【作者】 何佳清；

【导师】 王仁卉；

【作者基本信息】 武汉大学 ， 凝聚态物理， 2004， 博士

【摘要】 自从人类进入信息时代，与之相关的新材料也是日新月异，标志之一是计算机各种硬件更新换代之快让人刮目相看，例如高容量的硬盘和高速度的CPU。我们知道，硬盘与磁性材料相关而CPU处理器里有大量的与电介质相关的MOS管和电容器。对于磁性材料来说，从1993年人们在La_1-xBa_xMnO₃中发现了庞磁电阻特性后，这一研究方向更加火爆，我们注意到虽然它有巨大的应用前景，但是把这种复杂的氧化物推广到实际应用并非易事，还有很长的一段路要走。由于其成分的掺杂而引起的结构和性能的变化相当复杂，因此本论文的前一部分就是对它们的块状材料的结构和它们的薄膜特性进行初步的研究。对于电介质材料来说，我们知道，传统的动态随机存储器的设计，利用了在硅表面自然形成的SiO₂非晶层作为电介质材料，工艺上非常简单和成熟。但是动态随机存储器的存储密度需要不断的提高来满足市场的要求，这就要求不断的减小二氧化硅非晶层的厚度。然而，当电介质的厚度小到一定程度后，电子的隧道穿透效应将会使该器件无法工作。后来人们开始寻找其它解决途径，其中最有效的途径是用高电容率的电介质替换低电容率的二氧化硅来满足集成度提高的要求。它的优点是可以不改变电介质层，并且在工艺上能使用传统、成熟、简单的SiO₂流程。但是其缺点也是非常突出，这些材料同样需要克服许多的困难才可用于应用。如热稳定性，界面质量，漏电流的大小等等，这些复杂性将带来的是难以应用或器件制造成本的上升。正是在这样的背景下，人们试图寻找一种高电容率的电介质材料来代替传统的低电容率的非晶SiO₂而且又满足相关的条件，为继续提高存储器件的密度做准备，这正是本论文后半部分的主要研究内容。 本文对磁性和电介质材料分别进行了不同方面的研究，主要内容如下： 一、关于庞磁电阻材料La_1-xCa_xMnO₃正交到立方的相变研究 La_1-xCa_xMnO₃这种奇特氧化物非常地有趣，当x小于0.5时观察到的奇特物理性能是庞磁电阻效应，而当x大于0.5时，在居里温度以下能则能观察到电荷有序，自旋有序和轨道有序等特性，所以Chen等人认为在低温时观察到的La_1/3Ca_2/3MnO₃畴是电荷有序畴，但是王仁卉等用透射电子显微镜系统地研究这一成分后发现电荷有序没有发生时即在室温也能观察到这种畴，他们进一步用群论的知识分析了当在高温时发生立方到正交的相变时各种可能的取向畴，与他们的实验相当吻合。 这里我们利用多晶X射线衍射仪研究了锰基钙钛矿La_1-xCa_xMnO₃（X=3／8，1／2，2／3）的X射线衍射花样随成分和温度的变化。第一次观察到当温度达到750℃或900℃时有从正交到立方的相变。这一实验结果进一步地证明王仁卉等在室温时观察到的取向畴是由立方到正交相变引起，而不是电荷有序引起。同时我们还发现这些材料在室温时正交钙钦矿和在高温时立方钙钦矿的单胞参数都随成分的变化而变化，随X的减小而增大。 二、用PLD外延生长在SrTi伪和NdGaO3上的Lao.7Ca0JMn03薄膜的微结构 我们主要用电子显微学方法来研究薄膜在不同的衬底和厚度时表现出不同的结构特性。通过观察截面样品发现由于在Lao.7Cao.3Mno犷NdGa03系统中是小的晶格失配，它比生长在SrTio3衬底上的薄膜有较高的质量和完整性;对于Lao.7CaooMnO3/N dGa03，在很大的范围内很难发现失配位错，而对于La07ca0JMno声rTio3系统，频繁的观察到Burgers矢为b=a<olo>蚢<llo>牧?类失配位错。 通过综合用X射线能谱方法和电子衍射分析方法发现析出的第二相是MnO，它呈柱状，并且大部分分布在离界面几个纳米以上的薄膜中。在Lao.7CaooMnO犷NdGa03体系中，Mno析出相只在平面样品中观察到，密度很低。其它的人也报道过不同的Mn氧化物析出相，但是所有这些工作中，在母体中析出的第二相都是相应于在靶材料中Mn的过量。而这里靶材料是化学计量的，我们也发现了第二相MnO，它可以解释为局部的化学成分的波动而引起。柱状的析出相的形成也可以有助于释放来源于衬底的抑制而没有完全释放的应变所引起应力能，当位错数少于应力完全释放所要求的数目时它稳定位错的布局。 三、用MOCVD方法生长在si衬底上S叮103的微结构 从前面知道，在替代510:方面，一个很大的挑战是让高电容率的电介质材料直接生长在Si衬底上并且达到所要求的物理性能，而且要使它的生产工艺不能太复杂最好是和传统的是一样的。特别困难的是不让它生长出低电容率和高电子捕获能力的界面层。对于SrTio3，也正在想办法让它能够外延生长在si上，以避免多晶薄膜的晶粒边界导致电子特性的复杂化。 这里我们主要集中研究用MOCVD方法生长的SrTIO3薄膜，利用电子显微镜研究SrTIO3薄膜的微结构和界面。从平面样品上看，SrTio3薄膜是具有任意方向晶粒的多晶。从截面样品看，在薄膜和基底之间我们观察到一层非晶的界面层，它的厚度开始是一个快速的增加（在目前的先驱液下，在同样的生长条件下它比相应的510:非晶长得还要快），然后很快的达到一个准饱和值。在沉积过程中，这个非晶界面的厚度随氧气的分压增加而增加，如果氧的压力达到最小值更多还原

【Abstract】 When man came into informational era, great changes have taken place in advanced materials, which quickly renewed the hardware of computer. For examples, high speed hard-disc and CPU. As well known, hard-disc is relative to magnetic materials, and CPU device be consisted of MOS transistors and capacitors which are related to dielectric materials, therefore, new materials such as colossal magneto-resistance materials and high permittivity dielectric materials have been extensively studied in the past several years. For magnetic materials, since in 1993 colossal magnetoresistance (CMR) effect was found in La2/3Ba1/3MnO3 perovskite, great attention has been paid to this field. Though their prospects in application are very bright, the structures and properties of these complex oxides have not been very clear and many problems need to be resolved prior to application. The first part in this thesis will discuss the complex structure and the thin films of these metal oxides. For the high permittivity dielectric materials, we know, the design of conventional dynamic random access memory (DRAM) uses the amorphous SiO2 on the surface of Si substrate, a simple and ripe procedure. However, to accommodate the change of the market, the density of DRAM needs higher and higher. And this requires a decrease of thickness of amorphous SiO2 layer. However, scaling to some extent, the device can not be used because of the large tunneling current. Then, one of the best methods is to replace the low constant dielectric SiO2 by high permittivity materials. Its merit is that we can continue to use old procedure. However, many problems also need to be resolved, such .as thermodynamic stability, interfacial quality, leakage current and so on, these complications are making devices more complex and expensive. Under this circumstance, many researchers are trying their best to look for the high permittivity materials satisfying the above two requirements.In this thesis, we have investigated the magnetic materials and dielectric materials in different topics. The results are presented in the followings.1. Orthorhombic to Cubic Phase Transition in La1-xCaxMnO3 PerovskitesIt is very interesting in exotic oxides La1-xCaxMnO3, when x<0.5, colossal magnetoresistance (CMR) effect can be observed, however, when x>0.5 and the temperature is decreased to lower than Tc, charge-, orbital-, and spin-ordering and correlated properties may appear. Chen et al. believed the twin-related domains in charge-ordered La1/3Ca2/3MnO3 were charge-ordered domains. Wang et al. systematically studied orientation domains in La1/3Ca2/3MnO3 perovskite by transmission electron microscopy, and observed them even at room temperature (RT), when charge-ordering did not take place. By group-theoretical analysis they concludedthat these orientation domains were induced by a displacive phase transition from cubic to orthorhombic perovskite at higher temperature.Here the variations of the polycrystalline X-ray diffraction patterns of La1-xCaxMnO3 perovskites (x=3/8, 1/2, 2/3) with composition and temperature have been studied. When temperature increased to 750癈 or 900癈, a phase transition from orthorhombic to cubic perovskites was observed for the first time. This phase transition confirms further that the orientation domains in the La1/3Ca2/3MnO3 are induced by cubic to orthorhombic phase transition, but not by charge-ordering. Moreover, the unit cell parameters ao, bo and Co of these orthorhombic perovskites at room temperature, and also ap of these cubic perovskites at high temperature, decrease with increasing x.2. Microstructures of epitaxial La0.7Ca0.3Mn0.3 thin films grown on SrTiO3 and NdGaO3 substratesPerovskite La0.7Ca0.3MnO3 thin films were epitaxially grown on SrTiO3 and NdGaO3 substrates by pulsed laser deposition. The microstructure of these films was investigated by means of transmission electron microscopy. Due to the small lattice mismatch in the system of LCMO/NGO, the films showed a higher structural perfection than the films on更多还原