Abstract

Second-harmonic generation (SHG) provides a sensitive probe of the dynamics of adsorption on semiconductor surfaces. The technique has been applied to examine diffusion, desorption, and dissociation processes on silicon surfaces. Desorption has been investigated for hydrogen on Si(111)7×7 and Si(100)2×1 through isothermal measurements. The high sensitivity of SHG to adsorbed hydrogen allows us to perform such experiments at coverages well below 1% of a monolayer. For both surfaces, the results reveal a marked departure from the second-order kinetics expected for a recombinative desorption reaction. This observation is related to the details of the surface structure. Surface diffusion has been investigated by means of second-harmonic diffraction from a spatially modulated adsorbate density. For H/ Si(111)7×7, a diffusion barrier of 1.5 eV, ~50% of the H-Si binding energy, has been established. Such a high barrier for lateral motion stands in marked contrast to the usual behavior on metal surfaces. For NH₃ and O₂ on Si(111), SHG is found to be strongly dependent on the chemical state of these adsorbates, permitting real-time investigations of dissociation reactions.

© 1992 Optical Society of America