Pyroelectric Properties of Ferroelectric Thin Films: Effect of Internal Stresses
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Pyroelectric Properties of Ferroelectric Thin Films: Effect of Internal Stresses A. Sharma, Z. –G. Ban, and S. P. Alpay Department of Metallurgy and Materials Engineering and Institute of Materials Science University of Connecticut, Storrs, CT 06269 ABSTRACT A thermodynamic model is employed to analyze the effect of internal stresses on the pyroelectric response of ferroelectric thin films. The pyroelectric coefficient as a function of the misfit strain is calculated for (001) Ba0.6Sr0.4TiO3 epitaxial thin films by taking into account formation of misfit dislocations that relieve epitaxial stresses during deposition. It is shown that the pyroelectric response is highly dependent on the misfit strain in epitaxial thin films. Enhanced pyroelectric coefficient can be achieved by adjusting the misfit strain via substrate selection and film thickness especially in the vicinity of the ferroelectric to paraelectric phase transformation. INTRODUCTION One of the current interests in ferroelectric thin films is their pyroelectric property, which can be applied in focal plane arrays to obtain low cost, sensitive and fast responding infrared (IR) detectors. Thin ferroelectric elements are integrated on silicon substrate to form arrays in integrated circuits (ICs) [1]. Near the Curie temperature, the pyroelectric coefficient of bulk ferroelectrics is typically above 1 µC/cm2K, however, the response of ferroelectric thin film is in the range of 0.03-0.3 µC/cm2K [2] thus limiting the performance of the IR detectors. Electrical and electromechanical properties of various perosvkite-ferroelectrics in thin film form have been reported to diverge from those of bulk single crystals [3, 4]. It was also shown that, whereas the tensile stresses degrade the out-of-plane polarization, the compressive stresses in fact, enhance the out of-plane polarization, which enables the related properties to be manipulated. In this paper we show that the degradation in pyroelectric response is primarily due to the internal stresses, which arise as a result of lattice and thermal expansion mismatch between the film and the substrate. In particular, pyroelectric response of Ba0.6Sr0.4TiO3 (BST 60/40) thin films, which has a paraelectric↔ferroelectric transition temperature close to room temperature, is discussed in the presence of aforementioned internal stresses. Our goal is to provide a quantitative estimation of the pyroelectric response for films on different substrates as a function of the film thickness, thereby serving as a guide for the selection of film and substrate materials for future experimental work. THEORY
~ Based on a modified Landau potential, the thermodynamic potential G , of a (001) thin film deposited on a (001) thick substrate can be expressed in terms of the polarization Pi, the applied field Ei, and the misfit strain um=(aS-a0)/aS, where aS is the substrate lattice parameter and a0 is the cubic cell constant of the free standing film, and is given by [5]:
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~ * * G = a1* ( P12 + P22 ) + a3* P32 + a11* ( P14 + P2
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