Influence of adsorbates on UV-pulsed laser melting of Si in ultra-high vacuum

The dynamics of laser melting of atomically clean Si is investigated in ult ra-high-vacuum (UHV) by transient reflectivity with single-pulse sensitivit y in the presence of monitored amounts of chlorine, oxygen or propene. Adso rption of one monolayer (1 ML) leads to measurable variations of the meltin g dynamics, which are strongly adsorbate-dependent. The variations differ q ualitatively and quantitatively from those observed with heavy exposures to gases. The melting dynamics returns to that of clean Si upon subsequent ir radiation by laser pulses without readsorption. The required number of puls es for return to clean Si dynamics depends strongly on the type of adsorbat e. Adsorbate-induced changes of absorption and reflectivity, and/or incorpo ration of adsorbates into the substrate, do not explain the results. By con trast, the variations of the melting dynamics are correlated to the photoem itted electron yield, suggesting that laser melting is sensitive to the pre sence of electrons in the conduction band. These results show that accurate modelling of laser melting of Si interacting with gases should take into a ccount the presence of the gases.