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

该项目致力于表面的“epi-optic”光谱学-包括非线性二次谐波和和频产生（SHG/SFG）和线性反射差光谱学（RDS）-包括实验和理论之间的定量比较。在有限的光谱范围内，对几种吸附在Si（001）-（2 × 1）上的H、Ge和B的二次谐波谱进行了理论和实验的一致性研究，基于从头算赝势和全势方法的理论处理再现了二次谐波谱的基本特征。该项目有几个目标：1）使用新型飞秒钛：蓝宝石参量放大器系统，将SHG/SFG的光谱范围拓宽了近一个数量级，从而涵盖了0.5至4.5 eV的表面共振。2)从Si（001），Si（111）和Ge/Si（001）表面获得SHG/SFG光谱，这些表面是用单畴重构和小晶胞制备的，使用极化配置，其中只有非线性表面磁化率张量的单个分量起作用。3)在相同的各向异性表面上，在类似的光谱范围（1.5至5.0 eV）内与SHG/SFG并行采集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之后执行飞秒时间分辨的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相关性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形成尖锐弯曲的掩埋界面。光谱SHG/SFG将表征作为光发射和电荷捕获基础的界面状态。将与理论合作者保持密切联系，他们在单独资助下改进计算表面SHG/SFG光谱的方法。其主要目标是将非线性表面光谱学发展成为一门适用于许多表面系统的精确的定量科学。该项目涉及电子/光子材料科学领域的基础研究问题，具有高度的技术相关性。虽然拟议的工作将集中在半导体接口，预期的结果将有更广泛的影响，因为epi-optic光谱的通用性和多功能性。这些结果有望用于金属、聚合物薄膜和生物膜等多种材料的最终应用，并可能推动商业epi-optic光谱系统和计算epi-optic软件的发展。该项目的一个重要特点是对教育的高度重视，以及涉及研究生和本科生的研究和教育的整合。通过直接参与研究，学生将在半导体和相关材料，先进材料和界面表征领域拥有独特的学习和发现机会。该项目由CHE/EPC和DMR/EM计划共同支持。*

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.***