Optical And Electrical Properties Of Perovskite Epitaxial Ferroelectric Film Capacitors

Perovskite epitaxial ferroelectric films have not only piezoelectric and thermoelectric properties,but also photoelectric properties,which have attracted extensive attention of many scholars.When the ferroelectric film is irradiated by light,its physical properties will change,which have potential application in photoelectric detector,photovoltaic device,photoelectric storage and so on.The electrodes of ferroelectric films are generally used as precious metals such as Pt,Au,Ag or conductive oxides such as lanthanum strontium cobalt oxygen（LSCO）and lanthanum strontium manganese oxygen（LSMO）.Traditional precious metals and non-transparent conductive oxide electrode materials are not favorable for investigating the effect of light on the properties of optical and electrical properties of ferroelectric films due to their relative lower light transmittivity.Al-doped Zn O（AZO）film is used as the electrode due to its low resistivity,high transmittance,low cost,and non-toxicity.This enables the study of the effect of light on the optical and electrical properties of perovskite epitaxial ferroelectric films.The photoelectric properties of ferroelectric films are closely related to the structure,growth orientation,substrate,film thickness and other factors.Therefore,it is of great significance to study the electrical and optical properties of perovskite ferroelectric capacitors with different light intensities,different orientations,different substrates and different film thicknesses.In this paper,the AZO transparent conductive films were prepared by magnetron sputtering as the upper electrode of the capacitor.The epitaxial(Pb_0.9La_0.1)(Zi_0.4Ti_0.6)O₃（PLZT）ferroelectric films and Na_0.5Bi_0.5Ti O₃（NBT）ferroelectric films were grown on the single crystal substrate.The ferroelectric and photoelectric properties of AZO/PLZT/LSCO and AZO/NBT/LSCO ferroelectric capacitors are studied upon illumination.1.The AZO transparent conductive film was deposited on Corning glass substrate by magnetron sputtering.The resistivity of the AZO film is 2.632×10^-3Ω·cm,and the average transmittance from 400 nm to 1200 nm is greater than～90%,and in the ultraviolet region,the transmittance reaches more than 89.5%.The AZO thin films with high transmittance and low resistivity as electrode materials provide a possibility for studying the photoelectric properties of ferroelectric materials.2.The AZO/PLZT/LSCO capacitors were constructed by magnetron sputtering.The effects of orientation,substrate and film thickness on ferroelectric properties of AZO/PLZT/LSCO capacitors were studied and the dynamic hysteresis scaling behavior of thin films was also discussed.The results show that both LSCO and PLZT films have achieved epitaxial growth on single crystal substrates.The orientation of PLZT films directly affects the ferroelectric properties of AZO/PLZT/LSCO/Sr Ti O₃（STO）capacitors.（111）PLZT films have the best ferroelectric properties and the remanent polarization is 66.3μC/cm².The ferroelectric properties of AZO/PLZT/LSCO/STO capacitors with different substrates are also different.The PLZT films on STO substrate have the best ferroelectric property with the remanent polarization of 36.2μC/cm²,which may be attributed to the different interfacial stresses caused by lattice mismatch between PLZT films on different substrates.For PLZT ferroelectric films with different thicknesses,ferroelectric properties are the best when the film thickness is 250 nm,and the remanent polarization is 44.2μC/cm²,which is conducive to the separation of electron hole pairs generated by illumination.The dynamic hysteresis behavior of PLZT films can be divided into two stages:small loop and large loop,located in low E₀region and high E₀region respectively,and the scale relationship are（27）A（29）?f^-0.11E₀^2.72in small loop and（27）A（29）?f^-0.03E₀^1.27in large loop respectively.Domain structure and domain reversal play an important role in the dynamic hysteresis scaling behavior of thin films.3.The influence of illumination on the electrical properties,leakage current and photovoltaic properties of AZO/PLZT/LSCO/（001）STO capacitor was studied,and the effects of polarization,orientation,substrate and film thickness on the photovoltaic properties of AZO/PLZT/LSCO capacitors were investigated.The results show that light intensity can change the hysteresis loop of the capacitors,and the remanent polarization decreases obviously upon illumination which is due to the capture of photocarriers pinned to ferroelectric domains.When light incident on the surface of the film,the leakage current density increases with the increase of light intensity.Illumination does not affect the conductive mechanism of the capacitor,but it does affect the leakage current density.Light intensity can change the short-circuit current of the capacitor.With the increase of light intensity,the short-circuit current density increases and open circuit voltage does not change significantly.The photoresponse of capacitors are analyzed,and the capacitors are sensitive to the photoresponse and show good photoresponse.The analysis shows that the built-in electric field E_piinduced by the depolarization field plays a decisive role in photovoltaic of the capacitor.4.The AZO/NBT/LSCO capacitor was constructed on（001）STO substrate by magnetron sputtering.The influence of illumination on the electrical properties of AZO/NBT/LSCO capacitor was discussed,and the influence of polarization,illumination intensity and measurement temperature on the photovoltaic properties of AZO/NBT/LSCO capacitor was studied.The results show that LSCO film and NBT film grow epitaxial along（00l）direction on（001）STO substrate.Illumination can change the hysteresis loop of AZO/NBT/LSCO capacitor.When light incident onto the surface of the film,the leakage current density increases by an order of magnitude,indicating that photocarriers are generated inside the film.The photovoltaic properties of AZO/NBT/LSCO capacitors are mainly determined by the built-in electric field.Ferroelectric photovoltaic is largely dependent on the polarization state of ferroelectric film.The irreversibility of ferroelectric photovoltaic effect observed in our experiment is due to the fact that the built-in electric field induced by the interface barrier is larger than that induced by the depolarization field,and the polarization is not enough to completely change the direction of the built-in electric field.Light intensity can change the short-circuit current of the capacitor.The short-circuit current density increases from 42.2μA/cm²to 86.4μA/cm²with the increase of light intensity,and the open-circuit voltage changes little around-0.38 V.The measurement temperature also has an important effect on the photovoltaic properties of AZO/NBT/LSCO capacitors.