Broadband, High-Frequency Permittivity Characterization for Epitaxial <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mi>Ba</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi>Sr</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mrow><mml:mi>Ti</mml:mi><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mn>3</mml:mn></mml:msub></mml:math> Composition-Spread Thin Films
Eric J. Marksz, Aaron M. Hagerstrom, Xiaohang Zhang, Naila Al Hasan, J. C. Pearson, Jasper Drisko, James C. Booth, Christian J. Long, Ichiro Takeuchi, Nathan D. Orloff
Abstract
Next-generation millimeter-wave $(>30\phantom{\rule{0.1em}{0ex}}\mathrm{GHz})$ telecommunications electronics must be compact, energy efficient, and have good thermal management. Tunable materials may play a role in meeting these requirements for millimeter-wave front-end devices, but there are few models or even measurements of tunable dielectrics at these frequencies. Here, we report on the adaptation and development of high-frequency dielectric spectroscopy techniques for composition-spread thin films from 100 MHz to 110 GHz. Our comprehensive technique sequentially probes the composition, frequency, and electric field dependence of the complex permittivity in a combinatorial thin film library, which provides a platform to rapidly explore functional materials for emerging telecommunications electronics. This is achieved by modifying existing on-wafer transmission line permittivity measurement techniques to obtain a compact set of test devices that can be patterned to extract the complex permittivity in multiple regions of a thin film. We demonstrate this technique by applying it to composition-spread ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Ti}\mathrm{O}}_{3}$ thin films spanning compositions from $x=0$ to $x=1.$ The systematic approach to materials growth inherent in combinatorial synthesis allows for a comprehensive picture of the ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Ti}\mathrm{O}}_{3}$ system. Our continuous, quantitative measurements provide an encompassing view of the composition- and voltage-dependent trends in the room temperature dielectric properties at millimeter-wave frequencies---from strong, few-picosecond relaxations to no relaxation, and from large relative tunability (${n}_{r}>50\mathrm{%}$ at $75\phantom{\rule{0.1em}{0ex}}\mathrm{k}{\mathrm{V}}_{}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$) to zero tunability. Our work underscores both the utility of our technique, and the need to discover lower-loss, highly tunable electronic materials for next-generation telecommunications.