Interface and domain engineering for enhanced reliability in ferroelectric-based memory
Date of Issue2016
School of Materials Science and Engineering
In a ferroelectric random access memory (FeRAM), the two logic states, “1” and “0”, are represented by the spontaneous polarization directions in the ferroelectric material, which can be altered by an external electric field. FeRAM has many advantages, such as high speed and excellent data retention, over magnetic hard disk drive and flash memory. However, one drawback of the conventional FeRAM is that the reading process may erase the stored information and a re-write step is needed. This is because the read-out in a conventional FeRAM is achieved by sending a voltage pulse larger than the coercive field to the memory cell and detecting the current. For the past decade, researchers have been exploring various concepts for non-destructive read-out of FeRAM. In 2013, we have demonstrated the feasibility to use the ferroelectric photovoltaic response as the read-out signal of FeRAM, as both the signs of open-circuit voltage (Voc) and short-circuit photocurrent (Isc) depend on the polarization direction in the ferroelectric layer. Such a photovoltaic effect-based FeRAM features a non-destructive reading process, giving rise to low energy consumption and increased lifetime of the memory. In this study, we further investigate the fundamentals of fatigue in ferroelectric materials and clarify the origin of switchable photovoltaic effect, aiming to improve the performance of the proposed novel FeRAM. Polarization fatigue, i.e. the reduction of switchable polarization after repetitive electrical cycling, poses a serious problem for the performance and the lifetime of ferroelectric-based devices. The first part of this work is to study the mechanism of the polarization fatigue in a typical ferroelectric material, BiFeO3. By using planar BiFeO3-based capacitors, we have carried out in-situ study on the domain evolution and space charge redistribution in the ferroelectric layer during fatigue measurements. It is found out that charge injection/accumulation at the electrode/film interface is responsible for domain pinning and the macroscopic polarization fatigue in BiFeO3 films. Furthermore, the Schottky barrier at the electrode/BiFeO3 interface is likely to play a crucial role in the charge injection/accumulation process by deep-trapping the injected electrons under the localized high electric field. Lowering the barrier height, with either oxides or low work function metals as the electrodes, effectively suppresses or even eliminates the electron accumulation due to the high detrapping rate, and thus improves the fatigue performances of the device. The systematic study on vertical BiFeO3-based capacitors using different top electrodes further supports the Schottky barrier-controlled charge accumulation model for polarization fatigue. With the mechanism of polarization fatigue clarified, we move on to improving the ferroelectric photovoltaic response in BFO systems. Through controlling the interface conditions, we have studied the origin of the switchable ferroelectric photovoltaic effects in BFO heterostructures and both effects from bulk depolarization field and interface have been explored. In vertical Pt/BiFeO3/La0.7Sr0.3MnO3 capacitors, the polarization modulated built-in field at the Pt/BiFeO3 interface plays the dominating role in separating the photo-excited carries and producing photovoltages, whereas the contribution from bulk depolarization field proves to be relatively small. After clarifying the origin, we have also investigated the photovoltaic property of domain engineered epitaxial BiFeO3 films. Consistent with the photovoltaic effects being driven by the built-in field at the Pt/BiFeO3 interface, the domain structures in BiFeO3 films do not affect the Voc values in the vertical heterostructures. However, the improvements of Isc can be achieved by increasing the domain wall density, which is attributed to the larger photoconductivity of the domain walls. In addition, preliminary results on the enhancements of the photovoltaic responses by chemical substitution have also been obtained, though the mechanism is still unclear.