Microstructure and dielectric properties with CuO additions to liquid phase sintered BaTiO 3 thin films
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The refractory nature of BaTiO3 leads to limited densification and grain growth for films processed at low temperatures and a modest nonlinear dielectric response due to a marked sensitivity to physical scale and material quality. Adding liquid-forming sintering aids, common in bulk ceramics, to thin films enhances mass transport, leading to enhanced grain growth at lower temperatures. This work explores the effectiveness of a sputtered CuO buffer layer with BaO–B2O3 (BBO) fluxes to engineer the microstructure of BaTiO3 films. Grain size and homogeneity increase in the presence of even a ;1 nm CuO layer. In general, grain size increases from 75 to 370 nm with an addition of 2.2% BBO and 8 nm CuO. Room temperature capacitance in fluxed films increases by a factor of 5 over pure films, and ferroelectric phase transitions are clearly observable in dielectric measurements. CuO–BBO proves effective on (0001) Al2O3 and (100) MgO substrates, although all microstructures are notably finer for the latter. I. INTRODUCTION
The functional properties of oxides in ferroelectric, ferromagnetic, optic, and other electronic devices promise enhanced performance and novel applications. However, large challenges exist when integrating certain oxide materials with thin film systems as a consequence of restricting thermal budgets to minimize chemical reactions and maximize substrate and electrode compatibility. Due to the sensitivity of BaTiO3 to processing temperature,1 carbon removal,1,2 strain,3,4 and physical scale,1,2,5,6 this class of materials serves as a model system for understanding how refractory thin film oxide processing can be improved. Adding sintering aides to multilayer ceramic capacitors reduces firing temperatures to levels compatible with alloyed or base-metal electrodes.7 Liquid forming fluxes encourage densification and grain growth at lower temperatures for BaTiO3-based materials and other refractory complex oxides.8 Recently, liquid-phase work was extended to epitaxial films9 and polycrystalline Ba1x(Srx) TiO3 films10–12 grown using chemical solution deposition, radio frequency (RF)-magnetron sputtering, and pulsed laser deposition. Densification and grain growth occurs at significantly lower temperatures, improving the dielectric and ferroelectric properties of these complex oxide systems without increasing the thermal budget. However, for BaTiO3-thin films, the final average grain Contributing Editor: Ian M. Reaney a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.89 1018
J. Mater. Res., Vol. 31, No. 8, Apr 28, 2016
http://journals.cambridge.org
Downloaded: 10 May 2016
size is limited to around 300 nm, with further flux additions leading to deteriorated properties or abnormal grain growth and heterogeneous microstructures. Burn13 and Hennings14,15 first recognized combinations of copper oxide and other modifiers as effective sintering aides for BaTiO3. Both authors reported densification by 1100 °C, but explorations of the dielectric properties were initially limited. Later studies i
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