important conclusion is that the top LSMO film only reproduces the defects created during YSZ + CeO2 + BTO preparation. SEM analysis confirmed that surface morphology of BTO and top LSMO film (Fig. 3a,b) is identical. This is a clear evidence that the blocks originate in BTO films, while the top LSMO films do not play any role in this process. This observation supports the results of Perna et al. [4]. Their TEM analysis showed the presence of defects in the form of vertical grain boundary planes running across the entire thickness of LSMO, and originating at steps located at the bottom BTO/LSMO interface. Therefore, we have focused our attention mainly on the preparation and properties of the BTO + CeO2 + YSZ structure. Watanabe et al. [7] have prepared BTO films on various types of single-crystal oxide substrates by metal-organic chemical vapor deposition. They found out that BTO films tend to grow with a wide variety of crystal orientations,
3. Results and discussion Different types of morphology observed in LSMO films are shown in Fig. 1. Fig. 1a,b and c show different density of blocks outgrowing from the film. Fig. 1d shows smooth surface of the film without any outgrowths. To better understand the origin of the observed defects we have divided the fabrication process into the following separate steps: (1) YSZ deposition, (2) YSZ + CeO2 + BTO, (3) YSZ + CeO2 + BTO + LSMO. The detailed study of the first step (YSZ) and statistics of about 100 experiments has shown that a necessary (but not sufficient) condition for obtaining smooth LSMO film is a narrow rocking curve of the YSZ films (FWHM ≤ 1°). The treatment to reduce the native amorphous SiO2 layer, described in experimental part of this paper, is not valid in generally. YSZ films grown on different SOI substrates and those prepared in one vacuum cycle did not show the same FWHM (Fig. 2). In
Fig. 1. Different surface morphologies of LSMO films grown on the same substrate (polymorphism) are often observed. 2
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Fig. 3. The morphology of BTO + CeO2 + YSZ surface (a) and the final LSMO + BTO + CeO2 + YSZ multilayer structure (b) are identical. LSMO film was ex-situ deposited.
substrates with a total thickness above 300 µm. We asked ourselves about the reason of such a behavior, when the rocking curves of all substrates were comparable. We decided to examine the influence of substrate temperature because its value was not very exactly controlled due to radiation heating. Although we realize that other parameters, like the thickness of the BTO films and other buffer layers, energy of pulses etc., are also important factors, nevertheless, it was possible to keep them reproducibly constant in the experiments. We know that any impulse like imperfection of the YSZ layer, stress induced in the film can start a not optimal growth. Fig. 6 shows local area with the presence of different crystals. This remains in agreement with results of Watanabe et al. [8], who observed regions between the (1 0 0)/(0 1 0) and (1 1 8) orientations where both the (1 0 0)/(0 1 0) and (1 1 8) orientations of BTO coexisted. Such features were observed in all films deposited. We found out that the substrate temperature is indeed a very sensitive key parameter during the deposition and cooling of the BTO film to room temperature. We use Oxygen-Resistant SiC heater with rotating substrate without any contact silver paste. The advantage of such a heater is the resulting thickness homogeneity and no contamination of the backside of samples (for example residual of the silver paste). However, the disadvantage is that the thermocouple is located near the heater and its calibration by a pyrometer for not transparent sample at a pressure of 133 Pa, delivered by the manufacturer the device is not accurate enough. The actual temperature of the substrate placed in the hole of the Inconel holder and the holder itself were measured through a viewport by the pyrometer. The difference between the temperatures is about 10 °C (at the holder temperature 700 °C) due to different thermal capacity of the Inconel holder and employed Si substrate. Such a small difference has a dramatic effect on the surface morphology. We want to stress that the careful temperature control is necessary during all processes of film preparation, including the cooling steps. The morphology of two samples prepared subsequently - (one can expect the same properties of optical path and consequently the same energy of pulse) where all parameters of deposition process were the same except the temperature where we started to increase oxygen pressure during cooling process – exhibits significant difference. When we started to introduce the oxygen at higher temperature 630 °C, we observe sequential changes of the morphology (Fig. 7) with increasing distance from the edge of the sample (12 × 12 mm2) placed in contact with colder Inconel holder (symmetrically to all edges). In the case when we introduce the oxygen at lower temperature 620 °C, we expect less change (increase) of substrate temperature (due to introducing higher oxygen pressure into the chamber) and the morphology is much better. In this case smooth morphology area, like in Fig. 7a, spreads to the whole surface of the sample. We believe that this is the main reason why thick SOI and Si substrates of about 300 µm were more successful at the same level of radiation heating than the thinner ones (due to their
Fig. 4. XRD patterns of LSMO/BTO/CeO2/YSZ films (smooth LSMO- red line, LSMO with rotated blocks – black line, blue *, ♦, ∇ correspond to 111 BTO, 028 BTO, and 208 BTO diffraction maxima). In the case of rotated blocks LSMO diffraction peaks are missing. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
because of the large volume of the unit cell and anisotropic crystalline structure of the BTO [8]. Indeed, it is possible to observe different surface morphologies of BTO for more or less the same preparation parameters. In most cases, one observes outgrowing blocks perpendicular to the surface. XRD pattern of LSMO/BTO/CeO2/YSZ/Si shown in Fig. 4 confirms the presence of YSZ and BTO phases in the case of a smooth surface, as well as in the case of a surface with outgrowths (blocks). However, we register missing LSMO diffraction peaks in the case of rough surface morphology. In addition to 00l BTO diffraction line, maxima corresponding to crystallites with arbitrary orientations were detected in diffraction patterns of LSMO film with rough surface, e.g. lines 111, 028, and 208. On the other hand, YSZ diffraction peaks are not changed. This evidences that the YSZ film grown under outgrowths- rotated BTO blocks is still compact and homogenous. Due to very small thickness of CeO2 film (≤10 nm) we were not able to register diffraction maxima belonging to this material. The above conclusions are supported by φ-scans taken from the films. Fig. 5a,b show epitaxial cube on the cube growth of YSZ and BTO films. The orientation relations within the interface plane of the layers with smooth surface morphology can be described as Si [1 0 0]||YSZ [1 0 0]||BTO [1 0 0]||LSMO[1 1 0]. The 117 peaks of BTO coincide with 101 peaks of LSMO. However, for a layer with rotated BTO blocks we register wider peaks of the BTO maxima of lower intensity and no LSMO peaks, indicating poorer quality of the not continuous layer. Our statistics shows that the best smooth final LSMO films were obtained on SOI or Si (1 0 0) substrates with a thickness of about 300 µm. We conclude therefore, that the best way is to use SOI 3
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Fig. 5. φ-scans taken from the LSMO film with rotated blocks (a) with a smooth surface (b). LSMO film with rotated BTO blocks is of poorer quality (wider peaks of the BTO maxima of lower intensity and no LSMO peaks).
bigger thermal capacity). The correction of the temperature during preparation and cooling process in most cases resulted in a smooth morphology but for some types of substrates it did not. Therefore, there is still an open question about the influence of the substrate itself. We conclude that choosing of optimal parameters of the substrate, especially exactly controlling the substrate temperature, along with optimizing thickness of Si or SOI substrates, and deformation (bending) of the substrate after preparation of the multilayer structure [1], enables one to fabricate BTO films with smooth surface morphology. 4. Conclusion Bi4Ti3O12 films were epitaxially grown on YSZ/CeO2 buffered Si (1 0 0) and SOI substrates by PLD deposition. We have shown that rough surface morphology of the top LSMO film originates in the underlying BTO film. XRD analyses confirmed the expected composition and the fact that the YSZ film grown under BTO outgrowths (rotated
Fig. 6. The detail of the LSMO film surface with different crystallites in underlying BTO films.
Fig. 7. The morphology of YSZ + CeO2 + BTO surface at the edge of the sample (a), more to center (1.5 mm from the edge of the sample) (b) and in the middle of the sample (c). The substrate temperature was 730 °C when oxygen pressure was increased. 4
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BTO blocks) is still compact and homogeneous. However, beside 00l diffraction of BTO, maxima corresponding to crystallites with arbitrary orientations were detected in diffraction patterns of LSMO film with rough surfaces. SEM analyses showed frequently a different form of crystal orientation of BTO and consequently rough surfaces. We found out that the actual temperature of the substrate during all processes of BTO film preparation is very important and that a temperature difference only few degrees can start the growth of mixture crystal orientation despite having other parameters of processes optimized. The obtained results reveal the origin of rough LSMO film surface and enable more reliable fabrication of BTO films on YSZ buffered substrate. More detailed study of the substrate influence on BTO film surface morphology is necessary in future.
[2]
[3] [4]
[5] [6]
Acknowledgment [7]
This work was supported by the Slovak Research and Development Agency, APVV-14-0613, APVV-16-0315 and Slovak Grant Agency for Science, VEGA 2/0117/18.
[8]
References [1] T. Lalinský, J. Dzuba, G. Vanko, V. Kutiš, J. Paulech, G. Gálik, M. Držík, Š. Chromik,
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