Current trends in semiconductor technology demand that ferroelectric materials be used in thin film form, rather than bulk, for integration and scaling purposes. An inevitable consequence of integration is substrate induced constraint and stress. Sources of this stress are the lattice and thermal mismatch between film and substrate, structural phase transformation which leads to spontaneous strains, and dislocation cores at the film substrate interface. In addition to classical stress relaxation mechanisms all highly tetragonal ferroelectrics relax internal stress via formation of polydomain (90° domains and not 180° domains) structures below the phase transformation, which brings about a change in the microstructure of the film. Hence it is possible to control the resultant microstructure by controlling the degree of polydomain relaxation. Obviously this affects the electrical and electro-mechanical properties and in turn the device performance. The goal of this research is to study this structure-property relationship of ferroelectric thin films where in the structure has been systematically modified by changing the substrate-induced effect. To investigate the effect of the substrate, epitaxial films of PbZr 0.2Ti0.8O3 were grown by pulsed laser deposition (PLD). Epitaxial films reduce the complexities introduced grain boundaries and multiple domain orientations. By systematically changing the thickness the spontaneous strain or c/a ratio can be varied. As a consequence polydomain formation varies as a function of film thickness. Thus this is an effective yet simple method to fully understand the impact of stress on structure-property inter-relationships. The theoretical background for these experiments is first laid out by a thermodynamic analysis of the polydomain formation. It leads to the construction of a domain stability map and indicates a presence of a critical thickness for polydomain formation. This is followed by an investigation of the impact of polydomain formation on quasi-static and dynamic polarization switching. To correlate the material microstructure to switching, an activation field, α, is introduced. It is shown theoretically that α ∝ (c/a-1)3.5 and a good experimental fit can be obtained. However it is observed that polydomain formation does not impact the electromechanical and dielectric response significantly. It is shown experimentally and theoretically that stress-induced polarization varies only by 10%.; Therefore to study the impact of in plane stresses induced by substrate on piezoelectric and dielectric response we chose a “soft” relaxor ferroelectric (RFE) wherein the Curie temperature is close to room temperature. In this case even a small application of stress can change the properties significantly. The relaxor composition chosen was PbMg1/3Nb 2/3O3(90%)-PbTiO3(10%). By systematically changing the substrate and the thickness, stresses in the film the electromechanical constants is varied. High-resolution electron microscopy revealed a distinct change in the microstructure as a function of thickness, and a probable answer as to why thin films show inferior properties compared to bulk materials is proposed.; The last part of this thesis focuses on the effect of micro stresses. Two examples are demonstrated where the mechanical forces of interaction between the film and substrate are manipulated on a very local scale. We show that by inducing stresses at local regions one can induce polydomains in film thinner than previously calculated critical thickness, while by removing constraint at local regions we can enhance the d33 co-efficient to values higher than those shown by bulk ceramics.
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机译：半导体技术的当前趋势要求铁电材料以薄膜形式而不是整体形式用于集成和缩放目的。集成的必然结果是基板引起的约束和应力。这种应力的来源是薄膜与基材之间的晶格和热失配，导致自发应变的结构相变以及薄膜基材界面处的位错核心。除了经典的应力松弛机制外，所有高度四方铁电体还通过在相变以下形成多畴（90°畴而不是180°畴）结构来缓和内部应力，从而改变了薄膜的微观结构。因此，可以通过控制多畴弛豫的程度来控制所得的微结构。显然，这会影响电气和机电性能，进而影响设备性能。这项研究的目的是研究铁电薄膜的这种结构-性质关系，其中通过改变基体诱导的作用对铁电薄膜的结构进行了系统地修改。为了研究衬底的效果，通过脉冲激光沉积（PLD）生长了PbZr 0.2 Ti 0.8 O 3 的外延膜。外延膜降低了引入的晶界和多畴取向的复杂性。通过系统地改变厚度，可以改变自发应变或c / a比。结果，多畴形成随膜厚度的变化而变化。因此，这是一种有效而简单的方法，可以充分了解应力对结构-属性相互关系的影响。这些实验的理论背景首先是通过对多畴形成的热力学分析得出的。它导致了结构域稳定性图的构建，并表明存在用于多结构域形成的临界厚度。接下来是对多畴形成对准静态和动态极化切换的影响的研究。为了使材料的微观结构与转换相关，引入了一个激活场α。理论上证明α∝（c / a-1） 3.5 具有良好的实验拟合性。但是，观察到多畴形成不会显着影响机电响应和介电响应。实验和理论上表明，应力引起的极化仅变化10％。因此，为了研究衬底引起的面内应力对压电和介电响应的影响，我们选择了居里温度接近室温的“软”弛豫铁电体（RFE）。在这种情况下，即使很小的压力施加也可以显着改变性能。选择的松弛剂组成为PbMg 1/3 Nb 2/3 O 3 （90％）-PbTiO 3 （10％）。通过系统地改变衬底和厚度，膜中的应力，可以改变机电常数。高分辨率电子显微镜揭示了微观结构随厚度变化的明显变化，并提出了一个可能的答案，即为什么薄膜与散装材料相比性能较差。本文的最后一部分集中在微应力的影响上。演示了两个示例，其中在非常局部的范围内操纵了薄膜和基材之间相互作用的机械力。我们表明，通过在局部区域感应应力，可以在薄膜中诱导比先前计算的临界厚度更薄的多畴，而通过消除局部区域的约束，我们可以将d 33 系数提高到比那些更高的值如散装陶瓷所示。
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