Growth-mode induced defects in epitaxial SrTiO 3 thin films grown on single crystal LaAlO 3 by a two-step PLD process
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Tomoaki Yamada,b) Roman Gysel, Alexander K. Tagantsev, Paul Muralt,c) and Nava Setter Ceramics Laboratory, Swiss Federal Institute of Technology, Lausanne CH-1015, Switzerland
Nan Jiangd) Department of Physics, Arizona State University, Tempe, Arizona 85287-1504 (Received 6 September 2010; accepted 22 November 2010)
We grew epitaxial SrTiO3 (STO) thin films on (001) LaAlO3 substrates via a two-step procedure using pulsed laser deposition and studied them with transmission electron microscopy in plane-view and cross-sectional samples. We found that partial misfit dislocations are the main interfacial defects, whereas planar defects are the main defects in STO films. Our results suggest that a three-dimensional island mode dominates the growth of the STO film.
I. INTRODUCTION
Present address: Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973. b) Present address: Department of Innovative and Engineered Materials, Tokyo Institute of Technology, 4259-J2-43, Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan. c) This author was an editor of this journal during the review and decision stage. For JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs.org/ jmr_policy d) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.82
relax the strain induced by lattice mismatch between the film and its substrate. The two-step-grown STO film exhibited substantially higher permittivity and tunability than the conventional films grown by a single-step process, which were under compressive strain. We interpreted these dielectric responses of both types of film by Landau’s theory, taking account of the difference in the residual strain in them.10 To further understand the mechanism whereby the two-step growth process induces substantial strain relaxation, we undertook a comprehensive study on the microstructure of the STO film, as reported herein. Generally, there are three typical modes of growth11: (i) two-dimensional (2D) layer-by-layer growth (the Frank der Merwe mode.); (ii) three-dimensional (3D) island growth (the Volmer–Weber mode); and (iii) the 2D–3D island-on-layer growth [the Stranski–Krastanov (SK) mode]. For single-step STO thin films grown by PLD under our experimental conditions, seemingly growth follows the SK mode, as indicated by the existence of dislocations and the enhanced surface roughness. In contrast, for films generated via two-step growth, the amorphous-FL is being crystallized after a few nanometers' growth, signifying that its growth would not be in accord with the SK mode. Therefore, it is interesting to consider how this change in the pattern of growth might affect the defects in the film. In this work, we perform a detailed transmission electron microscopic (TEM) investigation on the microstructures of two-step-grown STO films on (001) LAO single crystals. We found that the defects in such films differ from those in conventionally single-step-grown film. We studied the surface morphology of t
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