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

该项目涉及表面的“表观光学”光谱--包括非线性二次谐波和和频产生(SHG/SFG)和线性反射-差谱(RDS)--包括实验和理论之间的定量比较。在超高真空条件下，几种半导体吸附体系(H、Ge、B吸附在Si(001)-(2x1)上)的倍频光谱在有限的光谱范围内得到了实验和理论的一致，而基于从头算赝势和全电势的理论处理再现了倍频光谱的基本特征。该项目有几个目标：1)使用新的飞秒钛宝石参数放大系统，将倍频/单模光纤的光谱范围扩大近一个数量级，从而包括从0.5 eV到4.5 eV的表面共振。2)利用只有一个非线性表面磁化率张量贡献的极化组态，从用单畴重构和小单胞制备的Si(001)、Si(111)和Ge/Si(001)表面获得SHG/SFG谱。3)在相同的各向异性表面上，在相似的光谱范围(1.5 eV到5.0 eV)上并行采集RDS和SHG/SFG。线性和非线性表面光谱将根据共同的理论基础进行计算，从而将这两种表面光谱联系起来。4)对二聚体相关表面带进行飞秒时间分辨倍频和RDS共振激发。这些实验旨在分离和表征表面二聚体相关偶极子和电子-空穴对关联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谱推广到镶嵌在SiO_2中的Si纳米晶体，与其形成陡峭弯曲的埋藏界面。光谱SHG/SFG将表征作为发光和电荷捕获基础的界面态。将与理论合作者保持密切联系，他们正在改进在单独资助下计算表面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适用于许多表面系统的精确、定量的科学。该项目致力于具有高度技术相关性的电子/光子材料科学的专题领域的基础研究问题。尽管拟议的工作将集中在半导体界面上，但由于表观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。这些结果有望对金属、聚合物薄膜和生物膜等材料的最终应用有所帮助，并可能推动商业表观光谱系统和计算表观光学软件的发展。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一体化。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获得独特的学习和发现机会。该项目由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.***