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 ultra-high-vacuum (UHV) by transient reflectivity with single-pulse sensitivity in the presence of monitored amounts of chlorine, oxygen or propene. Adsorption of one monolayer (1 ML) leads to measurable variations of the melting dynamics, which are strongly adsorbate-dependent. The variations differ qualitatively and quantitatively from those observed with heavy exposures to gases. The melting dynamics returns to that of clean Si upon subsequent irradiation by laser pulses without readsorption. The required number of pulses for return to clean Si dynamics depends strongly on the type of adsorbate. Adsorbate-induced changes of absorption and reflectivity, and/or incorporation of adsorbates into the substrate, do not explain the results. By contrast, the variations of the melting dynamics are correlated to the photoemitted electron yield, suggesting that laser melting is sensitive to the presence of electrons in the conduction band. These results show that accurate modelling of laser melting of Si interacting with gases should take into account the presence of the gases.