Fabrication Of Ferroelectric-based Heterojunction Devices And Regulation Of Physical Properties

Recent years,integrating ferroelectric and semiconductor materials into one device has been intensily studied to realize multifunctions.Actually,those integrated ferroelectric devices on heterojunction structures by fabricating ferroelectric thin film on a microelectronic/optoelectronic functional substrate,or depositing other functional films on the ferroelectric substrates.Therefore,it is critically important to understand the physical properties of ferroelectric-based heterojunction,in aim to develop new integrated ferroelectric devices while to improve their performance.Based on the recent physic progress on ferroelectric,optoelectric,semiconductor and phase transition,we designed and fabricated several ferroelectric-based heterojunction devices to enhance and manipulate their physical properties.The main contents are as follows:（1）Converting light energy to electrical energy in photovoltaic devices relies on the photogenerated electrons and holes separated by the built-in potential in semiconductors.Photo-excited electrons in metal electrodes are usually not considered in this process.In this work,we report an enhancedphotovoltaic effect in the ferroelectric lanthanum-modified lead zirconate titanate（PLZT）by using low work function metals as the electrodes.The results show that electrons in the metal with low work function could be photo-emitted into PLZT and form the dominant photocurrent in our devices.Under AM 1.5（100 mW/cm²）illumination,the short-circuit current and open-circuit voltage of Mg/PLZT/ITO are about 150 and 2 times of those of Pt/PLZT/ITO,respectively.The optical response of PLZT capacitor was expanded from ultraviolet to visible spectra,and it may have important impact on design and fabrication of high performance photovoltaic devices based on ferroelectric materials.（2）A controllable photoinduced current was obtained by poling the single-crystal ferroelectric film along the surface plane through pairs of interdigital electrodes under AM1.5（100 mW/cm²）illumination.We found that the electron-hole（e-h）pairs were opto-generated from both the poly（vinylidene fluoride-trifluoroethylene）[P（VDF-TrFE）]copolymer organic ferroelectric film and Si substrate,and the short-circuit current density(J_sc)is about 63.93 mA/cm².Our experimental results also show that both the direction and magnitude of the photovoltage and photocurrent can be tuned by the poling electric field,and the voltage and current can be able to be further improved with a larger gap of interdigital electrodes.Besides,we also noticed that the photovoltaic response was expanded from ultraviolet to visible spectra.（3）（020）-oriented VO₂and（111）-oriented Pb(Zr_0.52Ti_0.48)O₃ bi-layer films were fabricated on（111）La_0.67Sr_0.33MnO₃/（111）SrTiO₃ single crystal substrates.The VO₂thin films exhibited an abrupt metal-to-insulator phase transition characteristic in unpolarized state,with near four orders of magnitude change in the resistance from low-temperature to high-temperature.The change in the resistance with electric field showed a butterfly-like behavior:a typical characteristic exhibited by ferroelectric materials upon application of external electric field,implying that the resistance changes in VO₂film are induced by in-plane lattice strain states.The electric field modulated resistance change behavior was demonstrated in VO₂/PZT heterostructure with a small pulsed electric field.The modulated resistance change becomes larger at lower temperature.These results provide an alternative scheme for low power resistance switching for strain-sensitive materials and show potential could be applied in electronic devices.（4）We successfully deposited 020-oriented VO₂thin film on ferroelectric（001）LiTaO₃substrate with near four orders of magnitude change in the resistance at³40K and systematically investigated the epitaxial growth behavior at the interface by high-resolved synchrotron radiation X-ray diffraction（XRD）method,high resolution XRD confirmed the out-plane epitaxial matching relation,Φ-scan XRD confirmed the in-plane epitaxial matching relation and high temperature XRD identified the VO₂phase transition characteristic.Furthermore,we studied the in-plane strain at the interface by applying a bias electric field along LiTaO₃ thickness direction.The results indicate that the resistance of the VO₂thin film can be tuned by the lattice strain at room temperature.The combination of ferroelectric LiTaO₃ substrate and VO₂thin films may have abundant potentials for designing multi-functional micro devices.