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Growth mechanisms of ceria- and zirconia-based epitaxial thin films and hetero-structures grown by pulsed laser deposition

Growth mechanisms of ceria- and zirconia-based epitaxial thin films and hetero-structures grown... Mater Renew Sustain Energy (2013) 2:6 DOI 10.1007/s40243-012-0006-6 OR IGINAL PAPER Growth mechanisms of ceria- and zirconia-based epitaxial thin films and hetero-structures grown by pulsed laser deposition • • • Daniele Pergolesi Marco Fronzi Emiliana Fabbri Antonello Tebano Enrico Traversa Received: 28 October 2012 / Accepted: 7 December 2012 / Published online: 22 December 2012 The Author(s) 2013. This article is published with open access at Springerlink.com Abstract Thin films and epitaxial hetero-structures of Keywords Pulsed laser deposition  Hetero-structure doped and undoped CeO , and 8 mol% Y O stabilized Oxygen-ion conducting oxides  Reflection high energy 2 2 3 ZrO (YSZ), were fabricated by pulsed laser deposition on electron diffraction  Density functional theory different single crystal substrates. Reflection high energy electron diffraction was used to monitor in situ the growth mechanism of the films. Two distinct growth mechanisms Introduction were identified along the (001) growth direction for the Ce- and Zr-based materials, respectively. While the doped Thin films of doped CeO and Y O -stabilized ZrO (YSZ) 2 2 3 2 or undoped ceria films showed a 3-dimensional growth fabricated by different thin film deposition methods have mechanism typically characterized by a pronounced sur- been widely investigated as high temperature oxygen-ion face roughness, YSZ films showed an almost ideal layer- conductors. Among the most important applications of by-layer 2-dimensional growth. Moreover, when the two these materials is the fabrication of electrolyte membranes materials were stacked together in epitaxial hetero-struc- for solid oxide fuel cells (SOFCs). Particularly, large tures, the two different growth mechanisms were pre- oxygen ion conductivity characterizes doped CeO . Typi- served. As a result, a 2-dimensional reconstruction of the cal dopants are Gd and Sm with concentration ranging ceria-based layers determined by the YSZ film growing from 10 to 20 % [1]. The bulk oxygen-ion conductivity of -1 above was observed. The experimental results are 15 % Sm-doped CeO (SDC) is as large as 0.02 S cm at explained in terms of the thermodynamic stability of the about 600 C, making this material one of the most per- low-index surfaces of the two materials using computa- forming solid state electrolyte in the so-called intermediate tional analysis performed by density functional theory. temperature range (500–800 C) [2]. YSZ is also widely used as an electrolyte, for SOFCs but mostly for oxygen sensors used to control the air-to-fuel ratio in vehicles, as well as for the fabrication of thermal barrier coatings due to its low thermal conductivity [3]. D. Pergolesi (&)  M. Fronzi  E. Fabbri Pure zirconia (ZrO ) shows a complicated phase dia- International Research Center for Materials Nanoarchitectonics gram having a monoclinic crystal structure at temperatures (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan below about 1,000 C, with transitions to tetragonal and e-mail: pergolesi.daniele@nims.go.jp cubic structures with increasing the temperature. Such phase transformations induce very large stresses that cause A. Tebano pure zirconia to crack, limiting its practical application. On CNR-SPIN and Dipartimento di Informatica Sistemi e Produzione, University of Roma Tor Vergata, Rome, Italy the contrary, pure ceria (CeO ) has a stable cubic phase that can easily become non-stoichiometric in oxygen con- E. Traversa tent, showing important catalytic activity in oxygen International Research Center for Renewable Energy, State Key reduction reaction processes. This material finds many Laboratory of Multiphase Flow in Power Engineering, practical applications, of which the most important are in Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China 123 Page 2 of 9 Mater Renew Sustain Energy (2013) 2:6 catalysis [4]. Thin films of CeO have been also widely Table 1 Single crystal substrates used in this work, their crystallo- graphic structures, lattice parameters and crystallographic orientations used as buffer layers for the growth of thin film hetero- structures, such as insulating layers for microelectronic Material Crystal structure Lattice Crystal devices or as diffusion barriers to avoid chemical reactions parameters orientation (nm) at interfaces. The increasing miniaturization of solid state electro- MgO Cubic rock salt c = 0.421 (001) chemical devices leads to an increasing importance of the C-cut Al O Hexagonal a = 0.476 (0001) 2 3 thin film deposition technology for their fabrication. b = 0.476 Besides, it is well known that the microstructural and c = 1.300 morphological characteristics of thin films can strongly 9.5 mol % YSZ Cubic fluorite c = 0.512 (001) affect their physical and chemical properties. In particular, YAlO (YAO) Orthorhombic a = 0.518 (110) especially for doped ceria, the degree of crystallinity, the perovskite b = 0.531 average grain size and the relative grain boundary extent c = 0.735 can significantly modify the charge transport properties and SrTiO (STO) Cubic perovskite c = 0.3905 (001) the chemical stability in operating environment [5–7]. More recently, multilayered thin film hetero-structures comprising ceria-based oxides and/or YSZ have been fabricated in order to study the conducting properties of the deposition of the films, the substrates were kept at hetero-phase interfaces. A large increase in ionic conduc- about 800 C in about 40 Pa of high purity oxygen partial tivity was observed in case of incoherent interfaces due to pressure for about 20 min. It was observed that such a the faster conduction pathways along dislocation lines thermal treatment often resulted in an evident enhancement [8, 9]. More ordered oxygen-ion conducting hetero-inter- of the crystalline quality of the substrate surfaces. faces have been also fabricated to investigate the effect on The custom made PLD system (AOV Ltd) consisted of a -6 the conducting properties of the compressive or ten- vacuum chamber with a base pressure of about 10 Pa, sile strain occurring at quasi-coherent hetero-interfaces equipped with a load-lock chamber. The target carousel [10–12]. The fabrication of samples appropriately designed can hold up to six targets, and a stainless steel shield to allow isolating the effect of the interfacial strain field reduces cross contamination during the ablation process. requires a careful control of the deposition process and a Each target can simultaneously rotate and oscillate allow- deep understanding of the growth mechanisms. ing a uniform ablation of the target surface. A KrF excimer For this work, we used reflection high energy electron laser (Coherent Lambda Physik GmbH) with a wavelength diffraction (RHEED) and X-ray diffraction (XRD) analyses of 248 nm and a pulse width of 25 ns was focused on the to investigate the growth mechanism of doped and undoped target material in a spot area of about 5 mm . The energy ceria and YSZ thin films grown onto different single crystal of the laser shots onto the target surface was set at about substrates. Different growth mechanisms were observed for 160 mJ. A laser repetition rate between 2 and 5 Hz was the two materials depending on the crystallographic growth used. The deposition of the films was carried out in an direction. oxygen background pressure ranging from 0.5 to 5 Pa, at a Density functional theory calculations were used to substrate temperature of about 700 C. The target-to- theoretically investigate the thermodynamic stability of the substrate distance was 75 mm. (100) and (111) surfaces of the two materials in equilib- The samples were cooled from the deposition tempera- -1 rium with the gas phase, and a very good agreement with ture down to room temperature at 10 degrees min in an the experimental observations was found. oxygen background pressure of about 40 Pa. A high pressure reflection high energy electron diffrac- tion (RHEED) system (AOV Ltd), equipped with a dif- Experimental ferential pumping system was used to monitor in situ the surface evolution of the films. An accelerating voltage of Thin films of CeO , 15 % Sm-doped CeO (SDC) and 28 kV and an emission current of about 100 lA were used. 2 2 8 mol% Y O stabilized ZrO (YSZ) were grown by pulsed The RHEED patterns were recorded using a CCD camera. 2 3 2 laser deposition (PLD) on different single crystal substrates X-ray analysis (PANalytical X’pert Pro MPD) was used listed in Table 1. Sintered ceramic pellets prepared in our to calibrate the deposition rate by X-ray reflectometry laboratory were used as target materials. (XRR) and to investigate the out-of-plane crystalline The substrates were ultrasonically cleaned in de-ionized structure of the films by X-ray diffraction (XRD). water, acetone and methanol, and dried with pure nitrogen To confirm that the PLD process actually pro- prior to insertion into the deposition system. Before starting vided samples with the expected electrical properties, the 123 Mater Renew Sustain Energy (2013) 2:6 Page 3 of 9 electrical conductivities of the highly textured films of Figure 2b shows the XRR plot used for the calibration SDC and YSZ were measured by electrochemical imped- of the deposition rate of doped and undoped ceria. With the ance spectroscopy (EIS). Two Ti–Pt electrodes were selected deposition parameters, a deposition rate of about -1 deposited onto the film surfaces by electron beam deposi- 0.24 A shot was measured. Figure 2c shows that, also in tion. The electrical characterization was performed in air, this case, the RHEED patterns of the growing films showed using a multichannel potentiostat VMP3 (Bio-Logic), in the typical features of a 3D growth mechanism. the frequency range between 1 MHz and 100 mHz, vary- The spotty RHEED patterns characterizing the growth ing the temperature between 400 and 700 C. of pure and doped ceria films were observed over a rela- tively wide range of deposition parameters (substrate temperature of 700 ± 100 C, oxygen background pres- Results and discussion sure from 0.1 up to 5 Pa, deposition rate from 0.3 up to -1 about 2 A shot ). Analogous RHEED patterns were also The growth mechanism of ceria-based epitaxial observed for (001)-oriented CeO films grown by means of thin films different thin film deposition techniques [14]. To check whether such a 3D growth mechanism depends Among the substrates used in this work, and listed in on the growth crystallographic axis, C-cut Al O (0001)- 2 3 Table 1, (001)-oriented STO single crystals are particularly oriented single crystalline substrates were used. C-cut suitable for the epitaxial growth of doped and undoped (0001) and R-cut (1102) sapphire crystals have been widely CeO films [5]. CeO and SDC have a cubic fluorite crystal used for the growth of crystalline thin films of doped and 2 2 structure with a lattice parameter of about 5.41 and 5.44 A, undoped ceria. The R-cut surface is expected to favour the respectively. Epitaxial films can be obtained on STO with (001) orientation, while the C-cut should favour the (111) the in-plane orientation (100)CeO /(110)STO. Owing to this 45 in-plane rotation of the CeO unit cell with respect to the STO unit cell, the resulting lattice misfit with the STO substrate is about 1.4 % for SDC and about 2 % for pure ceria. Figure 1a shows the 2h-h scan of a 250 A-thick film of SDC epitaxially oriented with the STO substrate. Figure 1b shows the size effect interference fringes around the (002) SDC reflection, indicating the good crystallo- graphic quality of the film. The red curve in Fig. 1b rep- resents a simulation for a 45 unit cells thick SDC film (about 245 A), which is in very good agreement with the expected thickness, as derived from the calibration of the deposition rate performed by XRR (Fig. 2b). Figure 1c shows the typical RHEED patterns acquired for a film of SDC grown on STO, relative to the (100) in-plane orientation of the substrate. A spotty pattern asso- ciated with a 3D growth mechanism appeared after few laser shots implying that this island-like growth arose immedi- ately at the early stage of the formation of the first layers. Analogous results were obtained using (001)-oriented 9.5 YSZ single crystal substrates for the growth of epi- taxial CeO films, as shown in Fig. 2. In this case, the two materials have the same crystalline structure and the CeO has a lattice misfit of about -5.6 % with respect to the substrate. Such a relatively large lattice misfit can be accommodated by the introduction of a regular network of misfit dislocations at the interface [13] allowing a well- ordered cube-on-cube growth driven by the substrate along Fig. 1 a XRD analysis of an SDC film grown on STO. b XRD plot of the (002) SDC reflection with superimposed simulation (red curve)of the (001) direction. A mosaic spread of about 0.5 was the size effect interference fringes for an SDC film thickness of 45 evaluated by measuring the FWHM of the Gaussian fit of unit cells. c RHEED pattern recorded along the in-plane (100) the rocking curve acquired along the (002) reflection peak orientation of the substrate and RHEED pattern of the SDC film at the of the film. end of the deposition 123 Page 4 of 9 Mater Renew Sustain Energy (2013) 2:6 Fig. 3 XRD analysis of an SDC film grown on C-cut Al O single 2 3 crystal substrate. The inset shows the RHEED patterns recorded for the substrate and for the SDC film at the end of the deposition Finally, the electrical conductivity of an SDC film grown on sapphire was measured by EIS in air. The mea- -1 Fig. 2 a XRD analysis of a CeO film grown on a (001)-oriented sured conductivity ranged from 0.03 S cm at about -1 9.5 YSZ substrate. b XRR measurement performed for the calibration 680 C down to 0.001 S cm at about 400 C, with of the deposition rate of the film. c RHEED patterns of the substrate activation energy of about 0.70 eV. This result is in very and of the film recorded along the in-plane (100) orientation of the good agreement with the literature data relative to the bulk substrate conductivity of doped CeO films [6]. The electrical characterization of the films grown on STO and YSZ single film orientation [15, 16]. Nevertheless, both orientations crystal wafers cannot give reliable results due to the con- were obtained on both surfaces, as well as films showing ductive properties of the deposition substrates at high mixed (001)/(111) orientation, depending on process temperatures. parameters and deposition technique [17, 18]. Using PLD, the fabrication of highly (111)-oriented CeO films on C-cut The growth mechanism of YSZ epitaxial thin films sapphire has been reported for example in [6]. Figure 3 shows that preferentially (111)-oriented SDC The growth of YSZ films was studied using (110)-oriented films were grown with a minor (001) orientation. We YAO and (001)-oriented MgO single crystal substrates. observed that, in our experimental condition, (111)-ori- YSZ has a cubic fluorite structure with lattice parameter of ented films were obtained at relatively low oxygen partial about 5.14 A, which results in a lattice misfit of about 1.5 % pressure (in the order of 0.1–0.5 Pa), while for larger val- with (110)-oriented YAO (Table 1). Figure 4a shows the ues of oxygen partial pressure (few Pa), the films showed out-of-plane XRD analysis of a YSZ film grown on YAO. well-defined mixed (001)/(111) orientation. The presence of well-defined size effect interference fringes The RHEED patterns acquired during the growth at low around the (002) reflection line of the film (Fig. 4b) sug- oxygen partial pressure (inset in Fig. 3) showed the typical gests a very good crystallographic quality. The black curve features that characterize a quasi-2D layer-by-layer growth in Fig. 4b shows a simulation for a 35 unit cells thick YSZ mechanism. Such streaky features were never observed for film (about 180 A). From this simulation, we could estimate CeO or SDC film grown onto STO or YSZ substrates -1 a deposition rate of about 0.059 A shot . where the growth was along the (001) axis. Opposite to what observed in the case of (001)-oriented To summarize, these measurements showed that doped ceria-based films, the RHEED patterns of (001)-oriented or undoped ceria presents two different growth mecha- YSZ films clearly showed a different growth mechanism. nisms depending on the growth directions; the film grows Figure 3b shows the RHEED patterns of the substrate predominantly quasi-2D along the (111) direction, while an and of the YSZ film acquired toward the (100) in-plane evident 3D growth mechanism was observed along the direction of YAO. The RHEED patterns consisted of (001) direction. 123 Mater Renew Sustain Energy (2013) 2:6 Page 5 of 9 Fig. 5 a XRD analysis of a YSZ film grown on (001)-oriented MgO substrate. The inset shows the XRR measurement performed for the calibration of the deposition rate of the film. b RHEED patterns of the substrate and of the film recorded along the in-plane (100) orientation of the substrate thickness of about 20 A. However, even in the case of a particularly unfavourable crystalline matching with the Fig. 4 a XRD analysis of a YSZ film grown on (110)-oriented YAO. deposition substrate, YSZ showed a clear tendency to a 2D b XRD plot of the (002) YSZ reflection with superimposed simulation growth toward the (001) direction driven by the deposition (black curve) of the size effect interference fringes for a YSZ film thickness of 35 unit cells. c RHEED patterns of the substrate and the substrate. YSZ film recorded along the in-plane (100) orientation of the The electrical conductivity of a YSZ film grown on MgO substrate was measured in air by EIS and the conductivity was found -3 -5 -1 to range from 9.6 9 10 down to 8.5 9 10 Scm at well-defined streaks revealing a 2D layer-by-layer growth 700 and 400 C, respectively, showing an activation energy mechanism. of about 0.98 eV. The measured conductivity was in Very similar result was obtained analysing a 300 A thick excellent agreement with the conductivity of YSZ single film of YSZ grown on (001)-oriented MgO substrate. In crystals [19]. spite of a lattice misfit as large as -18 %, a highly textured growth was observed by XRD analysis (Fig. 5). A relatively Calculation of the low index surface energies large value of 0.64 was found for the FWHM of the (002) x-scan of the film. Figure 5a shows the XRR plot used to The comparison between the results obtained with YSZ measure the deposition rate of the film with the selected and those obtained with doped and undoped ceria films -1 deposition parameters. A value of about 0.053 A shot strongly suggests a significant difference in the thermo- was estimated in very good agreement with the value dynamic stability of the (001) surfaces of the two materials. obtained from the simulation of the size effect interference To understand the driving mechanism for the different fringes showed in Fig. 4b. The final RHEED pattern behaviours experimentally observed, a theoretical evalua- showed the typical features associated with a 2D layer-by- tion of the surface energy of the low-index surface orien- layer growth (Fig. 5b). However, in this case, the RHEED tation of CeO and cubic zirconia (c-ZrO ) was computed 2 2 pattern disappeared during the deposition of the first layers, using first-principles density functional theory (DFT). suggesting the formation of a disordered interface probably The low-index surface energies of CeO nano-particles characterized by a large density of misfit dislocations have been analysed in a previous work and the stable sur- introduced to release the excess interfacial strain [8]. faces were identified as a function of the temperature, the According to the measured deposition rate, a streaky chemical potential and the oxygen background pressure RHEED pattern originated from the surface of the YSZ [20]. It was found that unless the chemical potential of the film growing on MgO was not observed below a minimum oxygen is extremely low (i.e. very high temperature and very 123 Page 6 of 9 Mater Renew Sustain Energy (2013) 2:6 low oxygen partial pressure), the (111) surface orientation is Table 2 Low index surface Gibbs free energy of ceria and cubic zirconia stable if compared with the (100) and (110). By minimiza- tion of the Gibbs free energy as indicated by the Gibbs– Material Surface Surface Gibbs Surface energy References Wulff theorem, it was possible then to calculate the equi- index free energy (eV) difference (eV) librium crystal shape of a CeO nano-particles and show that CeO (001) 0.230 0.193 [20] under oxygen-rich conditions, the (111) surface is the only (111) 0.037 orientation present in the particle and the resulting mor- c-ZrO (001) 0.189 0.116 This work phology is described as the octahedron shown in Fig. 6a. (111) 0.073 For this work, an analogous analysis has been carried out to calculate the low-index surface energies of c-ZrO nano-particles. As previously mentioned, the cubic fluorite minimize the surface energy. In the case of highly textured zirconia is stable only at very high temperatures. In YSZ, thin films on single crystal substrates, the epitaxial stress the effect of the dopant (Y O ), besides creating oxygen can modify the extension of the different facets present in 2 3 vacancies allowing oxygen-ion diffusion, is to stabilize the the stress-free equilibrium shape, but it cannot create new cubic structure at lower temperatures. facets [22]. In this sense, we can say that the analysis Although YSZ is the material under investigation in this proposed here for nano-particles may give an indication of work, for the calculation of the low-index surface energies, the local preferential surface orientation of a thin film at we only considered the cubic undoped crystal, since it has thermodynamic equilibrium. been shown that the trend of the low-index surface relative In both cases (CeO and c-ZrO ), the (111) surface is 2 2 stability for c-ZrO and YSZ does not change [21]. stable if compared with other low-index orientations. Even Therefore, the computed surface energies obtained con- though the growth is driven toward the (001) direction by sidering undoped cubic zirconia are expected to bring, at crystalline constrains, the (111) facet is strongly favoured least qualitatively, to the same conclusions. for CeO , which will arrange the lattice in order to expose The surface energies of (111) and (001) surfaces of the (111) surface. As a result, the favoured (001) surface c-ZrO nano-particles under oxygen-rich condition were will consist on a mosaic of (111)-facet octahedra (001)- calculated. Table 2 compares the low-index surface ener- oriented, thus enhancing the surface roughness. On the gies calculated for CeO and c-ZrO nano-particles, other hand, c-ZrO , having a smaller difference between 2 2 2 showing that both crystalline structures favour the (111) the (001) and (111) surface energies will expose a mixture surface that strongly minimize the Gibbs free energy, but of these two orientations originating smoother surfaces. the difference between the (001) and (111) surface energies The growth mechanism of ceria- and zirconia-based in the case of c-ZrO turned out to be about a factor of two smaller than in the case of CeO . superlattices The equilibrium crystal shape of c-ZrO nano-particles, calculated according to the Wulff construction by mini- Multilayered hetero-structures and superlattices made by mizing the surface energy, can be described as a truncated coupling SDC or YSZ with an insulating phase [9, 11, 12] octahedron showing a mixture of (001) and (111) surfaces have been fabricated to study the conducting properties of (Fig. 6b). the interfaces [23]. To the best of our knowledge, for these The Wulff theorem states that for a given volume, the hetero-structures, high crystalline quality (biaxial texture, crystal exposes different surface orientations in order to no grain boundary) has been achieved only for (001)- oriented YSZ–STO multilayers grown by PLD on STO substrates [24], and for (001)-oriented CeO –YSZ hetero- structures grown on MgO [25, 26]. Highly (111)-oriented hetero-structures of Gd-doped ceria and Gd-doped zirconia, with minor phases from other orientations, were obtained on C-cut sapphire by molecular beam epitaxy [10]. In this case, the authors reported the observation of a streaky RHEED pattern up to a film thickness of about 40 nm with additional weak features corresponding to polycrystalline structure with increasing the thickness of the deposition. The literature does not report any detailed study of the Fig. 6 Morphology of the equilibrium crystal shape nano-particles of growth mechanism of these hetero-structures, particularly CeO (a) and c-ZrO (b) obtained by the minimization of the Gibbs 2 2 in the case of the (001) orientation. free energy, as indicated by the Gibbs-Wulff theorem 123 Mater Renew Sustain Energy (2013) 2:6 Page 7 of 9 Fig. 8 a XRD analysis of a CeO /YSZ superlattice grown on (001)- oriented MgO substrate using a 5 nm thick STO buffer layer. b RHEED patterns acquired during the growth of the superlattice Fig. 7 a XRD analysis of an SDC/YSZ superlattice grown on (001)- along the (100) in-plane orientation of the MgO substrate oriented STO substrate. b RHEED patterns acquired during the growth of the superlattice along the (100) in-plane orientation of the STO substrate thickness of each individual layer was about 15 unit cells. The very well-defined superlattice features allowed iden- tifying the diffraction satellite peaks up to the third order. For this work, we fabricated superlattices made of SDC For the fabrication of this sample, MgO was selected as and YSZ on (001)-oriented STO substrates and superlattices deposition substrate due to its good insulating properties at made of CeO and YSZ on (001)-oriented MgO substrates. high temperatures that allow the electrical characterization The growth of each layer was monitored by RHEED. along the planar direction of the hetero-interfaces. The The SDC/YSZ superlattice consisted of 30 bilayers of STO buffer layer provides the suitable lattice matching for the two materials with single layer thickness of about 30 the epitaxial growth [5]. unit cells, as estimated from the deposition rate measured An almost ideal 2D layer-by-layer growth with cube-on- by XRR. Figure 7a shows the XRD analysis of such a cube symmetry was observed for the STO buffer layer on superlattice. The superlattice was epitaxially (001)-oriented MgO. Also in this case, a regular array of spots charac- with the STO substrate and the XRD plot showed the terized the RHEED patterns of all the CeO layers, while typical superlattice peaks, as shown in the inset in Fig. 7a. streaky patterns were observed for all the YSZ layers. The RHEED patterns of several SDC/YSZ bilayers were Both Fig. 7 (SDC–YSZ on STO) and Fig. 8 (CeO –YSZ recorded during the growth of the superlattice. Figure 7b on STO-buffered MgO) show a mechanism of 2D surface shows the electron reflection patterns relative to the first reconstruction of the ceria layers determined by the YSZ layer and the 30th final bilayers. RHEED diagnostic revealed growing above. A similar mechanism of roughness suppres- that the two materials, SDC and YSZ, showed the two sion induced by the deposition of a second layer was already different growth mechanisms described above along this observed during the growth of superlattice structures [27]. crystallographic orientation, i.e. a 3D growth for SDC and a quasi-2D growth for YSZ. Figure 8 shows the XRD plot (Fig. 8a) and the RHEED Conclusions patterns (Fig. 8b) of the CeO /YSZ superlattice that con- sisted of 15 CeO /YSZ bilayers grown on MgO using a thin Thin films of CeO , 15 % Sm-doped CeO (SDC), as well 2 2 2 buffer layer of STO, about 5 nm thick. According to the as 8 mol% yttria-doped zirconia (YSZ) were grown on XRR measurement of the deposition rate, the expected different single crystalline substrates. RHEED diagnostic 123 Page 8 of 9 Mater Renew Sustain Energy (2013) 2:6 3. Chen, L.B.: Yttria-stabilized zirconia thermal barrier coatings—a allowed identifying two distinct growth mechanisms for review. Surf. Rev. Lett. 13, 535–544 (2006) the ceria films along the (001) and (111) growth directions. 4. Trovarelli, A.: Catalysis by ceria and related materials. In: Cat- A 3D growth mechanism, associated with a more pro- alytic science series, vol. 2. World Scientific Publishing Co, nounced surface roughness, clearly characterized the Singapore (2002) 5. Sanna, S., Esposito, V., Pergolesi, D., Orsini, A., Tebano, A., growth of the (001)-oriented surfaces, while a quasi-2D Licoccia, S., Balestrino, G., Traversa, E.: Fabrication and elec- growth was observed for the (111)-oriented surfaces. The trochemical properties of epitaxial samarium-doped ceria films 3D (001)-oriented growth of CeO and SDC was observed on SrTiO -buffered MgO substrates. Adv. Funct. Mater. 19, over a relatively wide range of deposition temperatures, 1713–1719 (2009) 6. Goebel, M.C., Gregori, G., Guo, X., Maier, J.: Boundary effects laser energy densities, oxygen background pressures, and on the electrical conductivity of pure and doped cerium oxide thin deposition rates, suggesting that this growth mechanism is films. Phys. Chem. Chem. Phys. 12, 14351–14361 (2010) intrinsically related with the physicochemical properties of 7. Goebel, M.C., Gregori, G., Maier, J.: Mixed conductivity in this material. On the contrary, YSZ films showed an almost nanocrystalline highly acceptor doped cerium oxide thin films under oxidizing conditions. Phys. Chem. Chem. Phys. 13, 10940– ideal 2D layer-by-layer growth along the (001) orientation, 10945 (2011) even in the presence of relatively unfavourable crystalline 8. Sillassen, M., Eklund, P., Pryds, N., Johnson, E., Helmersson, U., matching with the substrate, such as in the case of films Bøttiger, J.: Low-temperature superionic conductivity in strained grown on MgO. yttria-stabilized zirconia. Adv. Funct. Mater. 20, 2071–2076 (2010) The two distinct growth mechanisms along the (001) 9. Korte, C., Peters, A., Janek, J., Hesse, D., Zakharov, N.: Ionic orientation were, to the best of our knowledge, for the first conductivity and activation energy for oxygen ion transport time correlated with the low index surface Gibbs free in superlattices—the semicoherent multilayer system YSZ energies of the two materials, computed by first-principles (ZrO ? 9.5 mol% Y O )/Y O . Phys. Chem. Chem. Phys. 10, 2 2 3 2 3 4623–4635 (2008) DFT. Our calculation showed that both crystalline struc- 10. Azad, S., Marina, O.A., Wang, C.M., Saraf, L., Shutthanandan, tures favour the (111) surface that strongly minimizes the V., McCready, D.E., El-Azab, A., Jaffe, J.E., Engelhard, M.H., Gibbs free energy, but the difference between the (001) and Peden, C.H.F., Thevuthasan, S.: Nanoscale effects on ion con- (111) surface energies of CeO is twice that of cubic ZrO . 2 2 ductance of layer-by-layer structures of gadolinia-doped ceria and zirconia. Appl. Phys. Lett. 86, 1319061 (2005) The enhanced roughness of the (001)-oriented doped or 11. Korte, C., Schichtel, N., Hesse, D., Janek, J.: Influence of inter- undoped CeO surfaces should be taken into account in face structure on mass transport in phase boundaries between particular as far as the growth of highly textured multi- different ionic materials. Monatsh. Chem. 140, 1069–1080 (2009) layered hetero-structures is concerned. In this work, we 12. Schichtel, N., Korte, C., Hesse, D., Zakharov, N., Butz, B., Gerthsenc, D., Janek, J.: On the influence of strain on ion have shown that the ceria-based layers preserve their transport: microstructure and ionic conductivity of nanoscale characteristic 3D growth mechanism also when comprised YSZ/Sc2O3 multilayers. Phys. Chem. Chem. Phys. 12, 14596– in super-structures with YSZ leading to potentially signif- 14608 (2010) icant effects on the interface morphology. Therefore, 13. Wang, C.M., Thevuthasan, S., Peden, C.H.F.: Interface structure of an epitaxial cubic ceria film on cubic zirconia. J. Am. Ceram. especially for CeO and SDC, much better surface (and Soc. 86, 363–365 (2003) interface) quality might be achieved by developing the 14. Ikegawa, S., Motoi, Y.: Growth of CeO thin films by metal- expertise needed to obtain coherent film epitaxy and biaxial organic molecular beam epitaxy. Thin Solid Films 281, 60–63 texture along less common crystallographic directions, for (1996) 15. Zaitsev, A.G., Ockenfuss, G., Guggi, D., Wo ¨ rdenweber, R., instance on pseudo-cubic (110)- or (111)-oriented surfaces. Kru ¨ ger, U.: Structural perfection of (001) CeO thin films on (1102) sapphire. J. Appl. Phys. 81, 3069–3072 (1997) Acknowledgments This work was partly supported by the World 16. Bera, D., Kuchibhatla, S.V.N.T., Azad, S., Saraf, L., Wang, C.M., Premier International Research Centre Initiative of MEXT, Japan. Shutthanandan, V., Nachimuthu, P., McCready, D.E., Engelhard, M.H., Marina, O.A., Baer, D.R., Seal, S., Thevuthasan, S.: Open Access This article is distributed under the terms of the Growth and characterization of highly oriented gadolinia-doped Creative Commons Attribution License which permits any use, dis- ceria (111) thin films on zirconia (111)/sapphire (0001) sub- tribution, and reproduction in any medium, provided the original strates. Thin Solid Films 516, 6088–6094 (2008) author(s) and the source are credited. 17. Kurian, J., Naito, M.: Growth of epitaxial CeO thin films on r-cut sapphire by molecular beam epitaxy. Physica C 492, 31–37 (2004) 18. Nandasiri, M.I., Nachimuthu, P., Varga, T., Shutthanandan, V., References Jiang, W., Kuchibhatla, S.V.N.T., Thevuthasan, S., Seal, S., Kayani, A.N.: Influence of growth rate on the epitaxial orienta- 1. Esposito, V., Traversa, E.: Design of electroceramics for solid tion and crystalline quality of CeO thin films grown on Al2O3 oxides fuel cell applications: playing with ceria. J. Am. Ceram. (0001). J. Appl. Phys. 109, 013525 (2011) Soc. 91, 1037–1051 (2008) 19. Manning, P.S., Sirman, J.D., De Souza, R.A., Kilner, J.A.: The 2. Wachsman, E.D., Lee, K.Y.: Lowering the temperature of solid kinetics of oxygen transport in 9.5 mol % yttria stabilized zir- oxide fuel cells. Science 334, 935–939 (2011) conia. Solid State Ionics 100, 1–10 (1997) 123 Mater Renew Sustain Energy (2013) 2:6 Page 9 of 9 20. Fronzi, M., Soon, A., Delley, B., Traversa, E., Stampfl, C.: Sta- ionic conductivity at interfaces of epitaxial ZrO :Y O /SrTiO 2 2 3 3 bility and morphology of cerium oxide surfaces in an oxidizing heterostructures. Science 321, 676–680 (2008) environment: a first-principles investigation. J. Chem. Phys. 131, 25. Pergolesi, D., Tebano, A., Fabbri, E., Balestrino, G., Licoccia, S., 104701 (2009) Traversa, E.: Pulsed lased deposition of superlattices based on 21. Xia, X., Oldman, R., Catlow, R.: Computational modeling study ceria and zirconia. ECS Trans. 35, 1125–1130 (2010) of bulk and surface of yttria-stabilized cubic zirconia. Chem. 26. Pergolesi, D., Fabbri, E., Cook, S.N., Roddatis, V., Traversa, E., Mater. 21, 3576–3585 (2009) Kilner, J.A.: Tensile lattice distortion does not affect oxygen 22. Muller, P., Kern, R.: Equilibrium nano-shape change induced by transport in yttria-stabilized zirconia-CeO heterointerfaces. ACS epitaxial stress: effect of surface stress. Surf. Sci. 457, 229–253 Nano (2012). doi:10.1021/nn302812m (2000) 27. Tebano, A., Balestrino, G., Lavanga, S., Martellucci, S., Meda- 23. Fabbri, E., Pergolesi, D., Traversa, E.: Ionic conductivity in oxide glia, P.G., Paoletti, A., Pasquini, G., Petrocelli, G., Tucciarone, heterostructures: the role of interfaces. Sci. Technol. Adv. Mater. A.: Reflection high-energy electron diffraction oscillations during 11, 054503–054512 (2010) epitaxial growth of artificially layered films of (BaCuO )m/ 24. Garcia-Barriocanal, J., Rivera-Calzada, A., Varela, M., Sefrioui, (CaCuO )n. Physica C 355, 335–340 (2001) Z., Iborra, E., Leon, C., Pennycook, S.J., Santamaria, J.: Colossal http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Materials for Renewable and Sustainable Energy Springer Journals

Growth mechanisms of ceria- and zirconia-based epitaxial thin films and hetero-structures grown by pulsed laser deposition

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Material Science; Materials Science, general; Renewable and Green Energy; Renewable and Green Energy
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Mater Renew Sustain Energy (2013) 2:6 DOI 10.1007/s40243-012-0006-6 OR IGINAL PAPER Growth mechanisms of ceria- and zirconia-based epitaxial thin films and hetero-structures grown by pulsed laser deposition • • • Daniele Pergolesi Marco Fronzi Emiliana Fabbri Antonello Tebano Enrico Traversa Received: 28 October 2012 / Accepted: 7 December 2012 / Published online: 22 December 2012 The Author(s) 2013. This article is published with open access at Springerlink.com Abstract Thin films and epitaxial hetero-structures of Keywords Pulsed laser deposition  Hetero-structure doped and undoped CeO , and 8 mol% Y O stabilized Oxygen-ion conducting oxides  Reflection high energy 2 2 3 ZrO (YSZ), were fabricated by pulsed laser deposition on electron diffraction  Density functional theory different single crystal substrates. Reflection high energy electron diffraction was used to monitor in situ the growth mechanism of the films. Two distinct growth mechanisms Introduction were identified along the (001) growth direction for the Ce- and Zr-based materials, respectively. While the doped Thin films of doped CeO and Y O -stabilized ZrO (YSZ) 2 2 3 2 or undoped ceria films showed a 3-dimensional growth fabricated by different thin film deposition methods have mechanism typically characterized by a pronounced sur- been widely investigated as high temperature oxygen-ion face roughness, YSZ films showed an almost ideal layer- conductors. Among the most important applications of by-layer 2-dimensional growth. Moreover, when the two these materials is the fabrication of electrolyte membranes materials were stacked together in epitaxial hetero-struc- for solid oxide fuel cells (SOFCs). Particularly, large tures, the two different growth mechanisms were pre- oxygen ion conductivity characterizes doped CeO . Typi- served. As a result, a 2-dimensional reconstruction of the cal dopants are Gd and Sm with concentration ranging ceria-based layers determined by the YSZ film growing from 10 to 20 % [1]. The bulk oxygen-ion conductivity of -1 above was observed. The experimental results are 15 % Sm-doped CeO (SDC) is as large as 0.02 S cm at explained in terms of the thermodynamic stability of the about 600 C, making this material one of the most per- low-index surfaces of the two materials using computa- forming solid state electrolyte in the so-called intermediate tional analysis performed by density functional theory. temperature range (500–800 C) [2]. YSZ is also widely used as an electrolyte, for SOFCs but mostly for oxygen sensors used to control the air-to-fuel ratio in vehicles, as well as for the fabrication of thermal barrier coatings due to its low thermal conductivity [3]. D. Pergolesi (&)  M. Fronzi  E. Fabbri Pure zirconia (ZrO ) shows a complicated phase dia- International Research Center for Materials Nanoarchitectonics gram having a monoclinic crystal structure at temperatures (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan below about 1,000 C, with transitions to tetragonal and e-mail: pergolesi.daniele@nims.go.jp cubic structures with increasing the temperature. Such phase transformations induce very large stresses that cause A. Tebano pure zirconia to crack, limiting its practical application. On CNR-SPIN and Dipartimento di Informatica Sistemi e Produzione, University of Roma Tor Vergata, Rome, Italy the contrary, pure ceria (CeO ) has a stable cubic phase that can easily become non-stoichiometric in oxygen con- E. Traversa tent, showing important catalytic activity in oxygen International Research Center for Renewable Energy, State Key reduction reaction processes. This material finds many Laboratory of Multiphase Flow in Power Engineering, practical applications, of which the most important are in Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China 123 Page 2 of 9 Mater Renew Sustain Energy (2013) 2:6 catalysis [4]. Thin films of CeO have been also widely Table 1 Single crystal substrates used in this work, their crystallo- graphic structures, lattice parameters and crystallographic orientations used as buffer layers for the growth of thin film hetero- structures, such as insulating layers for microelectronic Material Crystal structure Lattice Crystal devices or as diffusion barriers to avoid chemical reactions parameters orientation (nm) at interfaces. The increasing miniaturization of solid state electro- MgO Cubic rock salt c = 0.421 (001) chemical devices leads to an increasing importance of the C-cut Al O Hexagonal a = 0.476 (0001) 2 3 thin film deposition technology for their fabrication. b = 0.476 Besides, it is well known that the microstructural and c = 1.300 morphological characteristics of thin films can strongly 9.5 mol % YSZ Cubic fluorite c = 0.512 (001) affect their physical and chemical properties. In particular, YAlO (YAO) Orthorhombic a = 0.518 (110) especially for doped ceria, the degree of crystallinity, the perovskite b = 0.531 average grain size and the relative grain boundary extent c = 0.735 can significantly modify the charge transport properties and SrTiO (STO) Cubic perovskite c = 0.3905 (001) the chemical stability in operating environment [5–7]. More recently, multilayered thin film hetero-structures comprising ceria-based oxides and/or YSZ have been fabricated in order to study the conducting properties of the deposition of the films, the substrates were kept at hetero-phase interfaces. A large increase in ionic conduc- about 800 C in about 40 Pa of high purity oxygen partial tivity was observed in case of incoherent interfaces due to pressure for about 20 min. It was observed that such a the faster conduction pathways along dislocation lines thermal treatment often resulted in an evident enhancement [8, 9]. More ordered oxygen-ion conducting hetero-inter- of the crystalline quality of the substrate surfaces. faces have been also fabricated to investigate the effect on The custom made PLD system (AOV Ltd) consisted of a -6 the conducting properties of the compressive or ten- vacuum chamber with a base pressure of about 10 Pa, sile strain occurring at quasi-coherent hetero-interfaces equipped with a load-lock chamber. The target carousel [10–12]. The fabrication of samples appropriately designed can hold up to six targets, and a stainless steel shield to allow isolating the effect of the interfacial strain field reduces cross contamination during the ablation process. requires a careful control of the deposition process and a Each target can simultaneously rotate and oscillate allow- deep understanding of the growth mechanisms. ing a uniform ablation of the target surface. A KrF excimer For this work, we used reflection high energy electron laser (Coherent Lambda Physik GmbH) with a wavelength diffraction (RHEED) and X-ray diffraction (XRD) analyses of 248 nm and a pulse width of 25 ns was focused on the to investigate the growth mechanism of doped and undoped target material in a spot area of about 5 mm . The energy ceria and YSZ thin films grown onto different single crystal of the laser shots onto the target surface was set at about substrates. Different growth mechanisms were observed for 160 mJ. A laser repetition rate between 2 and 5 Hz was the two materials depending on the crystallographic growth used. The deposition of the films was carried out in an direction. oxygen background pressure ranging from 0.5 to 5 Pa, at a Density functional theory calculations were used to substrate temperature of about 700 C. The target-to- theoretically investigate the thermodynamic stability of the substrate distance was 75 mm. (100) and (111) surfaces of the two materials in equilib- The samples were cooled from the deposition tempera- -1 rium with the gas phase, and a very good agreement with ture down to room temperature at 10 degrees min in an the experimental observations was found. oxygen background pressure of about 40 Pa. A high pressure reflection high energy electron diffrac- tion (RHEED) system (AOV Ltd), equipped with a dif- Experimental ferential pumping system was used to monitor in situ the surface evolution of the films. An accelerating voltage of Thin films of CeO , 15 % Sm-doped CeO (SDC) and 28 kV and an emission current of about 100 lA were used. 2 2 8 mol% Y O stabilized ZrO (YSZ) were grown by pulsed The RHEED patterns were recorded using a CCD camera. 2 3 2 laser deposition (PLD) on different single crystal substrates X-ray analysis (PANalytical X’pert Pro MPD) was used listed in Table 1. Sintered ceramic pellets prepared in our to calibrate the deposition rate by X-ray reflectometry laboratory were used as target materials. (XRR) and to investigate the out-of-plane crystalline The substrates were ultrasonically cleaned in de-ionized structure of the films by X-ray diffraction (XRD). water, acetone and methanol, and dried with pure nitrogen To confirm that the PLD process actually pro- prior to insertion into the deposition system. Before starting vided samples with the expected electrical properties, the 123 Mater Renew Sustain Energy (2013) 2:6 Page 3 of 9 electrical conductivities of the highly textured films of Figure 2b shows the XRR plot used for the calibration SDC and YSZ were measured by electrochemical imped- of the deposition rate of doped and undoped ceria. With the ance spectroscopy (EIS). Two Ti–Pt electrodes were selected deposition parameters, a deposition rate of about -1 deposited onto the film surfaces by electron beam deposi- 0.24 A shot was measured. Figure 2c shows that, also in tion. The electrical characterization was performed in air, this case, the RHEED patterns of the growing films showed using a multichannel potentiostat VMP3 (Bio-Logic), in the typical features of a 3D growth mechanism. the frequency range between 1 MHz and 100 mHz, vary- The spotty RHEED patterns characterizing the growth ing the temperature between 400 and 700 C. of pure and doped ceria films were observed over a rela- tively wide range of deposition parameters (substrate temperature of 700 ± 100 C, oxygen background pres- Results and discussion sure from 0.1 up to 5 Pa, deposition rate from 0.3 up to -1 about 2 A shot ). Analogous RHEED patterns were also The growth mechanism of ceria-based epitaxial observed for (001)-oriented CeO films grown by means of thin films different thin film deposition techniques [14]. To check whether such a 3D growth mechanism depends Among the substrates used in this work, and listed in on the growth crystallographic axis, C-cut Al O (0001)- 2 3 Table 1, (001)-oriented STO single crystals are particularly oriented single crystalline substrates were used. C-cut suitable for the epitaxial growth of doped and undoped (0001) and R-cut (1102) sapphire crystals have been widely CeO films [5]. CeO and SDC have a cubic fluorite crystal used for the growth of crystalline thin films of doped and 2 2 structure with a lattice parameter of about 5.41 and 5.44 A, undoped ceria. The R-cut surface is expected to favour the respectively. Epitaxial films can be obtained on STO with (001) orientation, while the C-cut should favour the (111) the in-plane orientation (100)CeO /(110)STO. Owing to this 45 in-plane rotation of the CeO unit cell with respect to the STO unit cell, the resulting lattice misfit with the STO substrate is about 1.4 % for SDC and about 2 % for pure ceria. Figure 1a shows the 2h-h scan of a 250 A-thick film of SDC epitaxially oriented with the STO substrate. Figure 1b shows the size effect interference fringes around the (002) SDC reflection, indicating the good crystallo- graphic quality of the film. The red curve in Fig. 1b rep- resents a simulation for a 45 unit cells thick SDC film (about 245 A), which is in very good agreement with the expected thickness, as derived from the calibration of the deposition rate performed by XRR (Fig. 2b). Figure 1c shows the typical RHEED patterns acquired for a film of SDC grown on STO, relative to the (100) in-plane orientation of the substrate. A spotty pattern asso- ciated with a 3D growth mechanism appeared after few laser shots implying that this island-like growth arose immedi- ately at the early stage of the formation of the first layers. Analogous results were obtained using (001)-oriented 9.5 YSZ single crystal substrates for the growth of epi- taxial CeO films, as shown in Fig. 2. In this case, the two materials have the same crystalline structure and the CeO has a lattice misfit of about -5.6 % with respect to the substrate. Such a relatively large lattice misfit can be accommodated by the introduction of a regular network of misfit dislocations at the interface [13] allowing a well- ordered cube-on-cube growth driven by the substrate along Fig. 1 a XRD analysis of an SDC film grown on STO. b XRD plot of the (002) SDC reflection with superimposed simulation (red curve)of the (001) direction. A mosaic spread of about 0.5 was the size effect interference fringes for an SDC film thickness of 45 evaluated by measuring the FWHM of the Gaussian fit of unit cells. c RHEED pattern recorded along the in-plane (100) the rocking curve acquired along the (002) reflection peak orientation of the substrate and RHEED pattern of the SDC film at the of the film. end of the deposition 123 Page 4 of 9 Mater Renew Sustain Energy (2013) 2:6 Fig. 3 XRD analysis of an SDC film grown on C-cut Al O single 2 3 crystal substrate. The inset shows the RHEED patterns recorded for the substrate and for the SDC film at the end of the deposition Finally, the electrical conductivity of an SDC film grown on sapphire was measured by EIS in air. The mea- -1 Fig. 2 a XRD analysis of a CeO film grown on a (001)-oriented sured conductivity ranged from 0.03 S cm at about -1 9.5 YSZ substrate. b XRR measurement performed for the calibration 680 C down to 0.001 S cm at about 400 C, with of the deposition rate of the film. c RHEED patterns of the substrate activation energy of about 0.70 eV. This result is in very and of the film recorded along the in-plane (100) orientation of the good agreement with the literature data relative to the bulk substrate conductivity of doped CeO films [6]. The electrical characterization of the films grown on STO and YSZ single film orientation [15, 16]. Nevertheless, both orientations crystal wafers cannot give reliable results due to the con- were obtained on both surfaces, as well as films showing ductive properties of the deposition substrates at high mixed (001)/(111) orientation, depending on process temperatures. parameters and deposition technique [17, 18]. Using PLD, the fabrication of highly (111)-oriented CeO films on C-cut The growth mechanism of YSZ epitaxial thin films sapphire has been reported for example in [6]. Figure 3 shows that preferentially (111)-oriented SDC The growth of YSZ films was studied using (110)-oriented films were grown with a minor (001) orientation. We YAO and (001)-oriented MgO single crystal substrates. observed that, in our experimental condition, (111)-ori- YSZ has a cubic fluorite structure with lattice parameter of ented films were obtained at relatively low oxygen partial about 5.14 A, which results in a lattice misfit of about 1.5 % pressure (in the order of 0.1–0.5 Pa), while for larger val- with (110)-oriented YAO (Table 1). Figure 4a shows the ues of oxygen partial pressure (few Pa), the films showed out-of-plane XRD analysis of a YSZ film grown on YAO. well-defined mixed (001)/(111) orientation. The presence of well-defined size effect interference fringes The RHEED patterns acquired during the growth at low around the (002) reflection line of the film (Fig. 4b) sug- oxygen partial pressure (inset in Fig. 3) showed the typical gests a very good crystallographic quality. The black curve features that characterize a quasi-2D layer-by-layer growth in Fig. 4b shows a simulation for a 35 unit cells thick YSZ mechanism. Such streaky features were never observed for film (about 180 A). From this simulation, we could estimate CeO or SDC film grown onto STO or YSZ substrates -1 a deposition rate of about 0.059 A shot . where the growth was along the (001) axis. Opposite to what observed in the case of (001)-oriented To summarize, these measurements showed that doped ceria-based films, the RHEED patterns of (001)-oriented or undoped ceria presents two different growth mecha- YSZ films clearly showed a different growth mechanism. nisms depending on the growth directions; the film grows Figure 3b shows the RHEED patterns of the substrate predominantly quasi-2D along the (111) direction, while an and of the YSZ film acquired toward the (100) in-plane evident 3D growth mechanism was observed along the direction of YAO. The RHEED patterns consisted of (001) direction. 123 Mater Renew Sustain Energy (2013) 2:6 Page 5 of 9 Fig. 5 a XRD analysis of a YSZ film grown on (001)-oriented MgO substrate. The inset shows the XRR measurement performed for the calibration of the deposition rate of the film. b RHEED patterns of the substrate and of the film recorded along the in-plane (100) orientation of the substrate thickness of about 20 A. However, even in the case of a particularly unfavourable crystalline matching with the Fig. 4 a XRD analysis of a YSZ film grown on (110)-oriented YAO. deposition substrate, YSZ showed a clear tendency to a 2D b XRD plot of the (002) YSZ reflection with superimposed simulation growth toward the (001) direction driven by the deposition (black curve) of the size effect interference fringes for a YSZ film thickness of 35 unit cells. c RHEED patterns of the substrate and the substrate. YSZ film recorded along the in-plane (100) orientation of the The electrical conductivity of a YSZ film grown on MgO substrate was measured in air by EIS and the conductivity was found -3 -5 -1 to range from 9.6 9 10 down to 8.5 9 10 Scm at well-defined streaks revealing a 2D layer-by-layer growth 700 and 400 C, respectively, showing an activation energy mechanism. of about 0.98 eV. The measured conductivity was in Very similar result was obtained analysing a 300 A thick excellent agreement with the conductivity of YSZ single film of YSZ grown on (001)-oriented MgO substrate. In crystals [19]. spite of a lattice misfit as large as -18 %, a highly textured growth was observed by XRD analysis (Fig. 5). A relatively Calculation of the low index surface energies large value of 0.64 was found for the FWHM of the (002) x-scan of the film. Figure 5a shows the XRR plot used to The comparison between the results obtained with YSZ measure the deposition rate of the film with the selected and those obtained with doped and undoped ceria films -1 deposition parameters. A value of about 0.053 A shot strongly suggests a significant difference in the thermo- was estimated in very good agreement with the value dynamic stability of the (001) surfaces of the two materials. obtained from the simulation of the size effect interference To understand the driving mechanism for the different fringes showed in Fig. 4b. The final RHEED pattern behaviours experimentally observed, a theoretical evalua- showed the typical features associated with a 2D layer-by- tion of the surface energy of the low-index surface orien- layer growth (Fig. 5b). However, in this case, the RHEED tation of CeO and cubic zirconia (c-ZrO ) was computed 2 2 pattern disappeared during the deposition of the first layers, using first-principles density functional theory (DFT). suggesting the formation of a disordered interface probably The low-index surface energies of CeO nano-particles characterized by a large density of misfit dislocations have been analysed in a previous work and the stable sur- introduced to release the excess interfacial strain [8]. faces were identified as a function of the temperature, the According to the measured deposition rate, a streaky chemical potential and the oxygen background pressure RHEED pattern originated from the surface of the YSZ [20]. It was found that unless the chemical potential of the film growing on MgO was not observed below a minimum oxygen is extremely low (i.e. very high temperature and very 123 Page 6 of 9 Mater Renew Sustain Energy (2013) 2:6 low oxygen partial pressure), the (111) surface orientation is Table 2 Low index surface Gibbs free energy of ceria and cubic zirconia stable if compared with the (100) and (110). By minimiza- tion of the Gibbs free energy as indicated by the Gibbs– Material Surface Surface Gibbs Surface energy References Wulff theorem, it was possible then to calculate the equi- index free energy (eV) difference (eV) librium crystal shape of a CeO nano-particles and show that CeO (001) 0.230 0.193 [20] under oxygen-rich conditions, the (111) surface is the only (111) 0.037 orientation present in the particle and the resulting mor- c-ZrO (001) 0.189 0.116 This work phology is described as the octahedron shown in Fig. 6a. (111) 0.073 For this work, an analogous analysis has been carried out to calculate the low-index surface energies of c-ZrO nano-particles. As previously mentioned, the cubic fluorite minimize the surface energy. In the case of highly textured zirconia is stable only at very high temperatures. In YSZ, thin films on single crystal substrates, the epitaxial stress the effect of the dopant (Y O ), besides creating oxygen can modify the extension of the different facets present in 2 3 vacancies allowing oxygen-ion diffusion, is to stabilize the the stress-free equilibrium shape, but it cannot create new cubic structure at lower temperatures. facets [22]. In this sense, we can say that the analysis Although YSZ is the material under investigation in this proposed here for nano-particles may give an indication of work, for the calculation of the low-index surface energies, the local preferential surface orientation of a thin film at we only considered the cubic undoped crystal, since it has thermodynamic equilibrium. been shown that the trend of the low-index surface relative In both cases (CeO and c-ZrO ), the (111) surface is 2 2 stability for c-ZrO and YSZ does not change [21]. stable if compared with other low-index orientations. Even Therefore, the computed surface energies obtained con- though the growth is driven toward the (001) direction by sidering undoped cubic zirconia are expected to bring, at crystalline constrains, the (111) facet is strongly favoured least qualitatively, to the same conclusions. for CeO , which will arrange the lattice in order to expose The surface energies of (111) and (001) surfaces of the (111) surface. As a result, the favoured (001) surface c-ZrO nano-particles under oxygen-rich condition were will consist on a mosaic of (111)-facet octahedra (001)- calculated. Table 2 compares the low-index surface ener- oriented, thus enhancing the surface roughness. On the gies calculated for CeO and c-ZrO nano-particles, other hand, c-ZrO , having a smaller difference between 2 2 2 showing that both crystalline structures favour the (111) the (001) and (111) surface energies will expose a mixture surface that strongly minimize the Gibbs free energy, but of these two orientations originating smoother surfaces. the difference between the (001) and (111) surface energies The growth mechanism of ceria- and zirconia-based in the case of c-ZrO turned out to be about a factor of two smaller than in the case of CeO . superlattices The equilibrium crystal shape of c-ZrO nano-particles, calculated according to the Wulff construction by mini- Multilayered hetero-structures and superlattices made by mizing the surface energy, can be described as a truncated coupling SDC or YSZ with an insulating phase [9, 11, 12] octahedron showing a mixture of (001) and (111) surfaces have been fabricated to study the conducting properties of (Fig. 6b). the interfaces [23]. To the best of our knowledge, for these The Wulff theorem states that for a given volume, the hetero-structures, high crystalline quality (biaxial texture, crystal exposes different surface orientations in order to no grain boundary) has been achieved only for (001)- oriented YSZ–STO multilayers grown by PLD on STO substrates [24], and for (001)-oriented CeO –YSZ hetero- structures grown on MgO [25, 26]. Highly (111)-oriented hetero-structures of Gd-doped ceria and Gd-doped zirconia, with minor phases from other orientations, were obtained on C-cut sapphire by molecular beam epitaxy [10]. In this case, the authors reported the observation of a streaky RHEED pattern up to a film thickness of about 40 nm with additional weak features corresponding to polycrystalline structure with increasing the thickness of the deposition. The literature does not report any detailed study of the Fig. 6 Morphology of the equilibrium crystal shape nano-particles of growth mechanism of these hetero-structures, particularly CeO (a) and c-ZrO (b) obtained by the minimization of the Gibbs 2 2 in the case of the (001) orientation. free energy, as indicated by the Gibbs-Wulff theorem 123 Mater Renew Sustain Energy (2013) 2:6 Page 7 of 9 Fig. 8 a XRD analysis of a CeO /YSZ superlattice grown on (001)- oriented MgO substrate using a 5 nm thick STO buffer layer. b RHEED patterns acquired during the growth of the superlattice Fig. 7 a XRD analysis of an SDC/YSZ superlattice grown on (001)- along the (100) in-plane orientation of the MgO substrate oriented STO substrate. b RHEED patterns acquired during the growth of the superlattice along the (100) in-plane orientation of the STO substrate thickness of each individual layer was about 15 unit cells. The very well-defined superlattice features allowed iden- tifying the diffraction satellite peaks up to the third order. For this work, we fabricated superlattices made of SDC For the fabrication of this sample, MgO was selected as and YSZ on (001)-oriented STO substrates and superlattices deposition substrate due to its good insulating properties at made of CeO and YSZ on (001)-oriented MgO substrates. high temperatures that allow the electrical characterization The growth of each layer was monitored by RHEED. along the planar direction of the hetero-interfaces. The The SDC/YSZ superlattice consisted of 30 bilayers of STO buffer layer provides the suitable lattice matching for the two materials with single layer thickness of about 30 the epitaxial growth [5]. unit cells, as estimated from the deposition rate measured An almost ideal 2D layer-by-layer growth with cube-on- by XRR. Figure 7a shows the XRD analysis of such a cube symmetry was observed for the STO buffer layer on superlattice. The superlattice was epitaxially (001)-oriented MgO. Also in this case, a regular array of spots charac- with the STO substrate and the XRD plot showed the terized the RHEED patterns of all the CeO layers, while typical superlattice peaks, as shown in the inset in Fig. 7a. streaky patterns were observed for all the YSZ layers. The RHEED patterns of several SDC/YSZ bilayers were Both Fig. 7 (SDC–YSZ on STO) and Fig. 8 (CeO –YSZ recorded during the growth of the superlattice. Figure 7b on STO-buffered MgO) show a mechanism of 2D surface shows the electron reflection patterns relative to the first reconstruction of the ceria layers determined by the YSZ layer and the 30th final bilayers. RHEED diagnostic revealed growing above. A similar mechanism of roughness suppres- that the two materials, SDC and YSZ, showed the two sion induced by the deposition of a second layer was already different growth mechanisms described above along this observed during the growth of superlattice structures [27]. crystallographic orientation, i.e. a 3D growth for SDC and a quasi-2D growth for YSZ. Figure 8 shows the XRD plot (Fig. 8a) and the RHEED Conclusions patterns (Fig. 8b) of the CeO /YSZ superlattice that con- sisted of 15 CeO /YSZ bilayers grown on MgO using a thin Thin films of CeO , 15 % Sm-doped CeO (SDC), as well 2 2 2 buffer layer of STO, about 5 nm thick. According to the as 8 mol% yttria-doped zirconia (YSZ) were grown on XRR measurement of the deposition rate, the expected different single crystalline substrates. RHEED diagnostic 123 Page 8 of 9 Mater Renew Sustain Energy (2013) 2:6 3. Chen, L.B.: Yttria-stabilized zirconia thermal barrier coatings—a allowed identifying two distinct growth mechanisms for review. Surf. Rev. Lett. 13, 535–544 (2006) the ceria films along the (001) and (111) growth directions. 4. Trovarelli, A.: Catalysis by ceria and related materials. In: Cat- A 3D growth mechanism, associated with a more pro- alytic science series, vol. 2. World Scientific Publishing Co, nounced surface roughness, clearly characterized the Singapore (2002) 5. Sanna, S., Esposito, V., Pergolesi, D., Orsini, A., Tebano, A., growth of the (001)-oriented surfaces, while a quasi-2D Licoccia, S., Balestrino, G., Traversa, E.: Fabrication and elec- growth was observed for the (111)-oriented surfaces. The trochemical properties of epitaxial samarium-doped ceria films 3D (001)-oriented growth of CeO and SDC was observed on SrTiO -buffered MgO substrates. Adv. Funct. Mater. 19, over a relatively wide range of deposition temperatures, 1713–1719 (2009) 6. Goebel, M.C., Gregori, G., Guo, X., Maier, J.: Boundary effects laser energy densities, oxygen background pressures, and on the electrical conductivity of pure and doped cerium oxide thin deposition rates, suggesting that this growth mechanism is films. Phys. Chem. Chem. Phys. 12, 14351–14361 (2010) intrinsically related with the physicochemical properties of 7. Goebel, M.C., Gregori, G., Maier, J.: Mixed conductivity in this material. On the contrary, YSZ films showed an almost nanocrystalline highly acceptor doped cerium oxide thin films under oxidizing conditions. Phys. Chem. Chem. Phys. 13, 10940– ideal 2D layer-by-layer growth along the (001) orientation, 10945 (2011) even in the presence of relatively unfavourable crystalline 8. Sillassen, M., Eklund, P., Pryds, N., Johnson, E., Helmersson, U., matching with the substrate, such as in the case of films Bøttiger, J.: Low-temperature superionic conductivity in strained grown on MgO. yttria-stabilized zirconia. Adv. Funct. Mater. 20, 2071–2076 (2010) The two distinct growth mechanisms along the (001) 9. Korte, C., Peters, A., Janek, J., Hesse, D., Zakharov, N.: Ionic orientation were, to the best of our knowledge, for the first conductivity and activation energy for oxygen ion transport time correlated with the low index surface Gibbs free in superlattices—the semicoherent multilayer system YSZ energies of the two materials, computed by first-principles (ZrO ? 9.5 mol% Y O )/Y O . Phys. Chem. Chem. Phys. 10, 2 2 3 2 3 4623–4635 (2008) DFT. Our calculation showed that both crystalline struc- 10. Azad, S., Marina, O.A., Wang, C.M., Saraf, L., Shutthanandan, tures favour the (111) surface that strongly minimizes the V., McCready, D.E., El-Azab, A., Jaffe, J.E., Engelhard, M.H., Gibbs free energy, but the difference between the (001) and Peden, C.H.F., Thevuthasan, S.: Nanoscale effects on ion con- (111) surface energies of CeO is twice that of cubic ZrO . 2 2 ductance of layer-by-layer structures of gadolinia-doped ceria and zirconia. Appl. Phys. Lett. 86, 1319061 (2005) The enhanced roughness of the (001)-oriented doped or 11. Korte, C., Schichtel, N., Hesse, D., Janek, J.: Influence of inter- undoped CeO surfaces should be taken into account in face structure on mass transport in phase boundaries between particular as far as the growth of highly textured multi- different ionic materials. Monatsh. Chem. 140, 1069–1080 (2009) layered hetero-structures is concerned. In this work, we 12. Schichtel, N., Korte, C., Hesse, D., Zakharov, N., Butz, B., Gerthsenc, D., Janek, J.: On the influence of strain on ion have shown that the ceria-based layers preserve their transport: microstructure and ionic conductivity of nanoscale characteristic 3D growth mechanism also when comprised YSZ/Sc2O3 multilayers. Phys. Chem. Chem. Phys. 12, 14596– in super-structures with YSZ leading to potentially signif- 14608 (2010) icant effects on the interface morphology. Therefore, 13. Wang, C.M., Thevuthasan, S., Peden, C.H.F.: Interface structure of an epitaxial cubic ceria film on cubic zirconia. J. Am. Ceram. especially for CeO and SDC, much better surface (and Soc. 86, 363–365 (2003) interface) quality might be achieved by developing the 14. Ikegawa, S., Motoi, Y.: Growth of CeO thin films by metal- expertise needed to obtain coherent film epitaxy and biaxial organic molecular beam epitaxy. Thin Solid Films 281, 60–63 texture along less common crystallographic directions, for (1996) 15. Zaitsev, A.G., Ockenfuss, G., Guggi, D., Wo ¨ rdenweber, R., instance on pseudo-cubic (110)- or (111)-oriented surfaces. Kru ¨ ger, U.: Structural perfection of (001) CeO thin films on (1102) sapphire. J. Appl. Phys. 81, 3069–3072 (1997) Acknowledgments This work was partly supported by the World 16. Bera, D., Kuchibhatla, S.V.N.T., Azad, S., Saraf, L., Wang, C.M., Premier International Research Centre Initiative of MEXT, Japan. Shutthanandan, V., Nachimuthu, P., McCready, D.E., Engelhard, M.H., Marina, O.A., Baer, D.R., Seal, S., Thevuthasan, S.: Open Access This article is distributed under the terms of the Growth and characterization of highly oriented gadolinia-doped Creative Commons Attribution License which permits any use, dis- ceria (111) thin films on zirconia (111)/sapphire (0001) sub- tribution, and reproduction in any medium, provided the original strates. Thin Solid Films 516, 6088–6094 (2008) author(s) and the source are credited. 17. Kurian, J., Naito, M.: Growth of epitaxial CeO thin films on r-cut sapphire by molecular beam epitaxy. Physica C 492, 31–37 (2004) 18. Nandasiri, M.I., Nachimuthu, P., Varga, T., Shutthanandan, V., References Jiang, W., Kuchibhatla, S.V.N.T., Thevuthasan, S., Seal, S., Kayani, A.N.: Influence of growth rate on the epitaxial orienta- 1. Esposito, V., Traversa, E.: Design of electroceramics for solid tion and crystalline quality of CeO thin films grown on Al2O3 oxides fuel cell applications: playing with ceria. J. Am. Ceram. (0001). J. Appl. Phys. 109, 013525 (2011) Soc. 91, 1037–1051 (2008) 19. Manning, P.S., Sirman, J.D., De Souza, R.A., Kilner, J.A.: The 2. Wachsman, E.D., Lee, K.Y.: Lowering the temperature of solid kinetics of oxygen transport in 9.5 mol % yttria stabilized zir- oxide fuel cells. Science 334, 935–939 (2011) conia. Solid State Ionics 100, 1–10 (1997) 123 Mater Renew Sustain Energy (2013) 2:6 Page 9 of 9 20. Fronzi, M., Soon, A., Delley, B., Traversa, E., Stampfl, C.: Sta- ionic conductivity at interfaces of epitaxial ZrO :Y O /SrTiO 2 2 3 3 bility and morphology of cerium oxide surfaces in an oxidizing heterostructures. Science 321, 676–680 (2008) environment: a first-principles investigation. J. Chem. Phys. 131, 25. Pergolesi, D., Tebano, A., Fabbri, E., Balestrino, G., Licoccia, S., 104701 (2009) Traversa, E.: Pulsed lased deposition of superlattices based on 21. Xia, X., Oldman, R., Catlow, R.: Computational modeling study ceria and zirconia. ECS Trans. 35, 1125–1130 (2010) of bulk and surface of yttria-stabilized cubic zirconia. Chem. 26. Pergolesi, D., Fabbri, E., Cook, S.N., Roddatis, V., Traversa, E., Mater. 21, 3576–3585 (2009) Kilner, J.A.: Tensile lattice distortion does not affect oxygen 22. Muller, P., Kern, R.: Equilibrium nano-shape change induced by transport in yttria-stabilized zirconia-CeO heterointerfaces. ACS epitaxial stress: effect of surface stress. Surf. Sci. 457, 229–253 Nano (2012). doi:10.1021/nn302812m (2000) 27. Tebano, A., Balestrino, G., Lavanga, S., Martellucci, S., Meda- 23. Fabbri, E., Pergolesi, D., Traversa, E.: Ionic conductivity in oxide glia, P.G., Paoletti, A., Pasquini, G., Petrocelli, G., Tucciarone, heterostructures: the role of interfaces. Sci. Technol. Adv. Mater. A.: Reflection high-energy electron diffraction oscillations during 11, 054503–054512 (2010) epitaxial growth of artificially layered films of (BaCuO )m/ 24. Garcia-Barriocanal, J., Rivera-Calzada, A., Varela, M., Sefrioui, (CaCuO )n. Physica C 355, 335–340 (2001) Z., Iborra, E., Leon, C., Pennycook, S.J., Santamaria, J.: Colossal

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