Elastic Deformations during Phase Transition in Freestanding BaTiO 3 Thin Films
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Elastic Deformations during Phase Transition in Freestanding BaTiO3 Thin Films. Jaya P.Nair, Natalie Stavitski, Ilya Zon, Konstantin Gartsman and Igor Lubomirsky Department of Materials and Interfaces, Weizmann Institute of Science Rehovot, 76100, Israel
ABSTRACT. Elastic deformations during phase transition in freestanding BaTiO3 thin films were investigated. BaTiO3 films were prepared by sol-gel technique or RF magnetron sputtering on silicon substrates, covered by randomly oriented 120 nm thick Al2O3. The as-deposited films were under tensile stress of 100-170 MPa and did not show neither pyroelectric nor piezoelectric properties. Partial substrate removal caused the freestanding films to expand laterally by 0.30.5% and corrugate. Dielectric constant of the freestanding films (620±10) was found to be significantly higher than that of the substrate supported films (110±20). The freestanding films showed detectable piezoelectric effect, which indicated that the lateral expansion was originated from the substrate-suppressed cubic-tetragonal phase transition. INTRODUCTION. Dielectric properties of BaTiO3 thin films are strongly affected by film-substrate interactions, among which the mechanical stress is considered to be the most influential factor. The phase diagram of a single domain ferroelectric film is strongly affected by mechanical stress, forming ferroelectric phases, which are unattainable for a bulk material [1-3]. A single domain film may transform to a polydomain one under sufficiently high misfit strain [4]. Further evolution of a polydomain ferroelectric film is determined by the reorientation of the ferroelectric domains, which presents a very efficient stress relief mechanism. Stress relaxation via domain reorientation was experimentally observed in PbZr0.2Ti0.8O3 [5, 6], but not in BaTiO3 thin films. The situation is more complex for polycrystalline films, where grain size (interface) effects may partially or completely suppress the para- to ferroelectric transition [7-9], retarding the formation and the reorientation of the ferroelectric domains. However, most of polydomain BaTiO3 films are found under significant stresses (hundreds of MPa) despite the built-in stress relaxation mechanism. This implies that the domain structure is clamped far from equilibrium. Therefore, one can expect, that the substrate removal may release the clamping and promote the phase transition and/or domains reorientation. We present the direct experimental evidence of this effect in freestanding polycrystalline BaTiO3 films, obtained by partial substrate removal. THEORY. Residual stress in BaTiO3 thin films comprises of a number of contributing stresses. Crystallization stress, σcr, develops during the deposition process and strongly depends on the deposition procedure [10-12]. Thermal stress appears due to the difference between thermal expansion coefficient of Si, αSi, and of BaTiO3, α BaTiO 3 . Since α BaTiO 3 -αSi=6·10-6 K-1>0, the thermal stress in BaTiO3 thin films, deposited on Si substrates, is tensile(σth = 510-
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