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

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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.***