Second-harmonic generation spectroscopy on thin metal films on Si(111)

Optical second-harmonic generation spectra have been recorded for various reconstructions of Si(111) surfaces as well as for thin metal films grown on these surfaces. Excitation of surface states gives rise to broad second harmonic spectra in the energy region below the Si direct band gap. Variations in the spectra with sample temperature and oxygen exposure have been used to identify resonances. Deposition of metal films leads to decay of surface state resonances and appearance of new resonances. In cases where the metal films grow epitaxially with a roughness of the order of a few atomic layers within the probed area, excitations of quantum well states can be observed. Quantization effects in second-harmonic generation are observed as oscillations in the signal as a function of film thickness with an oscillation period that depends on the pump frequency of the laser light.