New epitaxy paradigm in epitaxial self-assembled oxide vertically aligned nanocomposite thin films
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New epitaxy paradigm in epitaxial self-assembled oxide vertically aligned nanocomposite thin films Jijie Huang School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, USA
Judith L. MacManus-Driscoll Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 OFS, U.K.
Haiyan Wanga) School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, USA; and School of Electrical and Computer Engineering, West Lafayette, Indiana 47907, USA (Received 24 April 2017; accepted 28 June 2017)
Self-assembled oxide-based vertically aligned nanocomposite (VAN) thin films have aroused tremendous research interest in the past decade. The interest arises from the range of unique nanostructured films which can form and the multifunctionality arising from these forms. Hence, a large number of oxide VAN systems have been demonstrated and explored for enhancing specific physical properties, such as strain-enhanced ferroelectricity, tunable magnetotransport, and novel electrical/ionic transport properties. The epitaxial growth of the nanocomposite thin films and the coupling at the heterogeneous interfaces are critical considerations for future device applications. In this review, the advantages of strain coupling along vertical interfaces and film-substrate interfaces in nanocomposite films over conventional single phase films are discussed. Specifically, a unique strain compensation model enabling the epitaxial growth of two-phase nanocomposites having large lattice mismatch with substrates is proposed. Out-ofplane strain coupling between the two phases is also discussed in terms of designing strain states for desired functionalities.
I. INTRODUCTION
In standard planar oxide thin films, there has been tremendous research effort on strain-induced exotic physical properties. Novel functionalities and the development of next generation electronics, optoelectronics, and photonic devices have been widely explored.1–3 For example, multiferroics has been achieved by various oxide nanocomposite thin films, which can be used as electrically switchable magnetic memory.4–6 Such novel functionalities are strongly influenced by the defect structure in the oxide films, among which the strain state in the films play a much critical role in governing the overall material properties. Thin film strain results mainly from the lattice mismatch at heterointerfaces. Laterally strained films are generated by growing the films on particular substrates or in multilayer architectures, and strain enabled functionalities can be enhanced significantly.7–10 However, such lateral strain is limited to films within critical thicknesses which are generally on the order of few nanometers. Lateral strain is effectively Contributing Editor: Mmantsae Diale a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.281
relaxed by misfit dislocation formation as the films grow thicker beyond the critical thickness.11 In vertically aligned nanocomposite (VAN) systems, vertical strain
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