Coherent phonon spectroscopy of GaAs surfaces using time-resolved second-harmonic generation

The general theory and experimental considerations are presented for a novel all-optical time-domain technique for measuring low frequency vibrational modes of surfaces (phonons or adsorbate vibrational modes). A pump-laser pulse impulsively drives an initial displacement of the surface atoms. The subsequent free-induction decay of the coherent phonon modes of the surface atoms is detected by time-resolved second-harmonic generation (SHG). The spectral features are recovered by fitting the time-domain data to exponentially decaying sinusoids. This all-optical probe has advantages over inelastic particle scattering techniques because it can be applied at buried interfaces. It has signal to noise advantages over linear and spontaneous Raman techniques. This technique is demonstrated by measurement of surface optical phonon spectra on the clean GaAs (110)-reraxed-(1 x 1), GaAs (100)-(1 x 6), and GaAs (100)-(4 x 1) in UHV and of a local monolayer-scale interfacial mode at the buried native-oxide-covered GaAs (100). The chemical sensitivity is demonstrated by in situ oxidation of the GaAs (100)-(4 x 6). The general mechanism of generating coherent surface phonons is discussed in light of the symmetry selection rules. (C) 2000 Elsevier Science B.V. All rights reserved.