铁电氧化物薄膜的制备及其激光感生电压效应

【作者】 尚杰；

【导师】 张鹏翔；

【作者基本信息】 昆明理工大学 ， 材料学， 2010， 博士

【摘要】 铁电薄膜是一类重要的功能薄膜材料,对其制备工艺进行研究,是集成铁电学发展的基础。本论文主要围绕四种钙钛矿型铁电氧化物(1-x)Pb(Mg1/3Nb2/3)O3—xPbTiO3 (PMNT或PMN-PT)、Pb(ZrxTi1-x)O3 (PZT)、(Na1-xKx)0.5Bi0.5TiO3 (NKBT或NBT-KBT)和BaxSr1-xTiO3(BST)薄膜的脉冲激光沉积(PLD)工艺,以及薄膜中的激光感生电压(LTV)效应(后经验证该效应为热电效应,即激光感生热电电压效应(LITV))进行研究。获得的一些结果概括如下：一、通过一系列优化实验成功制备出了不含焦绿石相的PMNT铁电薄膜材料,并首次在此类薄膜中发现了LITV效应。具有钙钛矿结构的PMNT铁电薄膜在生长过程中,经常会产生一定量的焦绿石相,即生长的薄膜是由钙钛矿结构和焦绿石结构的混合相组成。钙钛矿相具有优越的铁电特性,而焦绿石相因结构是中心对称的则无此特性。因此,焦绿石相的存在会大大恶化PMNT铁电薄膜的铁电性能。文中通过一系列的优化实验,分别研究了各个工艺参数对薄膜生长的影响,包括沉积温度、沉积氧压、后退火、降温速率、脉冲激光能量以及频率等,结果发现各个工艺参数对薄膜生长都有着不同程度的影响,其中沉积温度的影响最大。对于不同组份的PMNT薄膜沉积温度大都不同,且纯钙钛矿相薄膜的生长温度范围很窄,大约在±2℃的范围以内。这样苛刻的生长条件在其它薄膜中是很少见的,这可能也是一直以来PMNT铁电薄膜利用PLD技术难以制备的主要原因。另外,在PMNT铁电薄膜中还首次发现了LITV效应。其中,在波长为248nm的紫外脉冲激光的辐照下0.50PMN-0.50PT薄膜中的LITV信号相对较好。二、首次研究了掺杂元素对PZT铁电薄膜中LITV信号的影响,结果发现某些掺杂元素(如Na)的引入可以明显增大薄膜中的LITV信号。通过研究组份和掺杂元素对信号大小的影响发现,在能量为0.16J/cm2的紫外脉冲激光辐照下,Pb(Zr0.03Ti0.97)O3铁电薄膜中的LITV信号最大,峰值电压为60mV,而在Pb(Zr0.53Ti0.47)O3铁电薄膜中,掺杂元素Na时LITV信号最大,峰值电压为61mV。这与不掺杂的Pb(Zr0.53Ti0.47)O3铁电薄膜中的LITV信号(峰值电压为41mV)相比,其峰值电压增大了近1/2倍。三、研究了衬底对NKBT铁电薄膜结晶质量的影响,并在20°倾斜的铝酸锶钽镧单晶衬底上生长的薄膜中首次发现了LITV效应。分别在LaA103 (LAO)、(La,Sr)(Al,Ta)O3 (LAST)及SrTiO3(STO)三种不同的单晶平衬底上制备了一系列无铅NKBT外延铁电薄膜。摇摆曲线的半高宽(FWHM)显示在LAST单晶衬底上生长的薄膜结晶质量最好。另外,在20°倾斜的(La,Sr)(Al,Ta)O3单晶衬底上生长的(Na1-xKx)0.5Bi0.5TiO3铁电薄膜中还首次观察到了LITV信号。发现在能量为0.48mJ/pulse的紫外脉冲激光辐照下,其最大激光感生电压为31mV。四、研究了氧压对Ba0.6Sr0.4TiO3铁电薄膜晶格常数的影响,制备出了表面最大均方根粗糙度仅为0.3048nm的BaTiO3/Ba0.6Sr0.4TiO3超晶格,并在倾斜单晶衬底上制备的不同周期BaTiO3Ba0.6Sr0.4TiO3超晶格中还首次发现了在单层膜及[(SrTiO3)15s/(Ba0.6Sr0.4TiO3)7.5s]20和[(BaTiO3)15s/(SrTiO3)7.5s]20超晶格中都没有发现的LITV效应。经一系列的优化实验,首先成功地制备了不同组分的BaxSr1-xTiO3外延膜。随着氧压在10-3-25 Pa范围内逐渐增大,Ba0.6Sr0.4TiO3薄膜的晶格常数与氧压之间近似满足Boltzmann函数关系。其次,在此优化条件下还制备出了高质量的BaTiO3/Ba0.6Sr0.4TiO3超晶格,AFM测试结果表明BaTiO3/Ba0.6Sro.4TiO3超晶格的最大均方根粗糙度仅为0.3048nm,薄膜表面达到了原子尺度的光滑。另外,在超晶格的X射线衍射(XRD)图谱中还清楚地观察到了卫星峰,根据(002)衍射峰周期调制的卫星峰计算了超周期,得到了BaTiO3和Ba0.6Sr0.4TiO3薄膜的沉积速率。最后,在倾斜单晶衬底上制备的BaTiO3/Ba0.6Sr04TiO3超晶格中还首次发现了在单层膜及[(SrTiO3)15s/(Ba0.6Sr0.4TiO3)7.5s]20和[(BaTiO3)15s/(SrTiO3)7.5s]20超晶格中都没有发现的LITV效应。更多还原

【Abstract】 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.更多还原