Second harmonic generation spectroscopy of chemically modified Si(1 1 1) surfaces

The spectrum of second harmonic radiation reflected from silicon surfaces with incident broad spectral bandwidth, femtosecond laser pulses has been measured for several chemical modifications of Si(1 1 1) surfaces. The modifications include surfaces with covalently attached –H and –C10H21(-decyl) monolayers, and a native oxide film. By normalizing the second harmonic spectrum with a quartz reference, the dispersion of the nonlinear susceptibility of the surfaces has been measured. The normalized spectra show well defined resonances associated with critical point structures in the linear susceptibility of silicon, in agreement with previous second harmonic generation (SHG) studies on oxidized silicon surfaces. It is shown that variations in peak position and normalized SHG intensity among the spectra are favorable for the application of SHG for monitoring the progress of surface chemical reactions. Preliminary results are presented that demonstrate such in situ monitoring for photoinduced reactions of H-terminated Si(1 1 1) surfaces in air and in liquid 1-decene. Estimates have been made of the absolute SHG efficiency of the chemically modified Si(1 1 1) surfaces. The significance of the spectroscopic results for the interpretation of the microscopic origin of the nonlinear response is considered.