Surface Diffusion and Ordering Processes Exploited for Directed Self-Assembly Using Amorphous Semiconductors

该纳米技术研究项目旨在建立一个与非晶材料表面扩散和表面有序相关的科学基础，最终目标是开发一种利用非晶半导体材料在纳米尺度上进行定向表面自组装的新方法。在非晶和其他能量非均质表面上的扩散在陶瓷烧结、回流工艺和微电子存储器件制造中起着重要作用。目前几乎没有关于非晶表面上扩散的文献，也很少有专门研究高缺陷晶体表面上连续分布的能量非均质性的文献。然而，这种不均匀性应该导致扩散系数D的值与定义明确的晶体表面的测量值有很大不同。对于非晶材料，制造工艺可以设计基于促进或抑制表面扩散驱动的表面有序。示例应用包括制造存储器件电极、高效太阳能电池和非晶陶瓷。常规技术无法在实验中遵循排序速率，这使得流程模型的创建变得困难。该项目的第一个目标是利用两个实验室的综合专业知识，为非晶态和其他能量异质表面的表面扩散和有序发展一个科学基础。以非晶态硅和陶瓷二氧化钛为例，将开发一种实验方法来量化描述能量非均质表面上表面扩散的分布函数。实验也将证实光学照明可以用来驱动表面非热扩散。在表面有序方面，波动显微镜将适应于近表面区域的研究，通过在扫描透射电子显微镜上实现，空间分辨率小至0.8 nm。此外，该方法将扩展到二元化合物的研究，如二氧化钛；到目前为止，应用仅限于单元素系统。该项目的第二个目标是开发一种新的表面自组装方法，在10- 200nm的长度尺度上，使用非晶半导体材料含有控制数量和尺寸分布的亚临界核。图像化的光学或电子束暴露应该产生空间变化的表面质量通量，当在结晶尖端的退火温度下进行时，提供额外的推力，使亚临界核在光通量决定的区域结晶。然后，成熟的晶体应该通过表面扩散和奥斯特瓦尔德成熟生长，直到所需的薄膜部分被吸积到原始核上。示范将集中在非晶硅和二氧化钛上。所提出的工作的一些计算方面将被纳入一个新的跨学科实验课程，重点是纳米材料合成的本科生。阐明非晶表面上的扩散现象，扩展波动显微镜的功能，以及展示自组装方法的一个版本的核心目标将保持不变。预算的下调将从几个方面影响拟议的工作，涉及所概述的个别任务的范围。降低了我们在非晶硅中确定最佳中量程顺序的条件轨迹的精度。2 .通过加氢量、离子能量和通量以及相关参数的变化来限制控制中量程顺序的研究范围。将定向自组装方法的实现限制为光学手段，而不是包括电子束。限制获得描述表面扩散的分布函数的精度。（数据越多，分布函数越准确。）

This Nanotechnology research project seeks to develop a science base connected with surface diffusion and surface ordering of amorphous materials, with the ultimate goal of developing a new method for directed surface self-assembly on the nanoscale using amorphous semiconducting materials. Diffusion over amorphous and other energetically heterogeneous surfaces plays a role in sintering of ceramics and in reflow processes and memory device fabrication in microelectronics. There presently exists virtually no literature for diffusion on amorphous surfaces, and very little that specifically addresses continuously distributed energetic heterogeneity on highly defected crystalline surfaces. Such heterogeneity should lead to values of the diffusivity D that differ significantly from those measured for well-defined crystalline surfaces, however. For amorphous materials, fabrication processes can be devised based upon promoting or inhibiting surface ordering driven by surface diffusion. Example applications include fabrication of memory device electrodes, efficient solar cells, and amorphous ceramics. The rates of ordering cannot be followed experimentally by conventional techniques, rendering creation of process models difficult.The first goal of this project is to develop a science base for surface diffusion and ordering on amorphous and other energetically heterogeneous surfaces, using the combined expertise of the two laboratories. An experimental method will be developed to quantify the distribution function describing surface diffusion on energetically heterogeneous surfaces, using amorphous silicon and the ceramic titanium dioxide as paradigm cases. Experiments will also confirm that optical illumination can be used to drive surface diffusion non-thermally. With respect to surface ordering, fluctuation microscopy will be adapted to the study of near-surface regions by implementation on a scanning transmission electron microscope, with spatial resolution as small as 0.8 nm. Furthermore, the method will be extended to the study of binary compounds like titanium dioxide; up to now application has been restricted to single-element systems.The second goal of this project is to develop a new surface self-assembly method at the 10-200 nm length scale using amorphous semiconducting materials containing controlled amounts and size distributions of subcritical nuclei. Patterned optical or electron beam exposure should yield a spatially varying surface mass flux that, when performed at an annealing temperature just at the cusp of crystallization, provides the extra nudge to crystallize subcritical nuclei in regions dictated by the light flux. The full-fledged crystallites should then grow by surface diffusion and Ostwald ripening until the desired fraction of the film has accreted onto the original nuclei. Demonstrations will focus on amorphous silicon and titanium dioxide. Some computational aspects of the proposed work will be incorporated into a new interdisciplinary laboratory course for undergraduates focusing on nano-materials synthesis.The core goals of elucidating diffusion phenomena on amorphous surfaces, extending the capabilities of fluctuation microscopy, and demonstrating a version of the self-assembly method will remain in place.The downward revision in budget will impact the proposed work in several ways involving the scope of the individual tasks outlined.1. Reduce the accuracy with which we can determine the locus of conditions giving optimal medium range order in amorphous silicon.2. Limit the scope of studies for controlling medium range order via variations in hydrogen addition, ion energy and flux, and related parameters.3. Limit the implementation of the directed self-assembly method to optical means, rather than including an electron-beam.4. Limit the accuracy with which distribution functions describing surface diffusion can be obtained. (The distribution functions are more accurate with more data.)