Strain Relaxation and Dislocation Confinement in Epitaxial SrTiO 3 by Two-step Growth Technique and the Resulting Dielec
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Strain Relaxation and Dislocation Confinement in Epitaxial SrTiO3 by Two-step Growth Technique and the Resulting Dielectric Response Tomoaki Yamada, Vladimir O. Sherman, Alexander K. Tagantsev, Dong Su, Paul Muralt, and Nava Setter Laboratoire de Céramique, Ecole Polytechnique Fédérale de Lausanne, EPFL, CH-1015 Lausanne, Switzerland ABSTRACT A two-step growth technique was used to achieve effective strain relaxation and dislocation confinement of epitaxial SrTiO3(STO) films and through this to improve their microwave dielectric properties. The crystallization of a very thin quasi-amorphous STO layer deposited at a low temperature in the initial growth step enhanced the strain relaxation from the lattice mismatch at the expense of the formation of high density of misfit dislocations. By varying the thickness of the first layer, different strain states of the films were systematically achieved while keeping the total film thickness unchanged. This allowed the study of the effect of strain on permittivity, and showed good agreement with theoretical predictions. Further more, the two-step growth technique suppressed significantly the threading dislocation density in the film, the dislocations being confined to the first layer. This in turn caused reduction in the extrinsic dielectric loss at microwave frequency. The loss reduction was analyzed and explained based on a dielectric composite model. INTRODUCTION Incipient ferroelectric films are suitable for microwave tunable applications [1]. For the practical operation of tunable devices, a high tunability and a low dielectric loss of the film are imperative. The tunability is basically controlled by the permittivity; a high permittivity leads to a high tunability. One of the effective ways to obtain a high permittivity at the desired operation temperature is to control the strain of the film. This is achieved by utilizing the setoff of the strain build-up due to the lattice mismatch and thermal expansion mismatch between the film and the substrate, and the strain relaxation from these mismatches by generation of dislocations. As to the dielectric loss of the ferroelectric films, the extrinsic loss contribution due to the defective structure of the film is usually much greater than the intrinsic loss contribution. Therefore, films of a high quality are needed. Although several attempts had been reported [2], the detailed effect of the defect structure on the loss behavior has not been completely clarified yet. The manipulation of dislocations is one of the key issues for achieving a high tunability and a low loss. We employed so-called two-step growth technique for the control of the dislocation formation behavior of epitaxial SrTiO3(STO) films [3,4]. The crystallization of a very thin quasiamorphous STO layer deposited in the initial growth step lead to effective strain relaxation by forming the misfit dislocations. The relaxed first layer also influenced the dislocations in the continuing upper STO film. In this technique, the thickness of the first laye
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