UC Berkeley

This dissertation presents a systematic study on idealized electrode/ferroelectric/electrode heterostructures, from film synthesis to capacitor fabrication. Single crystal BaTiO3 has shown a small switching field of ~ 1 kV cm-1 and small switching energy of ~ 0.1 J cm-3, making it an excellent candidate for next generation non-volatile ferroelectric applications. However, when BaTiO3 is incorporated into actual nanoscale devices, the ferroelectric properties get significantly worse. BaTiO3 thin films show either large coercive switching field, increasing the total power consumption, or diminished remanent polarization, putting non-volatility into question. In this dissertation, I demonstrate that by carefully controlling the O2 growth pressure in pulsed laser deposition process, we can synthesize SrRuO3/BaTiO3/SrRuO3 heterostructures with BaTiO3 ferroelectric properties comparable to bulk crystal. Thickness scaling is performed to explore the smallest coercive voltage, with BaTiO3 films with thicknesses ranging from 25-50 nm exhibiting best combinations of switching voltage (< 100 mV), switching energy (< 2 J cm−3) and remanent polarization (> 10 μC cm-2). Depolarization field is found to play an important role in suppressing the coercive field, leading to a deviation in coercive field scaling from the Janovec-Kay-Dunn law. Lateral scaling is performed to explore the fastest switching speed, with the projection of achieving sub-nanosecond switching time in capacitor sizes as small as ~ 10 μm2. Integration of SrRuO3/BaTiO3/SrRuO3 heterostructures onto silicon substrate is also shown, with similar switching behavior achieved, as well as good resistance to fatigue and retention. Besides heterostructure synthesis, different fabrication methods are also explored to bring out the best BaTiO3 capacitor performance in various ferroelectric functions. MgO hard mask process introduces surface defects at electrode/ferroelectric interface and results in a large imprint in the hysteresis loops. While the wet chemical etching method gives out idealized ferroelectric loops with almost no imprint, the high selectivity and isotropy makes it unsuitable for fabricating more complex device architectures with high precision. The ion mill methods, on the other hand, have better directionality and can be used to etch almost every type of material and fabricate advanced device structures. However, the Ar+ ion beam bombardment can result in horizontal imprints in the hysteresis loop. Transport studies and deep-level transient spectroscopies reveal the introduction of Ba-related defect types (???′′−??••, ???′′) during the milling process. Post-fabrication annealing in Ba-rich environment significantly lowers the defect levels and reduces the imprints. We also discuss what can be done to further optimize the ferroelectric properties (reducing coercive field, increasing remanent polarization, etc.) in ultrathin BaTiO3 films, for the ferroelectric material to function well for various application purposes.