Nonlinear Spectroscopy of Planar and Nano-Crystalline Silicon Interfaces: Experiments for ab initio Theory

本项目研究表面的“外光”光谱学——包括非线性二次谐波和和频产生（SHG/SFG）和线性反射率差光谱学（RDS）——包括实验和理论之间的定量比较。先前的研究在超高真空条件下，对几种半导体吸附体系(H、Ge和B吸附在Si（001)-(2x1)上）在有限光谱范围内的SHG光谱进行了实验与理论的一致，基于从头算赝势和全势方法的理论处理再现了SHG光谱的基本特征。该项目有几个目标：1)使用一种新的飞秒Ti：蓝宝石参数放大器系统，将SHG/SFG的光谱范围扩大近一个数量级，从而涵盖0.5至4.5 eV的表面共振。2)利用只有非线性表面磁化率张量的单一分量的极化配置，从单畴重构和小晶胞制备的Si（001）、Si（111）和Ge/Si（001）表面获得SHG/SFG光谱。3)在相同的各向异性表面上，与SHG/SFG在相似的光谱范围（1.5 ~ 5.0 eV）上并行获取RDS。线性和非线性表面光谱将从一个共同的理论基础计算，以连接这两个表面光谱。4)在二聚体相关表面带的共振激发下进行飞秒分辨SHG和RDS。这些实验旨在分离和表征表面二聚体相关偶极子和电子-空穴对在表面光谱中所起的作用，以阐明一些最具挑战性的理论问题。5)将SHG/SFG光谱扩展到嵌入SiO2中的Si纳米晶体，它们与SiO2形成陡峭弯曲的埋藏界面。光谱SHG/SFG将表征光发射和电荷捕获的界面状态。将与理论合作者保持密切联系，他们正在单独资助下改进计算表面SHG/SFG光谱的方法。广泛的目标是将非线性表面光谱学发展成一门精确的、定量的科学，适用于许多表面系统。该项目涉及电子/光子材料科学中具有高技术相关性的主题领域的基础研究问题。虽然提出的工作将侧重于半导体界面，但由于外延光谱学的通用性和多功能性，预期的结果将具有更广泛的影响。研究结果有望最终应用于金属、聚合物薄膜和生物膜等多种材料，并可能推动商业外光学光谱系统和计算外光学软件的发展。该项目的一个重要特点是非常重视教育，并将涉及研究生和本科生的研究与教育相结合。通过直接参与研究，学生将在半导体及相关材料、先进材料和界面表征领域获得独特的学习和发现机会。本项目由CHE/EPC和DMR/EM项目共同支持

This project addresses "epi-optic" spectroscopy of surfaces -including nonlinear second har-monic and sum-frequency generation (SHG/SFG) and linear reflectance-difference spectroscopy (RDS)-including quantitative comparison between experiment and theory. Prior work reached agreement between experiment and theory for SHG spectra of several semiconductor-adsorbate systems (H, Ge and B adsorbed on Si(001)-(2x1)) in ultrahigh vacuum within a limited spectral range, and theoretical treatment based on ab initio pseudopotential and full-potential approaches reproduced essential features of SHG spectra. This project has several aims: 1) widen the spectral range of SHG/SFG by nearly one order of magnitude, thereby encompassing surface resonances from 0.5 to 4.5 eV, using a new femtosecond Ti:sapphire parametric amplifier system. 2) acquire SHG/SFG spectra from Si(001), Si(111) and Ge/Si(001) surfaces prepared with single-domain reconstructions and small unit cells, using polarization configurations in which only a single component of the nonlinear surface susceptibility tensor contributes. 3) acquire RDS in parallel with SHG/SFG over a similar spectral range (1.5 to 5.0 eV) on the same anisotropic surfaces. Linear and nonlinear surface spectra will be calculated from a common theoretical basis, to link these two surface spectroscopies. 4) perform femtosecond-time-resolved SHG and RDS follow-ing resonant excitation of dimer-related surface bands. These experiments are designed to isolate and characterize the role played by surface dimer-related dipoles and electron-hole pair correla-tions in surface spectra to elucidate some of the most challenging theoretical issues. 5) extend SHG/SFG spectroscopy to Si nano-crystals embedded in SiO2, with which they form sharply-curved, buried interfaces. Spectroscopic SHG/SFG will characterize interface states that underlie light emission and charge trapping. Close contact will be maintained with theoretical collabora-tors who are improving methods for calculating surface SHG/SFG spectra under separate fund-ing. The broad goal is to develop nonlinear surface spectroscopy into an exact, quantitative sci-ence applicable to many surface systems. %%% The project addresses fundamental research issues in a topical area of electronic/photonic materi-als science having high technological relevance. Although the proposed work will focus on semi-conductor interfaces, anticipated outcomes will have broader implications because of the gener-ality and versatility of epi-optic spectroscopy. The results are expected to be useful for eventual applications to materials as diverse as metals, polymer films, and biological membranes, and could motivate the development of commercial epi-optic spectroscopy systems and computa-tional epi-optic software. An important feature of the project is the strong emphasis on education, and the integration of research and education involving graduate, and undergraduate students. Through direct involvement in research, students will have unique learning and discovery op-portunities in the areas of semiconductor and related materials, and advanced materials and inter-face characterization. This project is jointly supported by the CHE/EPC and DMR/EM programs.***