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Fabrication of Y doped BaZrO3 epitaxial film on YBa2Cu3Ox sacrificial buffer layer
Thin Solid Films 598 (2016) 25–32
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Fabrication of Y doped BaZrO3 epitaxial film on YBa2Cu3Ox sacrificial buffer layer Tomoya Horide ⁎, Kazuki Hara, Kaname Matsumoto Department of Materials Science and Engineering, Kyushu Institute of Technology, Sensui-cho 1-1, Tobata-ku, Kitakyushu 804-8550, Japan
a r t i c l e
i n f o
Article history: Received 7 May 2015 Received in revised form 25 November 2015 Accepted 27 November 2015 Available online 30 November 2015 Keywords: Yttrium doped barium zirconate Pulsed laser deposition Epitaxial film Etching Micro-SOFC
a b s t r a c t Fabrication of highly oriented Y doped BaZrO3 (Ba(Zr,Y)O3:BZY) films on YBa2Cu3Ox (YBCO) sacrificial buffer layers was investigated. To clarify requirements of orientation control, BZY films were fabricated on various substrates using pulsed laser deposition (PLD). Cube-on-cube orientation relationship was obtained in BZY films on SrTiO3, LaAlO3, and MgO(100) regardless of PLD conditions, but orientation of BZY was strongly dependent on PLD conditions on yttria stabilized zirconia(YSZ) and CeO2(100), showing that perovskite structure or almost the same lattice parameter as BZY is needed to obtain cube-on-cube orientated BZY films. Cube-on-cube orientated BZY films were fabricated on YBCO, whose structure was perovskite, under wide ranging PLD conditions, showing that YBCO sufficiently controls crystalline orientation of BZY films. In addition, highly oriented BZY films were obtained on YBCO/YSZ/Si(100), demonstrating epitaxial BZY films on Si. Since good selective etching ability is also needed in sacrificial buffers, YBCO/BZY films were etched using H3PO4. H3PO4 etching removed the YBCO sacrificial buffers without damaging BZY with etching rate of ~30 nm/s. The present results show that YBCO sacrificial buffer layers are promising for fabrication of the μSOFCs with highly oriented BZY. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are a promising power generator due to their high energy efficiency [1], and micro-SOFCs (μSOFCs) have high specific energy and energy density for mobile application [2]. Ionic conductivity in electrolyte as well as anode and cathode reaction determines cell performance in SOFCs. Although oxygen conductors such as Y stabilized ZrO2 (YSZ) and Gd doped CeO2 are well studied, proton conducting electrolyte is promising for reducing operation temperature of SOFCs. Proton conductors such as Y doped BaZrO3 (Ba(Zr,Y)O3:BZY) exhibit high ion conductivity even at intermediate temperature [3], but grain boundaries significantly degrade the ionic conductivity [3]. To realize high ionic conductivity in BZY, grain boundaries should be removed in ionic conduction path, indicating that at least uni-axially orientated electrolyte films are needed in μSOFCs. BZY films were fabricated using pulsed laser deposition (PLD) [4], and exsitu annealing of precursor films [5,6]. Better performance is expected in highly oriented films [4] since they generally exhibit high crystallinity and homogeneous ionic conductivity can be obtained without grain boundary effect. In addition, control of ionic conductivity using interface defects [7] was reported in highly oriented films, and therefore fabrication of highly oriented BZY films is needed for high performance μSOFCs.
⁎ Corresponding author. E-mail address: [email protected] (T. Horide).
http://dx.doi.org/10.1016/j.tsf.2015.11.072 0040-6090/© 2015 Elsevier B.V. All rights reserved.
In fabrication of μSOFCs, one of the most well-known processes is free-standing film process, where electrolyte films are deposited on substrates, the free standing electrolyte films are obtained by removing the substrate, and anode and cathode films are deposited on the electrolyte films [2,8]. Free-standing film process was investigated in Y doped BaCeO3 [9] and BZY [10] electrolyte films on Si substrates. However, the BZY films on Si were polycrystalline [11,12], and buffer layers are needed in order to deposit highly oriented BZY films on Si substrates. Buffer layers are standard technique extensively studied to control film growth, but etching ability of buffer layers is also required in the case of the free-standing film process in μSOFCs. Dry etching using laser or ion beam is applicable regardless of materials, but combination of etchant and film material determines etching ability and etching rate in wet etching. Negligible damage of BZY surface, etching selectivity, and complete removal of buffer layers are needed in the top sacrificial buffer layers next to BZY. Considering these points, wet etching which can selectively remove only the buffer layers with fast rate is desired in the top sacrificial buffer layers. Thus, crystalline orientation and wet etching ability should be discussed to develop the sacrificial buffer layers for BZY free-standing films, but the sacrificial buffer layers which met both requirements have not yet been proposed. In the present study, YBa2Cu3Ox(YBCO) sacrificial buffer layers were investigated for fabrication of BZY electrolyte films. Crystalline orientation in BZY films on various substrates was discussed to clarify requirements in buffer layer materials for highly oriented BZY films. Based on the requirements and previous studies on wet etching [13], YBCO
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sacrificial buffer layers were proposed. Crystalline orientation, microstructure of BZY films, and etching ability of YBCO films using H3PO4 were evaluated to demonstrate that YBCO could be used as the sacrificial buffer layers. Possibility of YBCO sacrificial buffer layers is also discussed in μSOFC fabrication with highly oriented BZY film. 2. Experimental Films were prepared using PLD. Sintered Ba(Zr0.8,Y0.2)O3 was used as a target, and BZY films whose thickness was ~200 nm were deposited on CeO2/YSZ, SrTiO3 (STO), LaAlO3 (LAO), MgO, YSZ, and YBCO/STO. Target substrate distance and repetition frequency were fixed at 6 cm and 10 Hz, respectively. Substrate temperature and oxygen pressure were 500–800 °C and 5–11 × 10−4 (denoted by 8 × 10−4 Pa in this paper), 0.2–0.3 (denoted by 0.25 Pa in this paper), and 13 Pa during BZY deposition. YBCO was fabricated on STO at 830 °C and 26 Pa, and PLD condition for CeO2/YSZ was 780 °C and 13 Pa. BZY/YBCO/YSZ was also deposited on Si(100) substrates. PLD conditions of BZY/YBCO/ YSZ/Si were as follows: YSZ (780 °C and 0.8 × 10−3 Pa); YBCO (780 °C and 26 Pa); BZY (700 °C and 0.26 Pa). X-ray diffraction (XRD; 2θ–ω, ω, and ϕ scan) was performed to evaluate the crystalline orientation, and surface and cross-section of films were observed using scanning electron microscopy (SEM). To discuss the etching ability of YBCO, YBCO/BZY/MgO films were prepared using PLD, where BZY was deposited at 700 °C and ~0.25 Pa. The YBCO/BZY/MgO films were etched by dipping the films in H3PO4 (25%) for 5–180 s, and the etched films were evaluated using XRD, SEM, and energy dispersive X-ray (EDX). Conventional photolithography [13] was performed on YBCO/BZY/ MgO films to discuss difference in etching rate between YBCO and BZY.
3. Results and discussions 3.1. Orientation of BZY films on various substrates In order to clarify the requirements for the buffer layers, BZY films were deposited on various substrates. Fig. 1 shows 2θ–ω scan results of BZY films which were deposited on single crystalline MgO(100), STO(100), LAO(100), YSZ(100), and CeO2(100)/YSZ(100) substrates at 700 °C and 0.25 Pa. BZY/MgO, BZY/STO, and BZY/LAO films were (100) oriented, but BZY/YSZ and BZY/CeO2/YSZ films exhibited (110) orientation (BZY/CeO2/YSZ film also contained slight amount of (100) oriented grains). To discuss difference between these two types of orientation relationships, Fig. 2(b) and (c) shows oxygen pressure dependence of 2θ–ω results in BZY/STO and BZY/YSZ films deposited at 700 °C. BZY did not form at 13 Pa, the BZY film was (110) orientated at 0.25 Pa, and (111) oriented BZY was obtained at 8 × 10−4 Pa on YSZ substrates. On the other hand, (100) oriented BZY films were obtained on STO regardless of oxygen pressure. This shows that film growth mode is significantly different between BZY/STO and BZY/YSZ. Fig. 2 shows ϕ scan result in the BZY/MgO, BZY/STO, BZY/LAO, BZY/ YSZ, and BZY/CeO2/YSZ films fabricated at 700 °C and 0.25 Pa in addition to BZY/YSZ deposited at 700 °C and 8 × 10−4 Pa. BZY(110) exhibited four-fold symmetry as is observed in MgO(111), STO(110), and LAO(110), indicating BZY highly oriented films on MgO, STO, and LAO ((BZO(100)||MgO(100), BZY[001]||MgO[001]; BZO(100)||STO(100), BZY[001]||STO[001]; BZO(100)||LAO(100), BZY[001]||LAO[001]). Eight fold symmetric BZY(200) was observed in (110) oriented BZY/YSZ and BZY/CeO2/YSZ films deposited at 0.25 Pa, and peak split of ~17° indicates BZY(110)||YSZ(or CeO2)(100), BZY½110 ||YSZ(or CeO2)[010],
Fig. 1. (a) 2θ–ω scan result in BZY films deposited on MgO, STO, LAO, YSZ, and CeO2/YSZ. Oxygen pressure dependence of 2θ–ω scan result in (b) BZY/YSZ and (c) BZY/STO.
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Fig. 2. ϕ scan results in (a) BZY/MgO, (b) BZY/STO, (c) BZY/LAO, (d) BZY/CeO2/YSZ, (e) BZY/YSZ, and (f) BZY/YSZ (deposited at 700 °C and 8 × 10−4 Pa). The BZY films in (a)–(e) were deposited at 700 °C and 0.25 Pa.
and BZY[001]||YSZ(or CeO2)[010]. On the other hand, BZY(200) displayed twelve fold symmetry in (111) oriented BZY/YSZ (8 × 10−4 Pa), indicating that BZY(111)||YSZ(100), BZY ½112 ||YSZ[001], and BZY½110 ||YSZ[010]. The results indicate that the BZY orientation mechanisms on (STO, LAO, and MgO) and (YSZ and CeO2) are different, and interface energy and/or growth kinetics should be discussed to understand the orientation mechanisms. Contribution of interface matching to interface energy is discussed based on near coincidence site lattice model (NCSL) [14,15]. Table 1 shows NCSL parameters of Σf, Σs ≤ 50 and misfit ≤ 4%, in addition to (ls, ms, lf, mf) = (1, 0, 1, 0) and (1, 0, 1, 1). Here, Σf = l2f + m2f , Σs = l2s + m2s and misfit = (Af − As) / (1/2(As + Af)), af(= 4.21 Å) [16] and as are lattice parameters of film (BZY) and substrate, where Af = √(l2f + m2f )af and As = √(l2s + m2s )as. BZY and MgO have the same crystalline symmetry (cubic) and almost the same lattice parameters (misfit = 0.17%), so that cube-on-cube orientation relationship was obtained in BZY/MgO films regardless of PLD conditions. Misfit is large between STO (LAO) and BZY compared with BZY/MgO, and NCSL matching may be better in BZY[110]||STO[100], but BZO(001)||STO(001) and
BZY[100]||STO[100] regardless of oxygen pressure. Although CeO2 has almost the same lattice parameter as STO and LAO, different orientation relationship was observed. This indicates that the same crystal structure
Table 1 NCSL analysis (Σ and misfit) for film[100]||substrate[100] and film[100]||substrate[110]. The values of Σ ≤ 50 and misfit b 4 % are shown.
MgO 4.203 Å
STO 3.905 Å
LAO 3.789 Å YSZ 5.139 Å
CeO2 5.415 Å
ls
ms
Σs = l2s + m2s
lf
mf
Σf = l2f + m2f
Misfit
1 1 7 1 1 3 1 1 1 4 5 1 6 1 3 4 1
0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0
1 1 49 1 1 9 1 1 1 16 25 2 36 1 9 16 1
1 1 5 1 1 2 1 1 1 5 6 1 5 1 4 5 1
0 1 5 0 1 2 0 1 0 0 0 0 5 0 0 0 1
1 2 50 1 2 4 1 2 1 25 36 1 50 1 16 25 2
0.17 34 1.2 7.5 41.6 1.6 10.5 44.4 −19.9 2.4 −1.7 14.7 −3.5 −25 3.6 −2.9 9.5
Fig. 3. (a) Oxygen pressure dependence of 2θ–ω scan result in BZY/YBCO/STO deposited at 700 ºC. (b) ϕ scan result of BZY(110) and YBCO(102) in BZY/YBCO/STO deposited at 700 ºC and 0.25 Pa.
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Fig. 4. (a) PLD condition dependence of FWHM in rocking curve of BZY(200) and ϕ scan of BZY(110).
(perovskite) in BZY and STO (and LAO) improved interface bonding in cube-on-cube orientation relationship in spite of large misfit. BZY was (111) and (110) oriented at 8 × 10−4 Pa and 0.25 Pa, respectively. If only surface and interface energy determined the orientation relationship, the orientation relationship did not depend on deposition condition, suggesting that kinetic effect affected the orientation in BZY/YSZ and BZY/CeO2. Previously, it was reported that (111) oriented CeO2 was
obtained on Al2O3(0001) at high growth rate, but (100) oriented CeO2 grew on Al2 O3 (0001) at low growth rate [17]. This was explained based on the difference in nucleation frequency between 2D layer and 3D island growth. Ablated atoms or clusters are excited particles with high kinetic energy. The kinetic energy defines incident energy of ablated atoms or clusters which accelerates their migration at surface. High oxygen pressure decreases the kinetic energy due to frequent collisions. Low oxygen pressure results in low energy state due to large kinetic energy, but high oxygen pressure may result in metastable state due to insufficient kinetic energy. At oxygen pressure of 13 Pa, crystalline phase was not observed on YSZ. (110) was obtained at 0.25 Pa, and (111) was observed at 8 × 10− 4 Pa. This suggests that (111) and (110) grains might be stable and metastable, respectively, and that (110) growth was dominated by the kinetic mechanism. Orientation dependence of system energy (orientation dependence of stability) is determined by interface and surface energy. Interface energy is determined by interface matching, and surface energy is affected by oxygen vacancy, polarity, and termination. Further studies are needed to understand the detailed mechanism of BZY/YSZ and BZY/CeO2, but competitive growth of stable and metastable grains is one possible explanation of the present results. Difference in structure (perovskite and non-perovskite) and lattice parameter between BZY and YSZ(CeO2) resulted in weak interface bonding, and the competitive growth was observed.
Fig. 5. Surface SEM images of BZY/YBCO/STO deposited at 500–800 °C and 8 × 10−4–13 Pa.
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Fig. 6. Oxygen pressure dependence of cross-sectional SEM image in BZY/YBCO/STO deposited at 700 ºC.
The present discussion indicates that perovskite buffer or buffer with almost the same lattice parameter can be used to obtain BZY highly oriented films. In addition to the requirements for orientation control, good etching ability is needed for the sacrificial buffer layers. In our previous study on superconductivity, YBCO could be easily removed by H3PO4 etching [13], showing that YBCO meets requirement for both orientation control and etching ability. Therefore, YBCO buffer layers are discussed in the next sections. 3.2. Deposition of BZY on YBCO buffer layers Fig. 3 shows XRD results of BZY/YBCO/STO deposited under various PLD conditions. BZY(l00), YBCO(00l), and STO(l00) peaks were observed in 2θ–ω scan, indicating (100) oriented BZY films. Fig. 3(b) shows ϕ scan results of YBCO(102) and BZY(110), and four fold symmetry of BZY(110) and YBCO(102) was observed, indicating that BZY(100)||YBCO(001)
Fig. 7. (a) 2θ–ω scan of the BZY/YBCO/YSZ/Si(100) film. (b) ϕ scan results of Si(111), YSZ(111), YBCO(103), and BZY(110) in the BZY/YBCO/YSZ/Si film.
and BZY[001]||YBCO[100]. Regardless of PLD conditions in the present study (500–800 °C, 8 × 10−4–13 Pa), similar crystalline orientation relationship was observed. These are consistent with the discussion in Section 3.1. Fig. 4 shows substrate temperature dependence of half width of full maximum (FWHM) in rocking curve of BZY(200) and ϕ scan of BZY(110). Small FWHM was obtained at temperature of 600–800 °C and pressure of 0.25 Pa. Small diffusivity at low temperature degraded crystallinity, and optimal oxygen pressure around 0.25 Pa resulted in good crystallinity. The smallest FWHM of Δω = 0.35° and Δϕ = 1.7° in the present study is as small as usual high-quality epitaxial films, indicating that YBCO can realize high-crystallinity BZY films. Fig. 5 shows surface SEM images of BZY/YBCO/STO films as functions of substrate temperature and oxygen pressure. Surface morphology was not so significantly dependent on temperature and pressure at 500– 700 °C and 8 × 10−4–0.25 Pa. Fig. 6 shows cross-sectional SEM images of BZY/YBCO/STO deposited at 700 °C. Dense BZY films were deposited regardless of oxygen pressure, but the BZY film contained many defects at 13 Pa. This is consistent with surface SEM images in Fig. 5. The island type particles in Fig. 5 correspond to the precipitates on surface in crosssectional SEM of Fig. 6, and they seemed to result from roughness induced by the precipitates in YBCO surface (bottom left of Fig. 5) or precipitation during BZY deposition. On the other hand, triangle shaped surface morphology was observed at 13 Pa regardless of temperature. It is considered that the triangle shaped morphology was due to the defective BZY structure observed in cross-sectional SEM of Fig. 6(c), and the surface roughness seemed to originate from the defective growth of BZY films. Similar surface morphology was previously observed in BZY/MgO deposited at high oxygen pressure [18]. The results indicate that YBCO can sufficiently control the crystalline orientation and crystallinity of BZY films, and the most high-quality film was obtained at substrate temperature of 700 °C and oxygen pressure of 0.25 Pa in the present study. Commercial substrates should be used for SOFC application. YBCO is not available as commercial substrates, but Si is one of the most widelyused substrates in film fabrication. Fig. 7 shows XRD result in BZY/YBCO/ YSZ/Si. Epitaxial growth of YBCO on YSZ/Si was reported by many researchers [19]. XRD results indicated epitaxial BZY films and the orientation relationship of BZY(100)||YBCO(001)||YSZ(100)||Si(100) and BZY[001]||YBCO[100]||YSZ[011]||Si[011]. This strongly suggests that epitaxial BZY films can be obtained if YBCO highly oriented buffer is deposited regardless of substrate. Fig. 8 shows surface and cross-sectional SEM images of BZY/YBCO/YSZ/Si. A 20 μm grain with flat surface and layer structure was observed. Thickness of YSZ, YBCO and BZY was 23 nm, 45 nm, and 167 nm, respectively. Thus, YBCO buffered Si is the candidate that meets the commercial availability and the highly oriented BZY growth although the structure is complicated. From the previous reports by many researchers [3], it is well known that grain boundaries degrade ionic conductivity in BZY. The present structural results suggest larger ionic conductivity in the present films than that in polycrystalline BZY films although electrical property measurement deepens the discussion.
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Fig. 8. (a) Surface SEM image of BZY/YBCO/YSZ/Si. (b) Cross-sectional SEM image of the film. Layer structure is observed.
3.3. Etching of YBCO buffer layer The YBCO/BZY/MgO films were etched using H3PO4, and wet etching rate, etching selectivity, and influence of etching on surface of BZY are discussed to clarify wet etching ability of YBCO. Fig. 9 shows etching time dependence of XRD results. As-deposited YBCO/BZY/MgO exhibited YBCO(00l) peaks as well as MgO(l00) and BZY(l00) peaks in 2θ–ω scan, and BZY(110) peaks in ϕ scan. The ϕ scan of BZY(110) was performed to observe the BZY peaks clearly since MgO(110) was not observed due to crystalline symmetry. Although MgO and BZY peaks were observed in 2θ–ω scan and ϕ scan, there is no YBCO(00l) peak in asdeposited BZY/MgO, etched YBCO/BZY/MgO(5 s), and etched YBCO/ BZY/MgO (30 s). Within XRD peak resolution, YBCO was removed by H3PO4, but BZY remained even after wet etching. Fig. 10 shows etching time dependence of surface SEM image. BZY/MgO and etched YBCO/ BZY/MgO films exhibited similar surfaces, and they are significantly different from that in as-deposited YBCO/BZY/MgO, also showing that YBCO was removed by H3PO4 and that BZY was not damaged by the wet etching. Table 2 shows EDX results(~100 μm × ~100 μm) of Zr, Y, Ba, and Cu, indicating that Cu was not observed in the etched films. Since Cu is included only in YBCO in the films, the absence of Cu in the
etched films means removal of YBCO from YBCO/BZY/MgO. The wet etching rate of YBCO using H3PO4 is ~ 30 nm/s which is sufficiently large. Fig. 11 shows cross-sectional SEM image of BZY/MgO with patterned YBCO. Inset shows surface image of BZY regions without YBCO. YBCO partially existed on BZY/MgO (only in the patterned region), and unpatterned YBCO was removed by H3PO4 without over-etching of BZY. 5–180 s etching did not change surface morphology in Figs. 10 and 11. If corrosion affects BZY significantly, surface is degraded by long time etching (for example, 180 s). These results show the strength of BZY against the corrosion from H3PO4. 3.4. Possibility of μSOFC fabrication process using YBCO sacrificial buffer layer The purpose of the present study is to investigate the YBCO sacrificial buffers, but total process of μSOFC using the YBCO sacrificial buffer is briefly discussed. Epitaxial BZY films were deposited on YBCO/YSZ/Si as shown in Fig. 7. YBCO sacrificial buffer layer is deposited only in the active area using stencil mask to avoid electron conduction through YBCO and to avoid side etching. After that, Si is etched using KOH, the bottom-buffer (YSZ) is removed by dry etching, and YBCO is
Fig. 9. XRD results(2θ–ω scan, ϕ scan) in YBCO/BZY/MgO films in the etching process.
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Fig. 10. Surface SEM images in (a) BZY/MgO, (b) YBCO/BZY/MgO(as deposited), (c) etched YBCO/BZY/MgO(5 s), and (d) etched YBCO/BZY/MgO (30 s).
4. Conclusion
Table 2 EDX result in YBCO/BZY/MgO (as-deposited, 5 s etching and 30 s etching)
Cu (atom%) Y (atom%) Zr (atom%) Ba (atom%)
As-deposited
5 s etching
30 s etching
14.9 4.44 3.49 7.92
0 1.16 5.5 1.2
0 1.17 5.48 2.49
etched with H3PO4 to fabricate free standing BZY films. Here, complete removal of the bottom-buffers without amorphous layer is not needed, since following wet etching of YBCO determines surface state of the freestanding BZY. After fabricating the free standing BZY films, anode and cathode films are deposited, and μSOFC with highly oriented BZY electrolyte is fabricated. Here, corrugation structure was not considered for simplicity, but it can be applicable to the present process easily [20].
Highly oriented BZY electrolyte on YBCO sacrificial buffer layer was investigated to propose the film structure of micro SOFCs. Cube-oncube orientation relationship was obtained on STO, LAO, MgO, but was not observed on YSZ and CeO2, showing that perovskite structure or almost the same lattice parameter as BZY is needed to control BZY orientation. Highly oriented BZY films were fabricated on YBCO films under wide ranging PLD conditions, showing that YBCO can be used as the sacrificial buffer layers. In addition, highly oriented BZY films were obtained on YBCO/YSZ/Si(100), demonstrating epitaxial BZY films on Si. Etching results of YBCO/BZY using H3PO4 indicated that YBCO could be etched by H3PO4 with high etching rate of ~30 nm/s and that the etching did not damage BZY surface. The present results show that YBCO is a buffer layer candidate for fabrication of the μSOFC with highly oriented BZY.
Fig. 11. Cross-sectional SEM images in YBCO/BZY/MgO at etching time of 30 s and 180 s YBCO was partially patterned by photolithography (right portion in the SEM images). Several SEM images were overlapped to show wide regions.
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[11] S. Ji, I. Chang, Y.H. Lee, M.H. Lee, S.W. Cha, Performance enhancement of thin-film ceramic electrolyte fuel cell using bi-layered yttrium-doped barium zirconate, Thin Solid Films 539 (2013) 117–121. [12] I. Chang, J.Y. Paek, S.W. Cha, Parametric study of Y-doped BaZrO3 thin film deposited via pulsed laser deposition, J. Vac. Sci. Technol. 33 (2015) 021515. [13] T. Horide, K. Matsumoto, K. Osamura, A. Ichinose, M. Mukaida, Y. Yoshida, S. Horii, Flux pinning properties of YBCO thin films deposited on SrTiO3(100) and MgO(100) substrates, Physica C 412-414 (2004) 1291–1295. [14] S.W. Chan, Degenerate epitaxy, coincidence epitaxy and origin of “special” boundaries in thin films, J. Phys. Chem. Solids 55 (1994) 1137–1145. [15] P.A. Langjahr, T. Wagner, F.F. Lange, M. Ruhle, Epitaxial growth and structure of highly mismatched oxides with rock-salt structure on MgO, J. Cryst. Growth 256 (2003) 162–173. [16] E. Fabbri, D. Pergolesi, S. Licoccia, E. Traversa, Does the increase in Y-dopant concentration improve the proton conductivity of BaZr1 − xYxO3 − δ fuel cell electrolytes? Solid State Ionics 181 (2010) 1043–1051. [17] S.V.N.T. Kuchibhatla, P. Nachimuthiu, F. Gao, W. Jiang, V. Shutthanandan, M.H. Engelhard, S. Seal, S. Thevuthasan, Growth-rate induced epitaxial orientation of CeO2 on Al2O3(0001), Appl. Phys. Lett. 94 (2009) 204101. [18] K. Bae, D.Y. Jang, S.M. Choi, B.K. Kim, J.H. Lee, J.W. Who, J.H. Shim, Influence of background oxygen pressure on film properties of pulsed laser deposited Y:BaZrO3, Thin solid films 552 (2014) 24–31. [19] D.K. Fork, D.B. Fenner, R.W. Barton, J.M. Philips, G.A.N. Connell, J.B. Boyce, T.H. Geballe, High critical currents in strained epitaxial YBa2Cu3O7 − δ on Si, Appl. Phys. Lett. 57 (1990) 1161. [20] C.C. Chao, C.M. Hsu, Y. Cui, F.B. Prinz, Improved solid oxide fuel cell performance with nanostructured electrolytes, ACS Nano 7 (2011) 5692–5696.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Fabrication of Y doped BaZrO3 epitaxial film on YBa2Cu3Ox sacrificial buffer layer Tomoya Horide ⁎, Kazuki Hara, Kaname Matsumoto Department of Materials Science and Engineering, Kyushu Institute of Technology, Sensui-cho 1-1, Tobata-ku, Kitakyushu 804-8550, Japan
a r t i c l e
i n f o
Article history: Received 7 May 2015 Received in revised form 25 November 2015 Accepted 27 November 2015 Available online 30 November 2015 Keywords: Yttrium doped barium zirconate Pulsed laser deposition Epitaxial film Etching Micro-SOFC
a b s t r a c t Fabrication of highly oriented Y doped BaZrO3 (Ba(Zr,Y)O3:BZY) films on YBa2Cu3Ox (YBCO) sacrificial buffer layers was investigated. To clarify requirements of orientation control, BZY films were fabricated on various substrates using pulsed laser deposition (PLD). Cube-on-cube orientation relationship was obtained in BZY films on SrTiO3, LaAlO3, and MgO(100) regardless of PLD conditions, but orientation of BZY was strongly dependent on PLD conditions on yttria stabilized zirconia(YSZ) and CeO2(100), showing that perovskite structure or almost the same lattice parameter as BZY is needed to obtain cube-on-cube orientated BZY films. Cube-on-cube orientated BZY films were fabricated on YBCO, whose structure was perovskite, under wide ranging PLD conditions, showing that YBCO sufficiently controls crystalline orientation of BZY films. In addition, highly oriented BZY films were obtained on YBCO/YSZ/Si(100), demonstrating epitaxial BZY films on Si. Since good selective etching ability is also needed in sacrificial buffers, YBCO/BZY films were etched using H3PO4. H3PO4 etching removed the YBCO sacrificial buffers without damaging BZY with etching rate of ~30 nm/s. The present results show that YBCO sacrificial buffer layers are promising for fabrication of the μSOFCs with highly oriented BZY. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are a promising power generator due to their high energy efficiency [1], and micro-SOFCs (μSOFCs) have high specific energy and energy density for mobile application [2]. Ionic conductivity in electrolyte as well as anode and cathode reaction determines cell performance in SOFCs. Although oxygen conductors such as Y stabilized ZrO2 (YSZ) and Gd doped CeO2 are well studied, proton conducting electrolyte is promising for reducing operation temperature of SOFCs. Proton conductors such as Y doped BaZrO3 (Ba(Zr,Y)O3:BZY) exhibit high ion conductivity even at intermediate temperature [3], but grain boundaries significantly degrade the ionic conductivity [3]. To realize high ionic conductivity in BZY, grain boundaries should be removed in ionic conduction path, indicating that at least uni-axially orientated electrolyte films are needed in μSOFCs. BZY films were fabricated using pulsed laser deposition (PLD) [4], and exsitu annealing of precursor films [5,6]. Better performance is expected in highly oriented films [4] since they generally exhibit high crystallinity and homogeneous ionic conductivity can be obtained without grain boundary effect. In addition, control of ionic conductivity using interface defects [7] was reported in highly oriented films, and therefore fabrication of highly oriented BZY films is needed for high performance μSOFCs.
⁎ Corresponding author. E-mail address: [email protected] (T. Horide).
http://dx.doi.org/10.1016/j.tsf.2015.11.072 0040-6090/© 2015 Elsevier B.V. All rights reserved.
In fabrication of μSOFCs, one of the most well-known processes is free-standing film process, where electrolyte films are deposited on substrates, the free standing electrolyte films are obtained by removing the substrate, and anode and cathode films are deposited on the electrolyte films [2,8]. Free-standing film process was investigated in Y doped BaCeO3 [9] and BZY [10] electrolyte films on Si substrates. However, the BZY films on Si were polycrystalline [11,12], and buffer layers are needed in order to deposit highly oriented BZY films on Si substrates. Buffer layers are standard technique extensively studied to control film growth, but etching ability of buffer layers is also required in the case of the free-standing film process in μSOFCs. Dry etching using laser or ion beam is applicable regardless of materials, but combination of etchant and film material determines etching ability and etching rate in wet etching. Negligible damage of BZY surface, etching selectivity, and complete removal of buffer layers are needed in the top sacrificial buffer layers next to BZY. Considering these points, wet etching which can selectively remove only the buffer layers with fast rate is desired in the top sacrificial buffer layers. Thus, crystalline orientation and wet etching ability should be discussed to develop the sacrificial buffer layers for BZY free-standing films, but the sacrificial buffer layers which met both requirements have not yet been proposed. In the present study, YBa2Cu3Ox(YBCO) sacrificial buffer layers were investigated for fabrication of BZY electrolyte films. Crystalline orientation in BZY films on various substrates was discussed to clarify requirements in buffer layer materials for highly oriented BZY films. Based on the requirements and previous studies on wet etching [13], YBCO
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sacrificial buffer layers were proposed. Crystalline orientation, microstructure of BZY films, and etching ability of YBCO films using H3PO4 were evaluated to demonstrate that YBCO could be used as the sacrificial buffer layers. Possibility of YBCO sacrificial buffer layers is also discussed in μSOFC fabrication with highly oriented BZY film. 2. Experimental Films were prepared using PLD. Sintered Ba(Zr0.8,Y0.2)O3 was used as a target, and BZY films whose thickness was ~200 nm were deposited on CeO2/YSZ, SrTiO3 (STO), LaAlO3 (LAO), MgO, YSZ, and YBCO/STO. Target substrate distance and repetition frequency were fixed at 6 cm and 10 Hz, respectively. Substrate temperature and oxygen pressure were 500–800 °C and 5–11 × 10−4 (denoted by 8 × 10−4 Pa in this paper), 0.2–0.3 (denoted by 0.25 Pa in this paper), and 13 Pa during BZY deposition. YBCO was fabricated on STO at 830 °C and 26 Pa, and PLD condition for CeO2/YSZ was 780 °C and 13 Pa. BZY/YBCO/YSZ was also deposited on Si(100) substrates. PLD conditions of BZY/YBCO/ YSZ/Si were as follows: YSZ (780 °C and 0.8 × 10−3 Pa); YBCO (780 °C and 26 Pa); BZY (700 °C and 0.26 Pa). X-ray diffraction (XRD; 2θ–ω, ω, and ϕ scan) was performed to evaluate the crystalline orientation, and surface and cross-section of films were observed using scanning electron microscopy (SEM). To discuss the etching ability of YBCO, YBCO/BZY/MgO films were prepared using PLD, where BZY was deposited at 700 °C and ~0.25 Pa. The YBCO/BZY/MgO films were etched by dipping the films in H3PO4 (25%) for 5–180 s, and the etched films were evaluated using XRD, SEM, and energy dispersive X-ray (EDX). Conventional photolithography [13] was performed on YBCO/BZY/ MgO films to discuss difference in etching rate between YBCO and BZY.
3. Results and discussions 3.1. Orientation of BZY films on various substrates In order to clarify the requirements for the buffer layers, BZY films were deposited on various substrates. Fig. 1 shows 2θ–ω scan results of BZY films which were deposited on single crystalline MgO(100), STO(100), LAO(100), YSZ(100), and CeO2(100)/YSZ(100) substrates at 700 °C and 0.25 Pa. BZY/MgO, BZY/STO, and BZY/LAO films were (100) oriented, but BZY/YSZ and BZY/CeO2/YSZ films exhibited (110) orientation (BZY/CeO2/YSZ film also contained slight amount of (100) oriented grains). To discuss difference between these two types of orientation relationships, Fig. 2(b) and (c) shows oxygen pressure dependence of 2θ–ω results in BZY/STO and BZY/YSZ films deposited at 700 °C. BZY did not form at 13 Pa, the BZY film was (110) orientated at 0.25 Pa, and (111) oriented BZY was obtained at 8 × 10−4 Pa on YSZ substrates. On the other hand, (100) oriented BZY films were obtained on STO regardless of oxygen pressure. This shows that film growth mode is significantly different between BZY/STO and BZY/YSZ. Fig. 2 shows ϕ scan result in the BZY/MgO, BZY/STO, BZY/LAO, BZY/ YSZ, and BZY/CeO2/YSZ films fabricated at 700 °C and 0.25 Pa in addition to BZY/YSZ deposited at 700 °C and 8 × 10−4 Pa. BZY(110) exhibited four-fold symmetry as is observed in MgO(111), STO(110), and LAO(110), indicating BZY highly oriented films on MgO, STO, and LAO ((BZO(100)||MgO(100), BZY[001]||MgO[001]; BZO(100)||STO(100), BZY[001]||STO[001]; BZO(100)||LAO(100), BZY[001]||LAO[001]). Eight fold symmetric BZY(200) was observed in (110) oriented BZY/YSZ and BZY/CeO2/YSZ films deposited at 0.25 Pa, and peak split of ~17° indicates BZY(110)||YSZ(or CeO2)(100), BZY½110 ||YSZ(or CeO2)[010],
Fig. 1. (a) 2θ–ω scan result in BZY films deposited on MgO, STO, LAO, YSZ, and CeO2/YSZ. Oxygen pressure dependence of 2θ–ω scan result in (b) BZY/YSZ and (c) BZY/STO.
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Fig. 2. ϕ scan results in (a) BZY/MgO, (b) BZY/STO, (c) BZY/LAO, (d) BZY/CeO2/YSZ, (e) BZY/YSZ, and (f) BZY/YSZ (deposited at 700 °C and 8 × 10−4 Pa). The BZY films in (a)–(e) were deposited at 700 °C and 0.25 Pa.
and BZY[001]||YSZ(or CeO2)[010]. On the other hand, BZY(200) displayed twelve fold symmetry in (111) oriented BZY/YSZ (8 × 10−4 Pa), indicating that BZY(111)||YSZ(100), BZY ½112 ||YSZ[001], and BZY½110 ||YSZ[010]. The results indicate that the BZY orientation mechanisms on (STO, LAO, and MgO) and (YSZ and CeO2) are different, and interface energy and/or growth kinetics should be discussed to understand the orientation mechanisms. Contribution of interface matching to interface energy is discussed based on near coincidence site lattice model (NCSL) [14,15]. Table 1 shows NCSL parameters of Σf, Σs ≤ 50 and misfit ≤ 4%, in addition to (ls, ms, lf, mf) = (1, 0, 1, 0) and (1, 0, 1, 1). Here, Σf = l2f + m2f , Σs = l2s + m2s and misfit = (Af − As) / (1/2(As + Af)), af(= 4.21 Å) [16] and as are lattice parameters of film (BZY) and substrate, where Af = √(l2f + m2f )af and As = √(l2s + m2s )as. BZY and MgO have the same crystalline symmetry (cubic) and almost the same lattice parameters (misfit = 0.17%), so that cube-on-cube orientation relationship was obtained in BZY/MgO films regardless of PLD conditions. Misfit is large between STO (LAO) and BZY compared with BZY/MgO, and NCSL matching may be better in BZY[110]||STO[100], but BZO(001)||STO(001) and
BZY[100]||STO[100] regardless of oxygen pressure. Although CeO2 has almost the same lattice parameter as STO and LAO, different orientation relationship was observed. This indicates that the same crystal structure
Table 1 NCSL analysis (Σ and misfit) for film[100]||substrate[100] and film[100]||substrate[110]. The values of Σ ≤ 50 and misfit b 4 % are shown.
MgO 4.203 Å
STO 3.905 Å
LAO 3.789 Å YSZ 5.139 Å
CeO2 5.415 Å
ls
ms
Σs = l2s + m2s
lf
mf
Σf = l2f + m2f
Misfit
1 1 7 1 1 3 1 1 1 4 5 1 6 1 3 4 1
0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0
1 1 49 1 1 9 1 1 1 16 25 2 36 1 9 16 1
1 1 5 1 1 2 1 1 1 5 6 1 5 1 4 5 1
0 1 5 0 1 2 0 1 0 0 0 0 5 0 0 0 1
1 2 50 1 2 4 1 2 1 25 36 1 50 1 16 25 2
0.17 34 1.2 7.5 41.6 1.6 10.5 44.4 −19.9 2.4 −1.7 14.7 −3.5 −25 3.6 −2.9 9.5
Fig. 3. (a) Oxygen pressure dependence of 2θ–ω scan result in BZY/YBCO/STO deposited at 700 ºC. (b) ϕ scan result of BZY(110) and YBCO(102) in BZY/YBCO/STO deposited at 700 ºC and 0.25 Pa.
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Fig. 4. (a) PLD condition dependence of FWHM in rocking curve of BZY(200) and ϕ scan of BZY(110).
(perovskite) in BZY and STO (and LAO) improved interface bonding in cube-on-cube orientation relationship in spite of large misfit. BZY was (111) and (110) oriented at 8 × 10−4 Pa and 0.25 Pa, respectively. If only surface and interface energy determined the orientation relationship, the orientation relationship did not depend on deposition condition, suggesting that kinetic effect affected the orientation in BZY/YSZ and BZY/CeO2. Previously, it was reported that (111) oriented CeO2 was
obtained on Al2O3(0001) at high growth rate, but (100) oriented CeO2 grew on Al2 O3 (0001) at low growth rate [17]. This was explained based on the difference in nucleation frequency between 2D layer and 3D island growth. Ablated atoms or clusters are excited particles with high kinetic energy. The kinetic energy defines incident energy of ablated atoms or clusters which accelerates their migration at surface. High oxygen pressure decreases the kinetic energy due to frequent collisions. Low oxygen pressure results in low energy state due to large kinetic energy, but high oxygen pressure may result in metastable state due to insufficient kinetic energy. At oxygen pressure of 13 Pa, crystalline phase was not observed on YSZ. (110) was obtained at 0.25 Pa, and (111) was observed at 8 × 10− 4 Pa. This suggests that (111) and (110) grains might be stable and metastable, respectively, and that (110) growth was dominated by the kinetic mechanism. Orientation dependence of system energy (orientation dependence of stability) is determined by interface and surface energy. Interface energy is determined by interface matching, and surface energy is affected by oxygen vacancy, polarity, and termination. Further studies are needed to understand the detailed mechanism of BZY/YSZ and BZY/CeO2, but competitive growth of stable and metastable grains is one possible explanation of the present results. Difference in structure (perovskite and non-perovskite) and lattice parameter between BZY and YSZ(CeO2) resulted in weak interface bonding, and the competitive growth was observed.
Fig. 5. Surface SEM images of BZY/YBCO/STO deposited at 500–800 °C and 8 × 10−4–13 Pa.
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Fig. 6. Oxygen pressure dependence of cross-sectional SEM image in BZY/YBCO/STO deposited at 700 ºC.
The present discussion indicates that perovskite buffer or buffer with almost the same lattice parameter can be used to obtain BZY highly oriented films. In addition to the requirements for orientation control, good etching ability is needed for the sacrificial buffer layers. In our previous study on superconductivity, YBCO could be easily removed by H3PO4 etching [13], showing that YBCO meets requirement for both orientation control and etching ability. Therefore, YBCO buffer layers are discussed in the next sections. 3.2. Deposition of BZY on YBCO buffer layers Fig. 3 shows XRD results of BZY/YBCO/STO deposited under various PLD conditions. BZY(l00), YBCO(00l), and STO(l00) peaks were observed in 2θ–ω scan, indicating (100) oriented BZY films. Fig. 3(b) shows ϕ scan results of YBCO(102) and BZY(110), and four fold symmetry of BZY(110) and YBCO(102) was observed, indicating that BZY(100)||YBCO(001)
Fig. 7. (a) 2θ–ω scan of the BZY/YBCO/YSZ/Si(100) film. (b) ϕ scan results of Si(111), YSZ(111), YBCO(103), and BZY(110) in the BZY/YBCO/YSZ/Si film.
and BZY[001]||YBCO[100]. Regardless of PLD conditions in the present study (500–800 °C, 8 × 10−4–13 Pa), similar crystalline orientation relationship was observed. These are consistent with the discussion in Section 3.1. Fig. 4 shows substrate temperature dependence of half width of full maximum (FWHM) in rocking curve of BZY(200) and ϕ scan of BZY(110). Small FWHM was obtained at temperature of 600–800 °C and pressure of 0.25 Pa. Small diffusivity at low temperature degraded crystallinity, and optimal oxygen pressure around 0.25 Pa resulted in good crystallinity. The smallest FWHM of Δω = 0.35° and Δϕ = 1.7° in the present study is as small as usual high-quality epitaxial films, indicating that YBCO can realize high-crystallinity BZY films. Fig. 5 shows surface SEM images of BZY/YBCO/STO films as functions of substrate temperature and oxygen pressure. Surface morphology was not so significantly dependent on temperature and pressure at 500– 700 °C and 8 × 10−4–0.25 Pa. Fig. 6 shows cross-sectional SEM images of BZY/YBCO/STO deposited at 700 °C. Dense BZY films were deposited regardless of oxygen pressure, but the BZY film contained many defects at 13 Pa. This is consistent with surface SEM images in Fig. 5. The island type particles in Fig. 5 correspond to the precipitates on surface in crosssectional SEM of Fig. 6, and they seemed to result from roughness induced by the precipitates in YBCO surface (bottom left of Fig. 5) or precipitation during BZY deposition. On the other hand, triangle shaped surface morphology was observed at 13 Pa regardless of temperature. It is considered that the triangle shaped morphology was due to the defective BZY structure observed in cross-sectional SEM of Fig. 6(c), and the surface roughness seemed to originate from the defective growth of BZY films. Similar surface morphology was previously observed in BZY/MgO deposited at high oxygen pressure [18]. The results indicate that YBCO can sufficiently control the crystalline orientation and crystallinity of BZY films, and the most high-quality film was obtained at substrate temperature of 700 °C and oxygen pressure of 0.25 Pa in the present study. Commercial substrates should be used for SOFC application. YBCO is not available as commercial substrates, but Si is one of the most widelyused substrates in film fabrication. Fig. 7 shows XRD result in BZY/YBCO/ YSZ/Si. Epitaxial growth of YBCO on YSZ/Si was reported by many researchers [19]. XRD results indicated epitaxial BZY films and the orientation relationship of BZY(100)||YBCO(001)||YSZ(100)||Si(100) and BZY[001]||YBCO[100]||YSZ[011]||Si[011]. This strongly suggests that epitaxial BZY films can be obtained if YBCO highly oriented buffer is deposited regardless of substrate. Fig. 8 shows surface and cross-sectional SEM images of BZY/YBCO/YSZ/Si. A 20 μm grain with flat surface and layer structure was observed. Thickness of YSZ, YBCO and BZY was 23 nm, 45 nm, and 167 nm, respectively. Thus, YBCO buffered Si is the candidate that meets the commercial availability and the highly oriented BZY growth although the structure is complicated. From the previous reports by many researchers [3], it is well known that grain boundaries degrade ionic conductivity in BZY. The present structural results suggest larger ionic conductivity in the present films than that in polycrystalline BZY films although electrical property measurement deepens the discussion.
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Fig. 8. (a) Surface SEM image of BZY/YBCO/YSZ/Si. (b) Cross-sectional SEM image of the film. Layer structure is observed.
3.3. Etching of YBCO buffer layer The YBCO/BZY/MgO films were etched using H3PO4, and wet etching rate, etching selectivity, and influence of etching on surface of BZY are discussed to clarify wet etching ability of YBCO. Fig. 9 shows etching time dependence of XRD results. As-deposited YBCO/BZY/MgO exhibited YBCO(00l) peaks as well as MgO(l00) and BZY(l00) peaks in 2θ–ω scan, and BZY(110) peaks in ϕ scan. The ϕ scan of BZY(110) was performed to observe the BZY peaks clearly since MgO(110) was not observed due to crystalline symmetry. Although MgO and BZY peaks were observed in 2θ–ω scan and ϕ scan, there is no YBCO(00l) peak in asdeposited BZY/MgO, etched YBCO/BZY/MgO(5 s), and etched YBCO/ BZY/MgO (30 s). Within XRD peak resolution, YBCO was removed by H3PO4, but BZY remained even after wet etching. Fig. 10 shows etching time dependence of surface SEM image. BZY/MgO and etched YBCO/ BZY/MgO films exhibited similar surfaces, and they are significantly different from that in as-deposited YBCO/BZY/MgO, also showing that YBCO was removed by H3PO4 and that BZY was not damaged by the wet etching. Table 2 shows EDX results(~100 μm × ~100 μm) of Zr, Y, Ba, and Cu, indicating that Cu was not observed in the etched films. Since Cu is included only in YBCO in the films, the absence of Cu in the
etched films means removal of YBCO from YBCO/BZY/MgO. The wet etching rate of YBCO using H3PO4 is ~ 30 nm/s which is sufficiently large. Fig. 11 shows cross-sectional SEM image of BZY/MgO with patterned YBCO. Inset shows surface image of BZY regions without YBCO. YBCO partially existed on BZY/MgO (only in the patterned region), and unpatterned YBCO was removed by H3PO4 without over-etching of BZY. 5–180 s etching did not change surface morphology in Figs. 10 and 11. If corrosion affects BZY significantly, surface is degraded by long time etching (for example, 180 s). These results show the strength of BZY against the corrosion from H3PO4. 3.4. Possibility of μSOFC fabrication process using YBCO sacrificial buffer layer The purpose of the present study is to investigate the YBCO sacrificial buffers, but total process of μSOFC using the YBCO sacrificial buffer is briefly discussed. Epitaxial BZY films were deposited on YBCO/YSZ/Si as shown in Fig. 7. YBCO sacrificial buffer layer is deposited only in the active area using stencil mask to avoid electron conduction through YBCO and to avoid side etching. After that, Si is etched using KOH, the bottom-buffer (YSZ) is removed by dry etching, and YBCO is
Fig. 9. XRD results(2θ–ω scan, ϕ scan) in YBCO/BZY/MgO films in the etching process.
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Fig. 10. Surface SEM images in (a) BZY/MgO, (b) YBCO/BZY/MgO(as deposited), (c) etched YBCO/BZY/MgO(5 s), and (d) etched YBCO/BZY/MgO (30 s).
4. Conclusion
Table 2 EDX result in YBCO/BZY/MgO (as-deposited, 5 s etching and 30 s etching)
Cu (atom%) Y (atom%) Zr (atom%) Ba (atom%)
As-deposited
5 s etching
30 s etching
14.9 4.44 3.49 7.92
0 1.16 5.5 1.2
0 1.17 5.48 2.49
etched with H3PO4 to fabricate free standing BZY films. Here, complete removal of the bottom-buffers without amorphous layer is not needed, since following wet etching of YBCO determines surface state of the freestanding BZY. After fabricating the free standing BZY films, anode and cathode films are deposited, and μSOFC with highly oriented BZY electrolyte is fabricated. Here, corrugation structure was not considered for simplicity, but it can be applicable to the present process easily [20].
Highly oriented BZY electrolyte on YBCO sacrificial buffer layer was investigated to propose the film structure of micro SOFCs. Cube-oncube orientation relationship was obtained on STO, LAO, MgO, but was not observed on YSZ and CeO2, showing that perovskite structure or almost the same lattice parameter as BZY is needed to control BZY orientation. Highly oriented BZY films were fabricated on YBCO films under wide ranging PLD conditions, showing that YBCO can be used as the sacrificial buffer layers. In addition, highly oriented BZY films were obtained on YBCO/YSZ/Si(100), demonstrating epitaxial BZY films on Si. Etching results of YBCO/BZY using H3PO4 indicated that YBCO could be etched by H3PO4 with high etching rate of ~30 nm/s and that the etching did not damage BZY surface. The present results show that YBCO is a buffer layer candidate for fabrication of the μSOFC with highly oriented BZY.
Fig. 11. Cross-sectional SEM images in YBCO/BZY/MgO at etching time of 30 s and 180 s YBCO was partially patterned by photolithography (right portion in the SEM images). Several SEM images were overlapped to show wide regions.
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