Nonlinear Spectroscopy of Silicon Nano-Interfaces

University Of Texas At Austin, Austin TX

Investigators

Abstract

Technical: This project is to study silicon nanocrystals (NCs) embedded in amorphous oxide materials and step edges at vicinal Si(001) and Si(111) surfaces mainly using nonlinear optical methods, particularly second harmonic generation (SHG) and sum-frequency generation (SFG) spectroscopy. The nonlinear optical spectroscopy is used to probe electronic structures, chemical modifications, and femtosecond dynamics of these materials. The experiments will be extended to a multi-electron-volt spectral range using an optical parametric amplifier to obtain interface state spectra. The research on embedded Si nanocrystals includes a comprehensive study of both radiative and nonradiative optical transitions between nanocrystals' interface states arising from, e.g., Si=O double bonds, Si dangling bonds, the amorphous Si transition region, and strained Si-Si and Si-O bonds, which are crucial for understanding luminescence of nanocrystals. Because composites of spherical silicon nanocrystals possess inversion symmetry, they lack a conventional electric-dipole SHG response, but possess an allowed, optically quadrupolar SHG response that is highly sensitive to interface chemical passivation. In studies of silicon step edges, SHG will be combined with reflectance-anisotropy spectroscopy (RAS), and analyzed with a simplified bond hyperpolarizability model to achieve bond-specific spectra. Non-technical: The project addresses basic research issues in a topical area of materials science with high technological relevance. The research is expected to yield much needed basic understanding on structures and properties of silicon nanocrystal interfaces and steps at crystalline silicon surfaces. The success of the project would help in the future progress of nanocrystal-based light-emitting and flash-memory devices and nanotechnology in general. Through this project, graduate and undergraduate students will receive training in a field of critical importance to U.S. industry and technology. The project also includes education and outreach activities in collaboration with the University of Texas - San Antonio campus, which has very high Hispanics student enrollment.