Dmitry Chezganov (Antwerp / BE), Nicolas Gauquelin (Antwerp / BE), Minh Than Do Nguyen (Enschede / NL), Gertjan Koster (Enschede / NL), Evert Houwman (Enschede / NL), Guus Rijnders (Enschede / NL), Sandra van Aert (Antwerp / BE), Johan Verbeeck (Antwerp / BE)
Abstract text (incl. figure legends and references)
Ferroelectric materials have been used in many applications such as non-volatile ferroelectric random access memories and micro-electromechanical systems. However, the suppression of the polarization in ferroelectric films under repeated electric field cycling (ferroelectric fatigue), leads to rapid degradation and reduced performance of these devices. Recently, a model has been proposed to clarify the causes and mechanisms, in which the Pb(Zr,Ti)O3 (PZT) ferroelectric capacitor fatigue was discussed as an interface-related effect [1]. However, there is little experimental information about the relation between the interface features and the fatigue behaviour as well as the modification of the interface during fatigue treatments.
S(TEM) is a powerful technique to study ferroelectric material with the highest spatial resolution. In-situ biasing in S(TEM) has been exploited successfully to trace domain and crystal structure evolution, phase transition, etc. So far only limited STEM investigation has been performed on fatigue mechanisms in ferroelectric devices.
Here we report an in situ S(TEM) study of ferroelectric capacitor fatigue-related phenomena under bipolar electric field cycling.
The samples were Pt/PbZr0.52Ti0.48O3/SrRuO3 (Pt/PZT/SRO) capacitors grown by pulsed laser deposition. The epitaxial PZT film was 250 nm in thickness and samples were prepared by FIB milling and placed on a DENS-solution In Situ TEM biasing MEMS-chip. HAADF-STEM was used for atomic resolution imaging of the interface degradation. DF-TEM in two-beam condition was utilized to track the domain distribution with a Gatan K2 IS direct electron detector. EDX spectroscopy was applied for element distribution mapping. The measurements were performed on Thermo Fisher Titan instruments operated at 300 kV. A function generator was employed for the application of up to 109 fatigue cycles of bipolar triangular voltage pulses.
A time series of HAADF-STEM images of the Pt/PZT interface were acquired after a different number of cycles. The images were aligned and stacked to trace the changes (Fig. 1a). The data demonstrates the metal/ferroelectric interface structural and elemental modification during cycling, resulting in a non-ferroelectric degraded layer [1] (Fig. 1).
We have analyzed the domain structure evolution during "slow" switching before and after fatigue by tracking image similarity relative to voltage amplitude. The sample's fatigue resulted in the appearance of partially non-switchable areas and increased threshold fields.
Fig. 1 (a) The evolution of metal/ferroelectric interface upon electric field cycling, (b) EDX elemental profile across the interface.
Our results enable studies of dynamic processes in ferroelectric thin films under an applied bias voltage and provide a path to identify the causes and clarify the physical mechanisms of polarization fatigue.
References
[1]. M. T. Do, N. Gauquelin, M. D. Nguyen et al., APL Mater. 9, 021113 (2021).
[2] The authors acknowledge the financial support from a TOP/BOF project from the University of Antwerp. This project has received funding from the European Union"s Horizon 2020 Research Infrastructure - Integrating Activities for Advanced Communities under grant agreement No 823717 – ESTEEM3.