Second harmonic generation spectroscopy of chemically modified Si(111) surfaces

The spectrum of second harmonic radiation reflected from silicon surfaces w ith incident broad spectral bandwidth, femtosecond laser pulses has been me asured for several chemical modifications of Si(1 1 1) surfaces. The modifi cations include surfaces with covalently attached -H and -C10H21 (-decyl) m onolayers, and a native oxide film. By normalizing the second harmonic spec trum with a quartz reference, the dispersion of the nonlinear susceptibilit y of the surfaces has been measured. The normalized spectra show well defin ed resonances associated with critical point structures in the linear susce ptibility 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(I 1 1) surfaces i n 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 orig in of the nonlinear response is considered. Crown Copyright (C) 2001 Publis hed by Elsevier Science B.V. All rights reserved.