Synthesis And Photovoltaic Effect Of PLZT Thin Films

Since the discovery of crystal Rochelle salt as one kind of ferroelectric materials in1920, ferroelectric thin films have attracted a great amount of attentions because of its excellent ferroelectric, pyroelectric and electrooptic properties. Also, ferroelectrics have a promising future on device applications. Lanthanum-doped lead zirconate titanate (PLZT) material is one of the perovskite-type (ABO3) ferroelectric materials. Based on PZT material, PLZT is formed by doping La3+ions. Its formula is (Pb1-xLax)(ZryTi1-y)1-x/4O3. As one of the most promising ferroelectric materials, PLZT has important applications on varieties of fields including semiconductor industry, micro-memory, pyroelectric infrared detector, ferroelectric field effect transistor, micro-mechanical and etc. Ferroelectric property of PLZT is sensitive to the concentration of La. And La content effects the dielectric property of PLZT material obviously. Recently, it's discovered that PLZT also has great photoelectric characteristics. PLZT material has potential application in the field of new energy devices.In this paper, PLZT thin films with different La content were deposited on several substrates by sol-gel method. The microstructures, electrical properties, optical properties and photovoltaic effect of ferroelectric PLZT thin films were investigated. Main results are given as follows:(1) Precursor solutions of PLZT with different La content were successfully obtained by Sol-gel method. PLZT thin films with certain thickness were deposited on different substrates (Si, SiO2/Si and LNO/Si) after Rapid Thermal Anneal (RTP) process.(2) Micro-structure was observed by X-ray diffraction (XRD). The influence of substrates and annealing temperature on PLZT thin films were studied. We find out that PLZT thin films annealed at600℃on LNO/Si substrate have better crystallization. PLZT films on LNO/Si show highly (100) and (200) oriented profile with narrower peaks and have rhombohedral-tetragonal phase.(3) The ferroelectric properties and dielectric properties of PLZT thin films were investigated. Hysteresis loop of the PLZT thin films on LNO/Si substrates is degraded because of more La content. The remanent polarization Pr and coercive field Ec decrease with increasing La content. These conclusions are agreed with other studies. When the bias voltage is40V, Pr and Ec of1mol%La modified PLZT thin films are39.2μC/cm-2and5.36kV/cm respectively. C-F curves show that when frequency is lower than200kHz, Cp increases with more La doped. This may be caused by the improvement of crystallization as a result of incorporation a small amount of La. And larger grain size causes the increase of capacitor. At the same time, Tanδ-F curves show that the increase of La content can lead more leakage of PLZT thin films.(4) Raman Spectroscopy, Spectroscopic Ellipsmetry (SE) Spectra and Infrared Reflection Spectrum (FTIR) were used to study the optical properties of PLZT films. We obtained the refractive index n, extinction coefficient k, absorption coefficient a and band gap Eg of PLZT thin films successfully by analyzing SE. SE shows that n, k and a decrease with increasing wavelength and finally approach unity. In the visible light range (-600nm), refractive index is approximately3.15. The band gap of PLZT thin films we get is about3.9eV. Raman spectrum shows that PLZT films on LNO substrates have E(3TO), Al(3TO) and Al(3LO) vibration modes which represent tetragonal phase, and R1mode which represents rhombohedral phase. The results we get from Raman spectrum confirm that PLZT thin films annealed at600℃have rhombohedral-tetragonal phase, which is consistent with the results of XRD analysis.(5) We measured the photovoltaic effect of PLZT thin films on LNO/Si substrate. The results show that the optical voltage reached a maximum value when La content is6%. And then, optical voltage deceases with increasing La content. Bond energy became stronger because of that the lattice constant of PLZT thin films increased with La content increasing. As a result, electrons need higher energy to become free electrons. This may explain the change trend of optical voltage. On the other hand, there were more carrier recombination centers because of defects and oxygen vacancies induced by La. As a result, optical voltage reduced because of shorter lifetime of free carriers.