Flexoelectric Properties And Photovoltaic Effects In Thin-film Heterostructures

Recently,flexoelectric effect,which is manifested as a strain gradient-induced electrical polarization,has attracted considerable attention due to the ubiquitous existence in dielectrics,regardless of the lattice symmetry.Taking the flexoelectric polarization as a handle,mechanical modulations of physical properties,such as charge distributions,electron transports,and ferroelectric polarizations,have been observed in semiconductors,transition-metal oxides,and perovskite oxides.While flexoelectric effect is applied to sensors and actuators,a breakthrough has been made in the field of optoelectronics.For the reason that it is not restricted by the thermodynamic Shockley-Queisser limit of p-n junction and the diversity of material selection,it has broad prospects.Due to the fact that the flexoelectric effect is inversely proportional to the size of materials,more and more studies tend to nanoscale films.In the previous studies,the flexoelectric effect of thin film is mostly induced by the inhomogeneous strain applied to the film by external force,so there is a lack of research on the inherent flexoelectric effect of thin film heterostructure.Therefore,a flexoelectric photodetector based on thin film heterostructure is proposed in this paper.In this structure,the LaFeO₃thin film is epitaxial onto LaAlO₃substrate with LaNiO₃as the bottom electrode.Through the lattice mismatch of 2.9%and the lattice relaxation of the film,the strain gradient is produced,which leads to the flexoelectric effect dominating the photovoltaic effect of the device.In this paper,the preparation,structure,mechanism and photoelectric detection characteristics of the device are characterized and studied.Firstly,high quality LaFeO₃epitaxial thin films were prepared on LaAlO₃substrate by pulsed laser deposition.The surface,pure phase composition and microstructure of the samples were characterized by X-ray diffraction,atomic force microscopy and scanning transmission electron microscope.The flexoelectric photodetector with strain gradient of10⁶/m was obtained.The LaFeO₃devices with different thicknesses and top electrodes were measured by the photoelectric test system,and the dark current curve of the devices was fitted by thermal emission model.It is revealed that the flexoelectric effect dominates the photoelectric properties by adjusting the height of Schottky barriers.In addition,the detectability of the energy density,the response time and the detectability at different temperatures were measured.Compared with other photodetectors,it is proved that the photodetector has the advantages of self-powered,15 nanosecond response time and wide operating temperature range.On this basis,LaFeO₃/LaNiO₃/LaAlO₃structure was epitaxially grown on flexible mica substrate.Although the orientation of the film was changed by the change of substrate,the film still had the photovoltaic characteristics caused by flexoelectric effect.X-ray diffraction and atomic force microscopy were also used to characterize the high quality of the films,and the photoelectric detectability was proved by photoelectric test system.It is worth noting that the photoelectric characteristic of the device is regulated by mechanical strain exerted by macro bending,which is stable and repeatable.The thermal emission model is used to fit the dark current curves of two bending states and the horizontal state,and the mechanism of flexoelectric effect is explained by Schottky barrier.It provides a way for the research of flexible electronic devices.