Photovoltaic property of bismuth ferrite thin films and its application in non-volatile memory

• Main Author: Guo, Rui
• Other Authors: Wang Junling
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Materials; DRNTU::Engineering::Materials::Energy materials; DRNTU::Engineering::Materials::Microelectronics and semiconductor materials::Thin films
• Online Access: https://hdl.handle.net/10356/55058
• Institution: Nanyang Technological University

In conventional junction-based photovoltaic cells, the photovoltage is usually smaller than the semiconductor band gap, due to the limitation of the energy barrier at the interface. Contrarily, ferroelectric photovoltaic effect is a bulk effect of which the photovoltage is not limited by the energy barrier. Therefore, ferroelectric photovoltaic effect has attracted much research attention due to its wide potential applications. BiFeO3, a multiferroic material with robust ferroelectric and magnetic orders at room temperature and a band gap within visible light range, provides a unique opportunity for bulk photovoltaic effect study. Earlier work on BiFeO3 photovoltaic property attributed the effect mainly to the ferroelectric polarization. However, a very large photovoltage was reported later in BiFeO3 films with regular 71° domains. The authors proposed that the effect arises from the electrostatic potential steps at the domain walls. Moreover, BiFeO3 films with regular 109° domain walls are expected to generate a significantly larger photovoltage, since the potential step at 109° domain walls was reported to be much larger than that at 71° domain walls. This project thus aimed to study the photovoltaic effect of BiFeO3 films with regular 109° domains, in order to clarify its correlation with the domain wall. To achieve this, we set our goal to control the domain structure of BiFeO3 thin films first. In fact, the ability to control domain structure is an essential work for various research topics. Ferroelectric domain walls in BiFeO3 have been shown to possess unique properties that do not exist in bulk material. Theoretical studies have predicted that a net electric field or magnetic moment could in principle exist in the center of domain walls while the domain themselves were non-electric or non-magnetic. However, a systematic investigation requires the precise control of domain structures in BiFeO3, which is still lacking. In this project the underlying mechanism of domain engineering will be addressed. Then, the photovoltaic effect of BiFeO3 films with regular 109° domains will be studied. The correlation of the photovoltaic effect with the domain walls will be discussed. Finally, after this, we studied the application of the photovoltaic effect of BiFeO3 films in memory device. The photovoltaic effect could be used to read the polarization status of BiFeO3 films non-destructively. The properties of the memory cells were studied and compared with other technologies. Pulsed Laser Deposition technique was used to grow BiFeO3 films. By using BiFeO3 target with different Bi content or adjusting the deposition parameters, films with regular 71° to 109° domains can be obtained. The domain structure was tested by Piezoelectric Force Microscopy, and the thin film composition was measured using Electron Probe X-ray Micro-analyzer. It is revealed that decreasing Bi content in the target or increasing substrate temperature changes the domain structure of BiFeO3from 71° to 109°. Domain structure is related to the Bi vacancies in the thin film. We suggest that a combination of interface effect and defect induced internal field causes this evolution. After the successful control of domain structure in BiFeO3 thin films, the photovoltaic property of BiFeO3 thin films with 109° domains was studied. BiFeO3 thin films that contain regular 109° domains with mainly two polarization variants were prepared by using miscut DyScO3 substrates. A direct correlation between the 109° domains switching and the photovoltaic response of the films was established, by conducting piezoelectric force microscopy study using a planar device. The polarity of the photovoltage is switchable upon polarization reversal. Besides, the photovoltage was also obtained in single domain BiFeO3 thin films. The results suggest that theoretical study might have overestimated the contribution of domain walls to BiFeO3 photovoltaic effect. After the study of the photovoltaic effect of BiFeO3 thin films, we investigated the possibility of using ferroelectric photovoltaic effect in non-volatile memory. Capacitors of Fe/BiFeO3 /(La0.7,Sr0.3)MnO3 were prepared on miscut SrTiO3 substrates. Photovoltage and photocurrent with opposite signs were obtained upon switching the polarization of the capacitor. The stored polarization information in BiFeO3 could be read non-destructively by sensing the photovoltage or photocurrent. The memory performance such as write energy, operation speed, retention time and fatigue cycles compare favorably with other memories, such as hard disk drive, flash memory, magnetoresistive random access memory and resistive switching random access memory.