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Chemical Synthesis of Multilayered Nanostructured Perovskite Thin Films with Dielectric Features for Electric Capacitors Mohamed Barakat Zakaria, Takahiro Nagata, Asahiko MATSUDA, Yudai YASUHARA, Atsushi Ogura, Md Shahriar A. Hossain, Motasim Billah, Yusuke Yamauchi, and Toyohiro Chikyow ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00310 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 14, 2018

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ACS Applied Nano Materials

Chemical Synthesis of Multilayered Nanostructured Perovskite Thin Films with Dielectric Features for Electric Capacitors

Mohamed Barakat Zakaria,1,2* Takahiro Nagata,1 Asahiko Matsuda,3 Yudai Yasuhara,1,4 Atsushi Ogura,4 Md. Shahriar A. Hossain,5 Motasim Billah,5 Yusuke Yamauchi,6,7* and Toyohiro Chikyow1,3,4*

1

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

2

Department of Chemistry, Faculty of Science, Tanta University, Tanta, Gharbeya 31527, Egypt

3

Materials Data & Integrated System (MaDIS), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan.

4

Graduate School of Science and Technology, Meiji University, Kawasaki, 214-8571, Japan

5

School of Chemistry & Australian Institute for Innovative Materials (AIIM), University of Wollongong, Squires Way, North Wollongong, NSW 2500, Australia

6

Department of Plant & Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero, Giheunggu, Yongin-si, Gyeonggi-do 446-701, South Korea

7

School of Chemical Engineering & Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia Emails: T.C. [email protected] M.B.Z. [email protected]; [email protected] Y.Y. [email protected]

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Abstract We report a simple sol-gel method for the preparation of multilayered hybrid thin films composed of nanostructured SrTiO3 (STO) and BaTiO3 (BTO) layers. A nanoporous thin film of STO is prepared by the spin-coating method on Si/SiO2/Ti/Pt substrate. When the BTO is processed, the BTO is crystalized between STO crystal pores. A significant strain field is induced on the interface between the STO and BTO surfaces. The concave shape of the nanopores could enable the realization of a highly strained interfacial heterostructure composed of STO/BTO. Compared to nanostructured STO, BTO, and bilayer STO/BTO films, multilayered STO/BTO films show the highest dielectric constant and a high Curie temperature (Tc) at around 280 ºC, which is suitable for electric capacitors applications.

Keywords; Nanoporous; Sol-gel; Multilayers; Films; Interfaces; Curie temperature; Dielectric.

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1. INTRODUCTION Many efforts have been made to develop high dielectric constant materials for their applications in capacitors.1-2 In particular, hybrid materials in the form of multilayered hybrid thin films are very attractive for increasing the interfaces between different layers in the heterostructures.3 Several researchers have prepared these layered structures using pulsed laser deposition (PLD)4 and molecular beam epitaxy (MBE).5-6 These methods are relatively expensive, however, and large-scale preparation by them is difficult. Moreover, understanding the growth process and consequently controlling the nanostructure and the number of layers are of fundamental importance to design multilayered hybrid films for the fabrication of devices.7-12 So far, SrTiO3 has been widely synthesized and developed because of its structural and dielectric properties. It is a well-known member of the perovskite family, which is crystallized in a cubic paraelectric phase at room temperature, in its pure form, and has a bulk dielectric constant around 300.13-15 There are some changes that should be taken into consideration, relating to the occurrence of rearrangements on the surface and variation of the stoichiometry of its constituents, depending on certain factors such as temperature and oxygen partial pressure in addition to the ratio between TiO2 and SrO.16 Surface deformities and other surface properties may provide the necessary conditions to incorporate atoms in a way that enables generation of new constituents. This opens the door to mixing the STO with other materials to obtain new hybrids with dual features or improved specific properties.16 BaTiO3 (BTO) is one of the most investigated as a hybrid material constituent, due to its similarities and high compatibility with STO in terms of structure and properties.17 Moreover, it has a high dielectric constant that is necessary for electronic devices.18 Compared to the conventional physical deposition techniques, the sol-gel method is an easy and simple process. By controlling their compositions and layer thicknesses, it is possible to

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induce high strain into hybrid materials.19 The layer thickness can be varied by altering the rotation rate in the case of spin-coating, or by controlling the dipping and/or withdrawal rates in the case of the dip-coating process. So far non-porous SrTiO3,20 LaAlO3,20 BaTiO3,21-22 Pb(Zr,Ti)O3,23 BiScO3-PbTiO3,24-25 BiFeO3,26 CaTiO3-BiScO3,27 and SrRuO328 thin films have been demonstrated. Our aim is to develop a chemical method to prepare multilayered hybrid structures composed of perovskite-based dielectric/ferroelectric (e.g., BaTiO3/SrTiO3) layers in the nanometer range. We have successfully prepared multilayered hybrid thin films composed of nanostructured STO and BTO layers.29 When a guest material (BTO) is incorporated into a nanoporous host material (STO), the interstitial surface area between the two materials greatly increases.30 This advantage gives us the opportunity for improving the dielectric properties of the composite material.31

EXPERIMENTAL DETAILS Fabrication of substrates. Si (100) wafer (p-type, 1-10 !"cm) with a thermal oxide film (SiO2, 200 nm in thickness) was purchased from AKI Corporation. For surface protection, OFPR-800LB photoresist (from Tokyo Ohka) was spin-coated on the wafer surface. After cleaning by applying O2 plasma for 3 min (O2 flow: 100 sccm, RF power: 300 W), the wafer was baked on a hotplate at 120 °C for more than 1 min. Hexamethyldisilazane (HMDS ‘OAP’ by Tokyo Ohka) was spin-coated as an adhesion promotor, at 3000 rpm for 5 s. Again, OFPR800LB photoresist was spin-coated as a surface protection layer, and the wafer was then baked on a hotplate to dry the resist film at 120 °C for 5 min. For dicing, the substrate was fixed on an adhesive sheet with the backside up and the following parameters were used: blade width: 30 µm, total thickness of wafer: 380 µm, and cut depth: 130 µm. For resist removal, the substrate was sonicated in acetone for 3 min, sonicated in isopropyl alcohol (IPA) for 3 min,

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and rinsed in deionized water 3 times. For treatment with a sulfur-peroxide mixture (SPM), H2O2 30 wt% and H2SO4 98% (100 ml, v/v = 1) were mixed, the wafer was dipped into this mixture for 45 min, rinsed in deionized water 4 times, and dried with dry nitrogen gas. A 50 nm-thick Ti layer was deposited under base pressure of ~3#10-5 Pa, with a substrate to evaporation source distance of 500 mm, by e-beam (Vacc = 10 kV) at ~0.1 nm"s-1, and finally deposition of a Pt layer (100 nm) was deposited by e-beam (Vacc = 10 kV) at ~0.1 nm"s-1.

Synthesis of STO/BTO hybrid thin films. In the first step, a nanostructured SrTiO3 (STO) thin film was prepared according to our previous sol-gel method.19 0.74 g of strontium acetate was dissolved in 6.0 mL of acetic acid under constant stirring at 50 °C until there was complete dissolution. Meanwhile, an ethanolic solution was prepared by dissolving 1.22 g of titanium butoxide in 6.0 g ethanol containing 0.4 g of Pluronic-P123 triblock copolymer. The two solutions were mixed under constant stirring for 30 min at room temperature (sol (A)). A nanostructured STO thin film was prepared by the spin-coating method on Si/SiO2/Ti/Pt substrate using the sol (A) at 4,000 rpm for 40 s. The as-prepared films were dried for 4 min at 120 °C, and after that, they were calcined at 800 °C for 10 min with a controlled heating rate of 1 °C min-1. Subsequently, a BTO layer was coated on the STO by an impregnation method. In a typical procedure, 0.92 g of barium acetate was dissolved in 6.0 ml acetic acid under constant stirring at 50 °C until it was completely dissolved. Meanwhile, an ethanolic solution was prepared by dissolving 1.22 g of titanium butoxide in 6.0 g ethanol until a clear solution was obtained. The two solutions were mixed under constant stirring for 30 min at room temperature (sol (B)). The above-prepared nanostructured STO thin film was immersed in the sol (B) for 10 min and the substrate then was removed, and the film was rotated at 4000 rpm for 40 s to remove the excess of BTO sol. The as-prepared films were dried for 4 min at 120 °C, and after that, they were

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calcined at 800 °C for 10 min with a controlled heating rate of 1 °C min-1. Finally, a STO/BTO bilayer film was obtained. For the preparation of multilayered STO/BTO hybrid thin films, the aforementioned process was repeated.

Fabrication of the top platinum electrode. A 150 nm thick, disc-shaped Pt layer with a diameter of 120 µm was deposited as the top electrode by DC sputtering using a shadow metal mask. On some films, a 50 nm layer of Si-rich oxide (SRO) was deposited at room temperature prior to Pt deposition. Current density (J-V) and capacitance-voltage (C-V) measurements were conducted using a source-measurement unit (Keithley, 2612B) and LCR meter (Agilent E4980A), respectively. The current compliance was set at 1 mA. The sample was placed in a vacuum chamber system, and the temperature was raised from 25-400 °C. The stability of the dielectric constant of the film against temperature is defined by Equation (1): !" "

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where !max, !min, and !RT are the maximum, minimum, and room temperature dielectric constants, respectively. The capacitance (C) is related to the film thickness (d) according to the following Equation (2),32 and capacitance-voltage (C-V) curves are used for calculating the dielectric constant (!r) using the same Equation (2): 33 2#

$3 $4 5

(2)

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where 78 is the dielectric constant, 70 is the permittivity of vacuum (8.854 . 10-14 F/cm), and s is the electrode’s surface area.

Characterization. Scanning electron microscope (SEM) images were collected with a low voltage Hitachi SU8000 scanning electron microscope at an accelerating voltage of 5 kV and current of 10 mA. Transmission electron microscope (TEM) observations were performed !


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using a JEM-2100F TEM system operated at 200 kV and equipped for energy-dispersive Xray (EDX) spectroscopy analysis. Wide-angle X-ray diffraction (XRD) patterns of powders were obtained with a Rigaku RINT 2500X diffractometer using monochromated Cu-K$ radiation (40 kV, 40 mA) at a scanning rate of 0.5 °C min-1. Two-dimensional (2D) X-ray diffraction (XRD) images and patterns were obtained using a Bruker D8 XRD system. The upper electrode was fabricated by DC sputtering using a shadow metal mask. Current density (J-V) and capacitance-voltage (C-V) measurements were conducted using the sourcemeasurement unit (Keithley, 2612B) and LCR meter (Agilent E4980A), respectively.

RESULTS AND DISCUSSION It is important to understand the thermal stability of such nanostructured thin films to avoid serious collapse of nanostructures by annealing. For this purpose, we prepared various nanostructured STO thin films on Si substrate at different temperatures from 400 °C to 900 °C. The top-surface was examined using SEM, as shown in Fig. S1 in the Supporting Information. It seems that the Pluronic-P123 template is not completely removed at 400 °C and 500 °C (Fig. S1a-b). When the applied temperature was further increased (Fig. S1c-f), the template was completely removed, and the nanoporous structures were generated with a random arrangement of pores. These results indicate the high thermal stability of the nanostructured STO thin films. As another control experiment, nanostructured BTO thin films were prepared on Si substrate using the P123-containing precursor solution.19 The top surface of each film was examined using SEM, as shown in Fig. S2. Similarly to the above STO system, when the applied temperature was less than 500 °C, the templates still remained on the film (Fig. S2a-b). After calcination at higher temperature, serious collapse of the nanoporous structure began to occur. It was found that the nanostructured BTO thin films offered less thermal stability compared to

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the above STO system. Therefore, in this study, nanostructured STO thin films were selected as the initial layers before incorporating the BTO phase. Our success in preparing nanostructured STO and BTO thin films on the Si substrate motivated us to apply Pt electrodes on their surfaces to study their dielectric properties for their potential for application in electric capacitors. A nanostructured STO thin film was prepared on the surface of the Pt electrode as shown in Fig. 1a. The thickness of the thin film was determined by cross-sectional SEM to be ~ 80 nm (Fig. 1b). A bilayer STO/BTO thin film was also successfully prepared, and the top-surface was examined by SEM (Fig. 1c). The overall film thickness is ~ 160 nm (Fig. 1c). A control experiment was carried out to determine the film thickness of nanostructured BTO on Pt electrode, and the film thickness estimated from cross-sectional SEM is ~112 nm (Fig. S3). Thus, the film thickness of the STO/BTO hybrid thin film (160 nm) is greater than those of the nanostructured STO (80 nm) and BTO (112 nm) thin films, because of the double coating of layers. Although the thin films on Si substrate or Pt electrode were prepared in the same way, the thickness of the films prepared on Si substrate is greater than those on Pt electrode (Figs. S4 and 1b,d). To determine the crystalline structure of STO, STO powders were prepared at different temperatures, ranging from 500 °C to 1000 °C, and analyzed using wide-angle XRD (Fig. S5). The XRD patterns are assignable to a typical SrTiO3 phase, and the crystallinity is improved by increasing the calcination temperature.34 The (110) peak with the highest intensity clearly appears at 2! = 32.8°, and the locations of all the peaks and their relative intensities are matched with JCPDS Card No 35-0734 (space group: Pm-3m).35 In the meantime, the STO and STO/BTO thin films on Pt electrode were further examined using high-resolution 2D XRD, as shown in Fig. 2. From the 2D images, there is a very strong peak at 40.5º corresponding to Pt (111) (Fig. 2a and c). Meanwhile, there are two peaks at 36.5º and 27.9º corresponding to Ti and Si, respectively. Regarding the STO film, there is a peak at 32.8º (Fig. 2a), which is also

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clear in the corresponding XRD pattern (Fig. 2b), with exactly the same value as the highest intensity peak obtained in the wide-angle XRD pattern of STO powder (Fig. S5), indicating the formation of well-crystalized SrTiO3 phase. In the case of the bilayer STO/BTO hybrid thin films, two obvious peaks corresponding to SrTiO3 and BaTiO3 phases are confirmed at 32.8° and 32.1°, respectively.34 The nanostructured STO thin film was further examine using TEM (Fig. 3a and S6). Moreover, the corresponding elemental mapping images indicate the homogenous distribution of Sr, Ti, and O atoms over the entire surface of the STO thin film (Fig. S6). The different contrast in the high-angle annular dark field – scanning TEM (HAADF-STEM) image clearly shows the nanoporous architecture (Fig. S6a). The average pores size was estimated to be ~ 5 nm (Fig. 3b). The clear periodic spots obtained from the electron diffraction (ED) pattern can be assigned to the STO crystal (Fig. 3c).36 After preparation of the bilayer STO/BTO thin film, the nanopores were filled with the BTO phase, as shown in Fig. 3d-e. The selected-area ED patterns reveal the interference of the periodic spots of STO with those of BTO, indicating the formation of highly strained STO/BTO composite thin films (Fig. 3f). The elemental distribution of Sr, Ba, Ti, and O atoms over the entire surface of the STO/BTO thin films was investigated by energy-dispersive X-ray (EDX) spectrometers attached to the SEM and TEM instruments (Fig. S7 and Fig. S8). The homogeneous distribution of each element over the surface of the STO/BTO hybrid thin films was clearly revealed. A schematic illustration of the fabricated electrode is shown in Fig. 4a. The multilayered STO/BTO hybrid thin film structure was examined using cross-sectional HAADF-STEM (Fig. 4b). The film, with a total thickness of ~360 nm, consists of 6 nanostructured layers. A nanoporous STO thin film was fabricated by the spin-coating process using a transparent sol of STO. The rotation rate was altered until the best nanoporous framework was obtained with a thickness of ~ 80 nm. The fabricated STO film was immersed in a solution of barium acetate

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and titanium tetrabutoxide for a while (~ 10 min) at 50 ºC to guarantee the impregnation of BTO. The nanoporous framework of the STO film was partially filled with BTO, leading to formation of STO, bilayer STO/BTO, and BTO layers. The heating helps in the fusion of BTO inside the pores of the STO film and produces a highly strained interfacial STO/BTO layer between the STO and BTO. To increase the layer numbers, the above process was repeated again. The diameter of the upper Pt electrode was estimated to be ~ 125 %m (Fig. 4c). Crosssectional HAADF-STEM and elemental mapping images are strong tools to examine the elemental distribution inside the film (Fig. 5). The overall thickness of the film is ~ 360 nm (Fig. 5a), which is in agreement with the above SEM results. From the corresponding elemental mapping images, it is clear that the film is composed of Sr, Ba, Ti, and O atoms (Fig. 5), as shown in Fig. 4a. Moreover, the only upper part of the STO is filled with BTO which is confirmed by the presence of pores, as is clear from the cross-sectional HAADF-STEM. This indicates that when the fabricated STO is immersed in the BTO solution, three layers are produced and ordered as follows: STO, bilayer STO/BTO, and BTO layers. Our previous paper has demonstrated the dielectric constant and dielectric loss of the bilayer STO/BTO composite thin films on silicon substrate as a function of frequency at temperatures varying from 0 ºC to 350 ºC by forming planar interdigital electrodes.19 There is around 50 % loss of the dielectric constant and a sharp shift in the Curie temperature (Tc) (230 ºC). There is no change in the peak position from changing the frequency. Moreover, the bilayer STO/BTO composite thin films show ferroelectric properties at room temperature, as obtained from piezoresponse force microscopy (PFM) measurements with a contact area of about 20 nm # 20 nm reported in our previous article.19 These measurements identified the nanoscale intrinsic properties of bilayer STO/BTO apart from the spatial effect of the nanoporous framework. They demonstrated the ferroelectric properties of the bilayer STO/BTO at room temperature because strong polarization was realized in the interfaces of STO/BTO. In discussing

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ferroelectricity, the piezoelectric response of STO/BTO to applied voltage would be of interest. So, in this study we conducted electrical measurements of bilayer STO/BTO composite thin films prepared on Pt electrode in a vertical direction with microscale electrodes. The STO and bilayer STO/BTO composite thin films showed high leakage currents above 0.01 A cm-2 (Fig. 6a), revealing the low stability of bilayer STO/BTO composite thin films and suggesting that the nanoporous framework is not able to effectively reduce the passage of current. In contrast, the multilayered STO/BTO hybrid thin films composite had a low leakage current of 1.3 #10-9 J/cm2 at 1 V. So, detailed C-V measurements were carried out on the multilayered STO/BTO hybrid thin films, as shown in Fig. S9. We did not find significant ferroelectric properties, and the dielectric constants remained the same, regardless of the bias voltage (Fig. 6b). The nanoporous framework decreased the dielectric constant by approximately 85%. There is a sharp shift in Tc at around 280 ºC (Fig. 6c), although a high Tc in a strained STO/BTO superlattices reported for electric capacitors fabrication was previously reported.19 Since the dielectric temperature stability of the capacitance ("C/C) from 25 to 200 °C is still high, approximately 24 %, the multilayered STO/BTO hybrid thin films offer the possibility of application in high temperature operational capacitors due to their suitability for cost-efficient and large-area fabrication. We anticipate that the improvement of the dielectric constant in the case of the multilayered STO/BTO is caused by the effective stress caused at the interfaces between the STO and BTO domains. This phenomenon is being revealed by the new research on strain engineering and its effects on the Tc and the dielectric features. For instance, Haeni et al. reported that the epitaxial strain from a DyScO3 substrate improved the Tc by hundreds of degrees.37 Moreover, in the same manner, Choi et al. demonstrated the improvement of Tc in strained BaTiO3 thin films.38 The strains in the bilayer STO/BTO thin films on silicon substrate were examined by unpolarized Raman spectra (Fig. S10a). In the low frequency region, a doublet peak was

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observed because of the interfacial bilayer STO/BTO structure.19 The frequency of the lowest E(1TO) soft mode shifted from 40 cm-1 for the bulk BTO to 115 cm-1 for the bilayer STO/BTO structure, which is attributed to the stress being sufficient to produce an observable straininduced effect on the dielectric features (Fig. S10b).19 On the other hand, researchers recently demonstrated the improvement of the dielectric constant in multilayered structures.3 Hybrid materials in the form of multilayered structured thin films increase the interface area, offer a novel microstructure, and create interstitial heterostructures. For example, Li et al. demonstrated the high thermal stability of multilayered 2D perovskite-type niobate nanosheets in terms of structure and dielectric properties.3 In our study, we believe, the multilayered structure is responsible for the high thermal stability, which is confirmed by the low leakage current. On the other hand, the single STO layer and the bilayer STO/BTO showed very high leakage current and low thermal stability, and there was no enhancement, however much we increased the film thickness. The multilayered structure, however, would exhibit constant high performance over wide ranges of frequency and temperature with low leakage current density, which makes it suitable for application in high-temperature electric capacitors.

CONCLUSION We reported a chemical synthesis method for multilayered STO/BTO thin films composed of STO, STO/BTO, BTO, STO, STO/BTO, and BTO, in this order, on the surfaces of Si/SiO2/Ti/Pt substrates. The layers were fabricated by a step-wise sol-gel method. In this method, the first layer of nanoporous STO was synthesized by a spin-coating processing using a sol-gel of strontium acetate solution, and an ethanolic solution of titanium tetrabutoxide and Pluronic-P123. By immersing samples with an STO top layer in a solution of barium acetate and titanium tetrabutoxide for a few minutes, the nanoporous framework of the STO film is partially filled with BTO, leading to formation of STO, STO/BTO, and BTO layers. The

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heating produces a highly strained interfacial STO/BTO layer between the STO and BTO. To increase the number of layer, the previous procedures were repeated again. Compared to STO, BTO, and bilayer STO/BTO, the multilayered STO/BTO shows the highest dielectric constant, with broadening of the Curie temperature, which has a value of around 280 ºC in strained multilayered STO/BTO superlattices, which is promising for their potential application in electric capacitors.

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/xxxxxx. !! SEM images of STO, BTO, and STO/BTO layers on silicon substrates !! Wide-angle XRD patterns of STO powders !! HAADF-STEM image and the corresponding elemental mapping images of STO and STO/BTO layers on Si/SiO2/Ti/Pt substrates !! EDX results: SEM image and the corresponding elemental mapping images of STO/BTO on Si/SiO2/Ti/Pt substrate !! Temperature dependence of the dielectric constant for bilayer STO/BTO thin films on silicon substrate !! Local piezoelectric hysteresis loops measured by PFM for BTO, STO, and STO/BTO !! C-V measurement curves of the multilayered STO/BTO on Si/SiO2/Ti/Pt substrate !! Unpolarized Raman spectra for STO/BTO, bulk BTO crystal, and bulk STO crystal; Polarized Raman spectra for bilayer STO/BTO and bulk BTO crystal

ACKNOWLEDGEMENTS Mohamed Barakat Zakaria and Toyohiro Chikyow acknowledge financial support from the

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Japan Society for the Promotion of Science (JSPS). The authors thank the staff members of the International Center for Materials Nanoarchitectonics (MANA) and the Namiki Foundry at NIMS for their support in sample preparation.

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