The Preparation Of Ferroelectric Oxide Thin Films And Their Laser Induced Voltage Effect,

The ferroelectric thin films are a kind of important functional materials. The research of preparation process is the base of the development of integrated ferroelectricity. In this dissertation, the pulsed laser deposition (PLD) process and the laser induced voltage (LTV) effect (Finally, the experimental results demonstrated the effect is thermoelectric effect, namely laser induced thermoelectric voltage (LITV) effect) are studied in four types of perovskite oxides, including (1-x)Pb(Mg1/3Nb2/3)O3—xPbTiO3 (PMNT or PMN-PT),Pb(ZrxTi1-x)O3 (PZT),(Na1-xKx)0.5Bi0.5TiO3 (NKBT or NBT-KBT) and BaxSr1-xTiO3 (BST) thin films. The main results in this dissertation are listed as following:Ⅰ. The pyrochlore-free PMNT ferroelectric thin films were successfully prepared through a series of optimization experiments and the LITV effect was observed for the first time.The PMNT ferroelectric thin films often contain a certain amount of pyrochlore phase. That is to say, the thin films are a mixed phase of perovskite phase and pyrochlore phase. Perovskite phase has superior ferroelectric properties. However, pyrochlore phase hasn't these characteristics because of centrosymmetric structure. Therefore, the existence of pyrochlore phase will greatly worsen the ferroelectric properties of PMNT ferroelectric thin films. In this dissertation, the effects of the technological parameters, such as deposition temperature, deposition oxygen pressure, post annealing, cooling rate, pulsed laser energy and pulsed laser repeated frequency were studied on the growth of thin film. It was found that the technological parameters could affect the growth of thin film in various degrees. The deposition temperature was very different for the different mole fraction of the thin films. The range of the deposition temperature of high-qualitied thin films is very narrow within±2℃. Such harsh conditions are rare in other thin films, which possibly is also the main reason that PMNT thin films is very difficult to prepare by using PLD technique. In addition, the LITV effect was observed in the PMNT ferroelectric thin films for the first time. Under the irradiation of KrF excimer laser with the 248nm wavelength, the better LITV signal was observed in the 0.50PMN-0.50PT films. Ⅱ. The influence of doped elements on the LITV signals was researched for the first time. Results showed that some doped elements (like Na) into PZT ferroelectric thin films could greatly improve the size of the LITV signal.Through researching the influence of doped elements on the size of the signal, the maximum LITV signal was found under UV light irradiation of 0.16J/cm2. For different component PZT ferroelectric thin films, the maximum peak voltage was 60mV if and only if the component was x=0.03. For different doped Pb(Zr0.53Ti0.47)O3 ferroelectric thin films, the maximum peak voltage was 61mV if and only if the doped element was Na, which increased nearly 1/2 times compared with the maximum peak voltage (41mV) of Pb(Zr0.53Ti0.47)O3 ferroelectric thin films.Ⅲ. The influence of substrates on the crystallinity of NKBT ferroelectric thin films was researched. Moreover, the LITV signals were observed in the films grown on 20°vicinal-cut (La,Sr)(Al,Ta)O3 monocrystalline substrates for the first time.A series of lead-free (Na1-xKx)0.5Bi0.5TiO3 ferroelectric thin films were fabricated on flat LaAlO3 (LAO), (La,Sr)(Al,Ta)O3 (LAST) and SrTiO3 (STO) monocrystalline substrates, respectively. The full wave at half maximum (FWHM) of rocking curve indicated that the crystallinity of thin films grown on LAST monocrystalline substrates was the best. In addition, the LITV signals were observed for the first time in NKBT ferroelectric thin films grown on 20°vicinal-cut LAST monocrystalline substrates. The maximum peak voltage was of NKBT ferroelectric thin films was 31mV under UV light irradiation of 0.48mJ/pulse.Ⅳ. The influence of oxygen pressures on the lattice constants of Ba0.6Sr0.4TiO3 ferroelectric thin films was researched. BaTiO3/Ba0.6Sr0.4TiO3 superlattice whose surface root-mean-square (RMS) roughness was only 0.3048 nm was prepared. The LITV effect was measured for the first time in BaTiO3/Ba0.6Sr0.4TiO3 superlattices with different periods grown on vicinal-cut single crystal substrates, which was not measured in simple layer films and superlattices of (Ba0.6Sr0.4TiO3)7.5s]20 and [(BaTiO3)15s(SrTiO3)7.5s]20.The epitaxial BaxSr1-xTiO3 thin films with different components were successfully prepared through a series of optimization experiments. The functional relation between the lattice constants of Ba0.6Sr0.4TiO3 thin films and the oxygen pressures was close to Boltzmann function from 10-3 to 25 Pa. Then the high-quality BaTiO3/Ba0.6Sr0.4TiO3 superlattices were prepared in these optimal conditions, too. The surface topography measurement by atomic force microscopy (AFM) demonstrated that the RMS roughness of BaTiO3/Ba0.6Sr0.4TiO3 superlattice was only 0.3048 nm. The surface smoothness has achieved the atomic scale. In addition, satellite peaks were clearly observed in the XRD spectrum. According to the position of satellite peaks that belong to (002) diffraction peak, the superperiod of superlattices was calculated and the deposition rates of BaTiO3 and Ba0.6Sr0.4TiO3 thin films were deduced. Finally, The LITV effect was measured for the first time in BaTiO3/Ba0.6Sr0.4TiO3 superlattices with different periods grown on vicinal-cut single crystal substrates, which was not measured in simple layer films and superlattices of (Ba0.6Sr0.4TiO3)7.5s]20 and [(BaTiO3)15s/(SrTiO3)7.5s]20.