Prof. Alexei Gruverman
Date of this Version
Buragohain, Pratyush, "Nanoscale Studies of the Ferroelectric and Electromechanical Properties of Hafnia-based Capacitors", PhD. Dissertation (2022).
The work presented in this dissertation aims to provide nanoscopic insights into the electrical and electromechanical behavior of the recently discovered ferroelectric HfO2 or hafnia-based capacitors. Hafnia-based ferroelectrics are highly promising for technological applications due to compatibility with the existing Si technology. To realize the full potential of hafnia, however, requires comprehensive understanding of its properties. In this regard, this dissertation hopes to bridge a gap between an understanding of the nanoscopic and macroscopic properties of hafnia by performing combined high-resolution piezoresponse force microscopy (PFM) and pulse switching studies.
More specifically, the dynamics of domain nucleation and wall motion during polarization reversal in hafnia was investigated. Polarization reversal was found to occur mainly via nucleation of new domains, albeit with limited expansion and sluggish domain wall motion, following the nucleation limited switching (NLS) model at low fields. At high fields, close to the thermodynamic activation fields, a convergence of the NLS and the Kolmogorov-Avrami-Ishibashi switching models was observed, signifying a uniform domain-less polarization reversal process.
Furthermore, negative d33 was demonstrated for the first time in hafnia after careful calibration of the PFM phase signal, providing confirmation of a theoretically predicted negative d33. However, the sign was found to be strongly sample dependent. Depending on the film thickness, electrode materials, deposition method used, or state of the capacitors (pristine vs field-cycled), hafnia-based capacitors exhibited either a uniformly negative or positive d33 response or a mixture of both positive and negative d33 responses.
In addition, a unique imprint behavior was identified in hafnia that was found to strongly depend on the switching pre-history. Our measurements highlight the critical role played by injected charges and mobile charges/defects in the imprint behavior of hafnia-based devices. Finally, application of PFM spectroscopy to ZrO2-based capacitors revealed dramatically different PFM amplitude response compared to hafnia that could be attributed to the divergence of dielectric susceptibility during field-induced antiferroelectric - ferroelectric phase transitions, providing a microscopic confirmation of antiferroelectricity in ZrO2.
Adviser: Alexei Gruverman