Self-selective Epitaxial Growth of BST Films: toward Nano-composite Structure for Microwave Tunable Devices
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0928-GG01-03
Self-selective Epitaxial Growth of BST Films: toward Nano-composite Structure for Microwave Tunable Devices Tomoaki Yamada, Vladimir O. Sherman, Andreas Noeth, Paul Muralt, Alexander K. Tagantsev, and Nava Setter Ceramics Laboratory, Swiss Federal Institute of Technology, EPFL, Lausanne, CH-1015, Switzerland
ABSTRACT Ba0.3Sr0.7TiO3 (BST) thin film composite structures were fabricated by means of a selective epitaxial growth process. The epitaxial growth of BST on SrRuO3 electrode surface was selectively achieved at 450~485ºC using a prepatterned ultra-thin amorphous BST layer that locally prevented crystallization. This self build-up mechanism resulted in a columnar composite structure, where epitaxial and amorphous BST columns are electrically connected in parallel. The epitaxial BST showed a high permittivity and a high tunability, whereas the amorphous BST showed a significantly lower permittivity and a very weak tunable behavior. The effective permittivity of the composite capacitors decreased linearly upon increasing the amorphous BST concentration q , while the tunability stayed fairly unchanged until q reached 70%, which agrees with the theoretical ferroelectric/dielectric parallel composite model. The results also give indirect evidence of a good separation between both BST phases. INTRODUCTION For tunable devices using ferroelectric films, a high tunability and a low dielectric loss of the films are definitely required [1, 2]. In addition, smaller capacitance of the ferroelectric capacitor is needed for impedance matching in microwave applications [3]. However, it is known that the tunability of a pure ferroelectric is a strong function of its permittivity [2]. Therefore the requirement of a high tunability and a low capacitance (low permittivity) seems contradictory in pure ferroelectrics. So far small amount of ions (such as Mn2+, Mn3+, Ni2+, Al3+, La3+ and Mg2+) has been added into the A2+B4+O2-3-type perovskite[2, 3], which often leads to a lower loss and a lower permittivity. However, it was not always possible to keep the tunability at the same level. Recently, modeling predicted that composite materials consisting of ferroelectrics with lowpermittivity dielectric inclusions can meet such requirements for two kinds of composite structures; the spherical dielectric inclusion model and the parallel dielectric inclusion model [2, 4, 5]. Particularly, in the latter case, one can expect to keep the tunability of pure ferroelectrics fairly unchanged even with a high concentration of inclusions; therefore, the effective permittivity can be significantly reduced without losing the tunability. Herein, we will experimentally demonstrate a significant reduction of the effective permittivity with keeping the tunability fairly unchanged using a parallel composite Ba0.3Sr0.7TiO3 (BST) capacitor. A selective epitaxial growth process is employed for the fabrication of the parallel composite structure consisting of epitaxially crystalline BST and
amorphous BST. The applied technique is a new way to re
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