Full Length ArticleOrdered misfit dislocations in epitaxial Gd doped CeO2 thin films deposited on (001)YSZ single crystal substrates
Introduction
It is generally accepted that strain plays an important role in determining the physical properties of epitaxial thin films. It has been shown that the functionality of these systems may be fine-tuned by means of strain engineering, see for example [1] and references therein. Beside the elastic strain, present in a commensurate lattice growth mode in large lattice mismatch heteroepitaxial multilayers, an additional strain component arises around the misfit dislocation cores. The latter are created to relieve the large elastic strain build-up during film growth. Recently, the critical role played by misfit dislocations has been reported in various oxide epitaxial thin films [2], [3], [4]. Also, point defects, such as oxygen vacancies play an important role in determining the structural properties of oxides [5]. Within this context, a complete description of lattice distortions is needed to fully understand and, eventually, control the properties of a heteroepitaxial system. Transmission electron microscopy techniques such as, geometric phase analysis [6], [7], peak pairs algorithm [8] and others [9], are excellent tools for providing a detailed quantitative description of strain fields, either elastic or originating around defects, such as misfit dislocations. However, TEM techniques provide information that is spatially very localized, while also being a destructive investigation method. X-ray diffraction experiments, one the other hand are non-destructive and offer a global sample investigation. In this field, several methods have been also developed that describe defect-induced strain [10], [11], [12], [13], [14].
In this paper we present a detailed investigation of the interface between epitaxial gadolinium doped ceria (Gd:CeO2) films deposited on (001) yttria stabilized zirconia (YSZ) single crystal substrates. In particular, we focus on the analysis of misfit dislocations located at the film/substrate interface. Using transmission electron microscopy (TEM) we determine the characteristics of the dislocations, in terms of Burgers vector and orientation, and the associated strain fields, using geometric phase analysis (GPA). We further demonstrate, by means of X-ray diffraction (XRD) experiments combined with Monte Carlo simulations, that the dislocations are periodically distributed at the interface and we determine the complete dislocation spacing distribution. Up to date, to the best of our knowledge, this is the first time that the employed X-ray diffraction model [12], [13] has been successfully applied in oxide heteroepitaxial systems. This model has so far been applied only to semiconducting systems [15], [16].
Ceria and ceria-based materials, are currently being considered as possible candidates for several applications such as, epitaxial buffer layers in long length high temperature superconducting coated conductor architecture [17], high level waste immobilization ceramics associated with nuclear power stations [18] and electrolyte layers in intermediate temperature solid oxide fuel cells (IT-SOFC) [19], [20], [21], [22], [23]. As is the case with other perovskite oxides, CeO2 thin films have properties that are strongly correlated with their respective strain state. For example, the ionic conductivity, which has a crucial importance in IT-SOFC applications, is affected by strain. Sun et al. [5] have shown that misfit dislocations slow down the oxide ion conduction in Gd doped CeO2 epitaxial thin films, by the segregation of charged defects, driven in part, by the elastic strain of the dislocations. The detailed knowledge of the strain state, including strain created by dislocations, of ceria thin films is needed for one to be able to understand and optimize its properties. Hence, our work is motivated on one hand by the novelty of the method used for X-ray data interpretation in the case of oxide systems, and on the other hand by the information we are able to extract in particular about the misfit dislocations present in Gd:CeO2 epitaxial thin films, which is one of the most promising materials for electrolyte layer fabrication in IT-SOFC.
Section snippets
Experimental details
The Gd:CeO2 films were elaborated using the polymer assisted deposition (PAD) method. PAD deposited thin films have shown to exhibit suitable morphologic and structural characteristics for buffer layers in superconducting architectures [24].The details for preparing the PAD solution are presented elsewhere [25]. After spin coating of the precursor solution onto YSZ (001) single crystal substrates, at 4000 rpm for 1 min, the as deposited film was pyrolyzed at 600 °C in a nitrogen atmosphere for 1 h.
TEM analysis
The cross sectional ADF-STEM image, Fig. 1b, reveals a CeO2 film thickness of approximately 25 nm on top of the YSZ. Thickness variations originate from the presence of pores on the film surface. At the film/substrate interface we observe extended regions of dark/bright contrast, periodically spaced by about 5–6 nm. Due to the lattice mismatch between CeO2 (aCeO2 = 0.5411 nm) and YSZ (aYSZ = 0.5145 nm), which corresponds to a misfit strain δ = (aYSZ − aCeO2)/aCeO2 = −4.9%, it has been observed that periodic
Conclusions
In the present paper we investigated Gd:CeO2/YSZ heteroepitaxial interface. Transmission electron microscopy analyses revealed that dislocation arrays found at the film/substrate interface are the predominant source of strain in the films. The orientation of the dislocations and their Burgers vector have been determined from TEM observations and, using the geometric phase analysis we mapped the corresponding strain components and compared their characteristics to the isotropic strain model
Acknowledgements
The authors gratefully acknowledge Prof. G. van Tendeloo for discussions and corrections. This work was performed within the framework of the Eurotapes project (FP7-NMP.2011.2.2-1 Grant no. 280432), funded by the European Union.
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