溶胶凝胶法制备银—钛酸铅复合薄膜及其电学性能的研究

【作者】 唐立文；

【导师】 杜丕一；

【作者基本信息】 浙江大学 ， 材料学， 2007， 博士

【摘要】 近年来，金属-介电体复合材料由于具有渗流效应，在高介电常数材料方面具有良好的应用潜力，获得了研究者广泛的关注。随着微电子器件向小型化发展，具有高介电常数的金属-介电体复合薄膜的制备研究也逐渐成为材料科学及固体物理学领域的一个重要研究热点。然而，对于金属-介电体复合薄膜，一方面薄膜在厚度方向上的尺度有时仅为几百纳米，如果薄膜内的金属颗粒尺寸较大，导电相颗粒就很容易在薄膜厚度方向上形成导电通路而将上下电极导通；另一方面由于渗流效应的产生本质是通过金属颗粒在材料中形成一系列表面面积大、间距相对较小的微电容器结构，以使材料表观介电常数大大增加，微电容器结构能否达到这一要求决定于薄膜内金属颗粒的形貌。可见，在薄膜材料中，寻找更有效的技术和方法对薄膜中金属相的含量和粒度进行控制，对渗流型高介电常数薄膜材料的成功突破将具有十分重要的意义。本论文研究目的在于采用溶胶凝胶法，对渗流型高介电常数的Ag-PbTiO₃复合薄膜进行制备研究。探讨薄膜内金属颗粒和介电基质的形成和结晶规律，控制在薄膜中形成纳米量级的金属颗粒和合适的颗粒分布；引入渗流理论的思想，掌握薄膜中渗流效应的产生原理，为高性能薄膜的制备打下坚实的基础。本文全面回顾了高介电常数复合材料、导体-介电体复合薄膜的研究进展，比较了常用导体-介电体复合薄膜的性能和制备方法，总结了制备导体-介电体复合薄膜的影响要素。用XRD、SEM、EDS、TG／DTA、FTIR、阻抗分析仪和高阻仪等分析了Ag-PbTiO₃复合薄膜的制备、微观结构、介电和电导性能。对Ag⁺的引入对钛酸铅晶相形成的影响、复合薄膜中金属银颗粒的形成机理、金属-介电体复合薄膜介电驰豫机制和电导机制进行了详细的研究，具体研究内容及研究结论如下：（1）研究了Ag⁺的引入对钛酸铅晶相形成的影响。制备掺银钛酸铅薄膜时，Ag⁺的引入对钛酸铅晶相形成的形成，主要表现为以下两个方面：一方面，硝酸银分解原位生成单质银晶相时，会优先消耗薄膜中的铅，造成钛酸铅晶相结晶时缺铅，促使薄膜中生成焦绿石相钛酸铅；另一方面，硝酸银在溶胶中能够发生醇交换反应生成Ag-O-Ti键，使Ag⁺在薄膜晶化过程中保持较高的价态，而进入钛酸铅晶格中。综合考虑不同热处理温度和不同升温过程对薄膜中相形成的影响时发现：快速升温过程对薄膜进行热处理有利于调节薄膜中的银含量和得到钙钛矿相较多的钛酸铅基质的晶相结构，热处理温度应该选择在600℃左右为宜。（2）研究了溶胶中过量铅的引入对银-钛酸铅复合薄膜晶相形成的影响。溶胶配方中引入过量铅后，有更多的Pb²⁺参与钛酸铅晶相形成，因而抑制了焦绿石相钛酸铅的形成，薄膜中生成了晶格完整的钙钛矿相钛酸铅；同时还降低了溶胶中Ag⁺的活度，减少钙钛矿相钛酸铅中固溶的Ag⁺量。溶胶中引入的过量铅，在热处理过程中会固溶入微米级单质银颗粒的晶格形成银铅合金，降低了微米级银颗粒的熔点，导致它们在远低于纯单质银熔点的温度下就开始挥发。对Pb／Ti=1.3，Ag／Ti=0.5时，薄膜中生成的银铅合金颗粒的挥发过程进行了具体研究，计算得到其挥发的活化能为88.69kJ／mol，表观频率因子为68.007s^-1。过量铅配方制备的薄膜中，固溶有铅的微米级银挥发后，钙钛矿相钛酸铅晶格中固溶的Ag⁺会被继续还原形成纳米银颗粒。纳米银颗粒的生成量受薄膜中银挥发量以及热处理温度两个因素影响。在600℃下热处理时，纳米银颗粒的生长机制介于颗粒与基体边界扩散控制的机制和基体内部体相扩散控制的机制之间。根据这一结果，通过在后处理过程中控制低熔点银铅合金的挥发过程，能够在薄膜中原位形成纳米银颗粒，得以避免在制备纳米银-介电体复合薄膜时为实现纳米银颗粒的形成而需要对溶胶条件进行繁杂的控制，为制备纳米银-介电体复合薄膜提供了一个简单的方法及一个新思路。（3）研究了络合剂及水解条件对银-钛酸铅复合薄膜晶相形成的影响。分别在溶胶中引入LA，DEA和CA这三种多配位基络合剂后，均能够增强Pb²⁺与Ti-O网络的结合，抑制焦绿石相钛酸铅的生成，促进钙钛矿相钛酸铅的生成。薄膜内形成的银颗粒的大小，受Ag⁺与Ti-O的结合能力影响，随着Ag⁺与Ti-O网络的结合能力按CA＞DEA＞LA的次序减弱，薄膜中银颗粒的大小按CA＜DEA＜LA的次序减小。不同水解条件下，薄膜内单质银颗粒的粒径随凝胶网络结构致密度的减小而增大。（4）研究了热处理气氛对Ag-PbTiO₃复合薄膜中银颗粒形成的影响：对不同热处理条件下，薄膜内银颗粒的形成机制进行了分析，发现在空气气氛和还原气氛中单次热处理时，单质银颗粒均通过Ag⁰的扩散而成核和长大；在空气气氛中600℃先处理，再在还原气氛中后处理时，单质银颗粒通过形成Ag3²⁺为成核中心而长大。对还原气氛和空气气氛单次热处理下，银颗粒形成的热力学条件进行了理论计算，得到了不同热处理气氛下薄膜中银颗粒生成的热力学必要条件。同时对银颗粒形成的动力学条件进行了理论计算，指出还原气氛下薄膜中银颗粒的成核密度远大于空气气氛，因而在相同热处理条件下，还原气氛下形成的银颗粒尺寸远小于空气气氛下形成的银颗粒，为纳米金属-介电体复合薄膜的制备提供了理论依据。（5）研究了Ag-PbTiO₃复合薄膜电导机制：银-钛酸铅复合薄膜直流和交流电导机制主要受不同热处理气氛的影响，与银含量的大小无关。在空气气氛热处理的情况下，薄膜的直流电导在低电压和高电压下分别受局域化跃迁载流子导电机制和隧道电流机制控制；薄膜的交流电导在所研究的频率范围内均随着温度的增加从偶极子德拜弛豫跃迁向肖特基电流机制转化。在还原气氛热处理的情况下，薄膜的直流电导在低电压和高电压下同样也分别受局域化跃迁载流子导电机制和隧道电流机制控制，但局域化跃迁载流子导电所产生的电流要比空气气氛热处理情况制备的薄膜要大好几个数量级。受局域化跃迁载流子所引起的电导率增加的影响，还原气氛中制备的薄膜交流电导机制随温度的变化规律受测量频率的影响：当测量频率较低时，交流电导机制随着温度的升高先增加后减小最后增加，在相应的阶段分别受载流子跃迁电导、钙钛矿相再氧化以及肖特基势垒电流控制；当测量频率较高时，在较低温度下，交流电导受弛豫时间分布较宽的偶极子德拜弛豫电流控制，在此之后，随着温度的升高，分别经历载流子跃迁电导、钙钛矿相再氧化以及肖特基势垒电流的控制。对于不同银含量的薄膜，薄膜的电导机制随电压和温度的变化规律相同，但漏电流的大小随银含量的增加而增加，同时薄膜直流电导中载流子跃迁电导机制向Fower-Nordheim隧穿电导机制转换的临界电压随着银含量的增加而减小。（6）研究了Ag-PbTiO₃复合薄膜介电驰豫机制：空气气氛热处理制备的薄膜介电弛豫机制为德拜弛豫机制，而还原气氛热处理制备的薄膜介电弛豫机制在低频下受局域化载流子极化机制控制，在高频下则受德拜弛豫机制控制。两种热处理气氛下制备的薄膜，介电驰豫机制均不随着银含量的增加而发生改变。（7）研究了Ag的引入对Ag-PbTiO₃复合薄膜介电和电导性能的影响：空气气氛下热处理和还原气氛下热处理的银-钛酸铅复合薄膜中，薄膜的介电和电导性能随着银含量的变化具有相似的变化规律：在银含量较低时，由于Ag⁺进入钛酸铅晶格，薄膜的电导率和介电常数随银含量的增加而略有减小；在银含量大于Ag⁺在钛酸铅晶格中的固溶度后，Ag⁺进入钛酸铅晶格造成电导率和介电常数降低的作用逐渐可以忽略不计，薄膜的电导率和介电常数在银含量较大时均按照渗流理论的规律随银含量的增加而增加。空气气氛下热处理的薄膜渗流阈值为0.04左右，渗流阈值附近薄膜的介电常数最高可以达到纯钛酸铅薄膜的1.27倍；而还原气氛下热处理的薄膜渗流阈值在0.17附近，渗流阈值附近薄膜的介电常数最高可以达到纯钛酸铅薄膜的4.13倍。更多还原

【Abstract】 Metal-dielectric composite material is a novel kind of high-dielectric composite materials. It is widely researched because of its percolation phenomenon. With the miniaturization of electronic devices, metal dispersed dielectric film with a high dielectric constant has attracted many interests in material science and modern physics area. However, since the thickness of film is in several nanometers range, and metal particles can easily aggregate and form a conducting path perpendicular to the film, it is hard to prepare metal dispersed dielectric film with a percolation phenomenon. Therefore, it is very meaningful to investigate the prepartion of metal dispersed dielectric film, and take good control of the size and content of metal particles in the films.The aim of this work is to fabricate Ag dispersed PbTiO₃ film via. sol-gel method, which follow the rules of percolation law and performs a high dielectric constant. Efforts have been devoted to control the size and dispersity of Ag particles and the crystallization of PbTiO₃ phase. Effect of dispersed Ag particles on the dielectric properties of the film has been investigated.（1） the effect of Ag⁺ on the phase formation of lead titanate has been investigated. After Ag⁺ has been introduced into the sol, Pb²⁺ will be consumed during Ag particle formation, which will facilitate the formation of pyrochlore lead titanate. Moreover, Ag⁺ will form Ag-O-Ti bond in the sol, which will make Ag⁺ dissolutes into the crystalline lattice of PbTiO₃. After comparing the effect of heating temperature and heating process on phase formation in the films, a rapid heating process and a heating temperature of 600℃ is the most ideal condition for PbTiO₃ phase formation.（2） the effect of excess Pb on the phase formation of Ag dispersed PbTiO₃ film has been investigated. With excess Pb in the sol, more Pb²⁺ can take part in the phase formation of lead titanate, therefore formation of pyrochlore lead titanate can be inhibited. Moreover, the dissolution of Ag⁺ into PbTiO₃ phase can be lowered. The excess Pb will dissolve into Ag particle phase and as a result form Ag-Pb alloy. which will evaporate at a much lower temperature than the melting point of Ag. Detailed investigation of Ag-Pb alloy evaporation in the film with a content of Pb/Ti=1.3 Ag/Ti=0.5 has been conducted, and the activation energy is calculated to be 88.69kJ/mol, apparent frequency factor is calculated to be 68.007s^-1. In the films prepared with exess Pb, after the evaporation of Ag-Pb alloy, Ag⁺ dissolved in PbTiO₃ lattice will transform into Ag nanoparticles. The transformation is controlled by evaporation of Ag-Pb alloy and the heating temperature. At a heating temperature of 600℃, the growth mechanism is either a interfacial diffusion of particle and matrix boudary, or a bulk diffusion in the matrix.（3） The effect of complexing agent and hydrolysis condition on the formation of Agdispersed lead titanate has been investigated. After LA, DEA, DA has been introduced into the sol, the interaction between Pb2+ and Ti-0 can be enhanced, therefore the formation of pyrochlore lead titanate can be inhibited. The size of Ag particle is controlled by the interaction between Ag⁺ and Ti-O. As the interaction increases in a sequece of CA>DEA>LA, the size of Ag particle decreases in a sequence of CA<DEA<LA. Under different hydrolysis condition, the size of Ag particles will increase as the density of gel network decreases.（4） The effect of heating atmosphere on the formation of Ag particles has been investigated. The formation mechanism of Ag particles under different heating process has been discussed. During direct heating process in ambient or reducing atmosphere, Ag particle is formed by diffusion and congregration of Ag⁰; during a 600°C preannealing in ambient atmosphere + a postannealing in reducing atmosphere, Ag₃2+ nuclei will be formed before the growth of Ag particle. For direct heating in ambient or reducing atmosphere, the Gibbs energy for Ag particle formation has been calculated, and the necessary thermodynamic condition for Ag particle formation has been provided. Dynamic condition of Ag particle formation has been calculated. It is pointed out that the reason for Ag particle size difference in the films prepared under different atomphere originates from the nucleation density inequality, which provide a theoretical foundation for the preparation of metal nanoparticle dispersed dielectric film.（5） The conduction mechanisn of Ag dispersed PbTiO₃ film has been discussed. DC and AC conduction mechanism of the film is controlled by the heating atmosphere and has no relationship with Ag content. For the films prepared under ambient atmosphere, DC conductivity of the film is controlled by localized hopping electron mechanism and Fowler-Nordheim tunneling mechanism at low V and high V region respectively. As the measruing temperature increases, the AC conductivity shifted from a Debye relaxation current mechanism to a Schottky emission mechanism. For the films prepared under reducing atmosphere, DC conductivity of the film is controlled by localized hopping electron mechanism and Fowler-Nordheim tunneling mechanism at low V and high V region respectively, but the contribution of localized hopping electron is much larger than the films prepared under ambient atmosphere. For this reason, the AC conduction mechanism of the films prepared in reducing atmosphere is different at a low measuring frequecy and a high measuring frequency. When the measuring frequecy is low, with the increase of measuring temperature, the conduction mechanism shift from localized hopping electron mechanism to reoxidation mechanism and finally to Schottky emission mechanism. When the measuring frequecy is high, with the increase of measuring temperature, the conduction mechanism shift from Debye relaxation mechanism, to localized hopping electron mechanism, then to reoxidation mechanism and finally to Schottky emissionmechanism. The varaition tendency of condution mechanism with voltage and temperature didn’t change with the Ag content. But the value of leakage current increase as the Ag content increases. Moreover, the transition voltage in DC conduction between localized hopping carrier mechanism and Fowler-Nordheim tunneling mechanism increases as the Ag content increases.（6） The dielectric relaxation mechanism of Ag dispersed PbTiO₃ film has beeen investigated. For films prepared in ambient atmosphere, the dielectric relaxation mechanism is controlled by Debye relaxation mechanism. For films prepared in reducing atmosphere, the dielectric relaxation is controlled by localized hopping electron mechanism at a low measuring frequency, and is controlled by Debye relaxation mechanism at a high measuring frequency. Ag content has no effect on the dielectric relaxation mechanism of the films.（7） The effect of dispersed Ag on the dielectric constant and conductivity of Ag dispersed PbTiO₃ film has been discussed. When Ag content is low, the conductivity and dielectric constant will decreased since a small amout of Ag⁺ is dissolved into the lattice of PbTiO₃ phase. But after the Ag content exceeds the solid solubility, the conductivity and dielectric constant of Ag dispersed PbTiO₃ film will both increase, following the percolation law. For films prepared under ambient atmosphere, the dielectric constant can be increased to 1.27 times of that of the pure lead titanate, around a percolation threshold of 0.04. For films prepared under reducing atmosphere, the dielectric constant can be increased to 4.13 times of that of the pure lead titanate, around a percolation threshold of 0.17.更多还原