Studies of Semiconductor Surfaces and Interfaces by Three-Wave Mixing Spectroscopy

Abstract

The surface-sensitive nonlinear optical processes of second-harmonic generation (SHG) and sum-frequency generation (SFG) are powerful tools for investigating the structural and electronic properties of semiconductor surfaces and interfaces. Information about the symmetry and structural properties of the surface or interface can be obtained from an analysis of the polarization dependence of the nonlinear optical response, as has been illustrated for clean Si(111) surfaces prepared with different surface reconstructions. The approach has been applied to real-time, in-situ studies of thin film growth and annealing, from which barriers for adatom motion can be inferred. Information concerning the electronic properties of surfaces and interfaces can be obtained from spectroscopic measurements of resonant three-wave mixing (SHG or SFG). The method has been utilized, for example, to probe electronic transitions at the insulator/semiconductor interface formed by CaF2/Si(111). These measurements led to a determination of the “interface” band gap, i.e. the minimum energy separation between filled and empty interface states. The interface band gap is found to differ strongly from that of either of the bulk materials, highlighting our ability to probe the distinctive properties of this quasi-two-dimensional region.