Collaborative Research: A Combined Experimental and Theoretical Investigation of Plasma Deposition of Nanocrystalline Silicon Films

以含有硅烷（SiH4）和氢（H2）的原料气为原料，通过等离子体增强化学气相沉积（PECVD）生长的氢化纳米晶硅（nm - si:H）薄膜在电子、光电和光伏器件制造技术方面具有巨大的潜力。PECVD过程中的等离子体表面相互作用和随后的H2等离子体处理决定了这些薄膜的结构和性能。薄膜可以是多晶的，其中纳米尺寸的晶粒被晶界分开，或者是多晶的，其中纳米晶体嵌入在氢化的非晶Si （a-Si:H）基体中。在沉积或沉积后处理过程中，等离子体表面相互作用控制着纳米晶相的成核和生长，以及纳米晶晶粒尺寸分布的控制，对调整沉积薄膜的电子和光学特性至关重要。本研究旨在开发控制纳米si: h膜的晶粒尺寸和晶体分数的策略，这些纳米si: h膜是由在H2中大量稀释的SiH4通过低温PECVD形成的，或者是通过等离子体解离H2产生的H原子后处理a-Si:H膜。为了实现这些目标，pi提出了一项研究计划，将原位等离子体和表面诊断与原子尺度模拟相结合。他们寻求对氢在纳米晶硅薄膜成核和生长中的作用的基本和定量理解，这将有助于操纵合成方法和选择等离子体处理参数，以获得比目前可能的更好的薄膜性能控制。因此，提出的研究将为建立薄膜结构（如晶粒尺寸和结晶分数）和等离子体加工参数（如H通量和衬底温度）之间的定量关系奠定基础。本文的实验工作将集中于通过对PECVD沉积的a-Si:H薄膜进行H原子后处理来合成含有纳米晶体的硅薄膜。氢原子的通量将使用视距阈值电离质谱法测量。原位多次全内反射傅里叶变换红外光谱（MTIR-FTIR）将用于检测生长膜及其晶界上的硅氢化物。高分辨率透射电子显微镜（HRTEM）、x射线衍射（XRD）和拉曼光谱将提供有关晶粒尺寸和晶粒分布以及晶体分数的信息；在沉积和后处理过程中，将使用光谱椭偏仪来监测这些相同参数的演变。结合实验工作，将进行a-Si:H薄膜生长和H2等离子体后处理的分子动力学（MD）模拟，旨在对生长和结晶机制进行基本理解，并全面确定化学反应和扩散过程，以便随后进行定量能量和速率分析。由此产生的反应/扩散数据库将用作实现混合晶格外动力学蒙特卡罗（KMC）模拟的输入，该模拟能够捕获硅膜生长和H2等离子体处理后的长时间动力学。计算结果将与实验数据进行直接比较；从模拟中获得的见解将用于指导新的实验研究和设计新的沉积策略。智力价值-拟议的研究在将实验诊断测量和结构表征分析与化学反应和结晶机制的计算原子尺度研究联系起来方面具有开创性。鉴于我们最近在监测等离子体表面相互作用和原子尺度模拟工具的原位实验技术的发展，这项研究特别及时。pi预计他们的研究结果将为控制薄膜结晶度和纳米晶硅薄膜的晶粒尺寸分布提供系统的工程策略，这反过来决定了薄膜的电子和光学性质。此外，他们希望他们的研究策略和方法将适用于研究各种其他技术上重要的材料的生长和加工。更广泛的影响——纳米结构材料合成和薄膜沉积与加工的科学基础是多学科的，跨越了物理学、化学、化学工程、材料科学以及应用数学和数值数学之间的传统界限。因此，提出的硅薄膜沉积和沉积后加工的系统研究为训练学生使用综合的、最先进的实验和计算方法来解决技术上重要的问题提供了理想的手段。研究结果将通过出版物和会议报告在物理、化学、电子材料和等离子体工程界广泛传播。提出的研究有可能使低温等离子体沉积nc-Si:H薄膜的技术进步，这将对可再生能源生产的太阳能电池制造和消费电子产品的柔性显示器制造产生巨大影响。

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films grown by plasma-enhanced chemical vapordeposition (PECVD) from feed gases containing silane (SiH4) and hydrogen (H2) have tremendous potential for electronic, optoelectronic, and photovoltaic device fabrication technologies. Plasma-surface interactions during PECVD and subsequent H2 plasma treatment of these films determine their structure and properties. The films may be either polycrystalline, where nanometer-size grains are separated by grain boundaries, or polymorphous where the nanocrystals are embedded in a hydrogenated amorphous Si (a-Si:H) matrix. Fundamental understanding of the plasma-surface interactions that govern the nucleation and growth of the nanocrystalline phase during deposition or post-deposition processing, as well as control of the nanocrystalline grain size distribution are essential for tailoring the electronic and optical properties of the deposited films.This research aims at developing strategies for controlling the grain size and crystalline fraction in nc-Si:Hfilms formed through low-temperature PECVD from SiH4 heavily diluted in H2 or through post treatment of a-Si:H films with H atoms created by plasma dissociation of H2. Toward these goals, the PIs propose a research plan that integrates in situ plasma and surface diagnostics with atomic-scale simulations. They seek a fundamental and quantitative understanding of the role of hydrogen in the nucleation and growth of nanocrystalline silicon films that will aid in manipulating synthesis methods and choosing plasma-processing parameters to gain a better control over the film properties than is currently possible. As a result, the proposed study will set the stage for establishing quantitative relationships between the film's structure (e.g., grain size and crystalline fraction) and plasma processing parameters, such as the H flux and the substrate temperature.The proposed experimental work will focus on synthesizing silicon films containing nanocrystals throughH-atom post treatment of a-Si:H films deposited by PECVD. Fluxes of H atoms will be measured using line-of-sight threshold-ionization mass spectrometry. In situ multiple total internal reflection Fourier transform infrared (MTIR-FTIR) specroscopy will be used to detect silicon hydrides in the growing film and on its grain boundaries. High-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and Raman spectroscopy will provide information on the grain sizes and grain-size distribution, as well as crystalline fraction; spectroscopic ellipsometry will be used in situ during deposition and post treatment to monitor the evolution of these same parameters. In conjunction with the experimental work, molecular-dynamics (MD) simulations of a-Si:H film growth and H2 plasma post treatment will be carried out aiming at both fundamental understanding of the growth and crystallization mechanisms and comprehensive identification of chemical reaction and diffusion processes for subsequent quantitative energetic and rate analysis. The resulting reaction/diffusion database will be used as input for implementing hybrid off-lattice kinetic Monte Carlo (KMC) simulations that are capable of capturing the long-time-scale dynamics of silicon film growth and H2 plasma post treatment. The computational results will becompared directly with the experimental data; the insights gained from the simulations will be used to guide new experimental studies and design new deposition strategies.Intellectual Merit - The proposed research is pioneering in linking experimental diagnostic measurements and structural characterization analyses with computational atomic-scale studies of chemical reactions and crystallization mechanisms. The research is particularly timely given our recent developments of in situ experimental techniques for monitoring plasma-surface interactions and atomic-scale simulation tools. The PIs anticipate that their research findings will enable systematic engineering strategies for controlling thin-film crystallinity and the grain size distribution of nanocrystalline silicon films, which in turn determine the films' electronic and optical properties. In addition, they expect that their research strategy and methodology will be applicable to studying the growth and processing of various other technologically important materials.Broader Impact - The scientific underpinnings of nanostructured materials synthesis and thin-film deposition & processing are multidisciplinary and cut across traditional boundaries between physics, chemistry, chemical engineering, materials science, as well as applied and numerical mathematics. Thus, the proposed systematic study of silicon thin-film deposition and post-deposition processing provides ideal means for training students to address technologically important problems using an integrated, state-of-the-art experimental and computational approach. The results of the research will be disseminated broadly in the physics, chemistry, electronic materials, and plasma engineering communities through publications and conference presentations. The proposed research has the potential to enable technological advancements in low-temperature plasma deposition of nc-Si:H films which will have tremendous impact on fabrication of solar cells for renewable energy production and flexible display manufacturing for consumer electronics.