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J. Phys. Chem. B 2002, 106, 3778-3787
Inducing Polycrystallinity within Supported Oxide Thin Films Using Template Buffer Layers Dean C. Sayle*,† and Graeme W. Watson‡,§ Department of EnVironmental and Ordnance Systems, Cranfield UniVersity, Royal Military College of Science, ShriVenham, Swindon SN6 8LA, U.K., and Department of Chemistry, Trinity College, Dublin 2, Ireland ReceiVed: September 5, 2001
A simulated amorphization and recrystallization technique has been used to explore how a polycrystalline metal-oxide thin film can be synthesized selectively on an MgO(100) substrate. The simulations indicate that by introducing a polycrystalline SrO “buffer-layer”, sandwiched between the support and overlying thin film, polycrystalline thin films of CaO and BaO can be fabricated. Conversely, our findings suggest that without the buffer layer, neither the CaO nor the BaO exhibit polycrystallinity when supported on an MgO(100) substrate. This suggests that the polycrystalline buffer layer acts as a template in directing the structure of the overlying metal oxide resulting in polycrystalline thin films.
Introduction Many of the properties of oxide ceramics are controlled largely by structural modifications present within the oxide. These may include, for example, grain boundaries, dislocations, vacancies, interstitials, and substitutionals. For example, oxygen diffusivity in yttria-stabilized zirconia is significantly lower at the grain boundaries compared with that in the bulk,1 and doping ceria with zirconia leads to an increase in the oxygen storage capacity with important implications for the catalytic properties of the material.2,3 Clearly, the ability to exact some kind of control over the microstructure of a material provides an avenue to tailor certain desirable chemical physical or mechanical properties. Two requirements are therefore apparent. One desires, first, the ability to correlate the relationships that exist between the structural modifications with the corresponding material properties and, second, the capability to control selectively the introduction of such structural modifications within the material to optimize certain desired properties. The second goal is pehaps implicit within the first. For example, the elucidation of the relationships that exist between microstructure and properties necessitates the systematic and controlled introduction of various structural modifications together with the measured changes in material properties. Experimentally, such a procedure is very difficult. Indeed, at present, even the structural characterization at the atomic level of various structural modifications such as dislocations and grain boundaries can prove problematic. And while much effort has been directed in this area4 with considerable progress made, it has been remarked that “one reason for the relative lack of scientific output of studies of oxide surfaces is the lack of availability of high quality oxide surfaces together with the difficulties associated with the insulating nature of these materials”. However, an enabling development4 has been the fabrication of high-quality oxide thin films supported on metallic substrates using thin-film growth techniques such as molecular beam epitaxy (MBE). This circumvents both problems in that * To whom correspondence should be addressed. E-mail: sayle@ rmcs.cranfield.ac.uk. † Cranfield University. ‡ Trinity College. § E-mail:
[email protected].
high-quality oxide films can be prepared and that adequate conductivities are attainable owing to the metallic substrate. That high-quality oxides can now be prepared by supporting them on an appropriate substrate is an important step in realizing the desire to correlate microstructure with material properties. One then needs the ability to modify selectively the microstructure of the supported oxide. Various mechanisms are available including a suitable choice of substrate,4 the inclusion of buffer layers,5,6 growth method (i.e., molecular beam epitaxy, vapor deposition) and conditions (i.e., temperature, deposition rate) of preparation; the latter reflect the versatility and flexibility of the mode of experimental fabrication. However, as alluded to previously, analysis of the resulting microstructure is difficult. Accordingly, an alternative approach, and one that we employ in this present study, is that of atomistic computer simulation. Here, because the structure can be defined precisely, it offers a valuable complimentary technique to experiment. In this present study, we focus specifically on polycrystalline thin films. Previously, Phillpot et al. developed a simulation methodology that enabled the “synthesis of polycrystalline thin films” using a molecular dynamical approach7 as a preliminary step in exploring the relationships between microstructure and properties. The study proved successful in generating models for thin polycrystalline FeO films, together with a comprehensive characterization of the various structural features within the thin film at the atomistic level. Once such models have been fabricated and characterized, various chemical and physical properties can then be calculated8 and correlated with the structural modifications present within the material. The final step is then to determine whether such structural modifications (or polycrystalline structures considered in this present study) can be introduced selectively to optimize the desired material properties. In this study, we explore the possibility of exacting control over the structure of a supported thin film. In particular, we aim to extend the work of Phillpot et al. by exploring whether one can selectively synthesize a polycrystalline thin film by modifying only the structure of the substrate material. Because the structure of thin films is influenced considerably by the structure of the underlying substrate, it is pertinent to suggest that a polycrystalline (in this context, we refer to polycrystallinity
10.1021/jp0133945 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/26/2002
Inducing Polycrystallinity within Oxide Thin Films
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3779
at the nanometer scale) substrate might induce polycrystallinity within the thin film deposited thereon. In a previous study, models for a MO (M ) Ca, Ba) thin film supported on a (perfect) MgO(100) substrate were generated using a simulated amorphization and recrystallization methodology. The resulting structure for the MO thin film comprised many screw-edge dislocations resulting in low-angle domains as the MO responds structurally to the lattice misfit and interfacial interactions of the MgO substrate.9 However, neither the BaO nor the CaO thin films could be classed as polycrystalline when supported on the MgO(100) substrate. Many experimental studies that explore the fabrication of high-quality thin films on a substrate material employ a buffer layer to help control the crystallinity of the overlying thin film. For example, Lisoni et al. used a MgO buffer to facilitate the growth of high-quality BaTiO3 thin films on sapphire,10 the rationale being that BaTiO3(001) grows epitaxially on MgO(100) single crystals owing to the small (
