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. PACS: 61.82.Fk; 79.20.Ds; 81.40.Wx Laser processing of semiconductors in the presence of gases has been a continuing field of research over the last 20 years. Adsorbates on Si experience strong effects induced by UVlaser melting: desorption from Si, diffusion in liquid Si, and segregation at the solid/liquid frontier during recrystallization. While diffusion in the liquid phase does not vary significantly with adsorbate, desorption and segregation at the Si liquid/solid interface depend strongly on the chemical nature of the adsorbates. After a single pulse, the undesorbed fraction of adsorbates may be located at the surface or in the region of the volume that was melted, influencing strongly the desorbed fraction at the next pulse. This interplay between ∗Corresponding author. (Fax: +33-1/6915-6777, E-mail: [email protected]) ∗∗Present address: Chemistry Department, Indiana University at Bloomington, Chemistry Building C125, Bloomington, IN 47405-7102, USA desorption and segregation results in very different types of surface modifications: cleaning, etching, doping, dopant profile redistribution. The melting dynamics can be measured through time-resolved measurements of the Si reflectivity at wavelengths where solid and liquid Si have very different reflectivities [1]. Numerical simulations assume the instantaneous conversion of electronic excitation into heat, and calculate the temperature depth and time profiles as a result of laser absorption and heat flow. The result can be compared to transient reflectivity (TR) and time-unresolved measurements, like impurity depth profile [2], time-of-flight mass spectrometry, and Auger electron spectroscopy (AES) [3]. The competition between desorption and diffusion to the bulk was studied in the case of Cl experimentally [4] and numerically [3], showing that the branching ratio between desorption and diffusion to the bulk is “decided” in the first ns of melting. The surface is almost completely depleted from adsorbates during melting. We observed previously that the melting dynamics of Si depends strongly on chlorine pressure in high-vacuum experiments [3]. The melting threshold varies by 70 mJ/cm2 and the melting duration varies by nearly a factor of two. Since the presence of impurities inherent to high vacuum precluded an interpretation of this experiment, it has prompted us to realize TR measurements under a clean UHV environment. It is the aim of this paper to report results concerning the effect of adsorbates on the melting dynamics in UHV. We studied first clean Si in a large range of laser fluence, and for several temperatures, with an experimental setup allowing to record TR in a single pulse [5]. We included in numerical simulations the dependence of reflectivity on the temperature depth profile of the solid, allowing us to simulate accurately the TRs below and above the melting threshold. Surprisingly, we found that the melting dynamics of clean Si is faster than calculated with a standard thermal model [6]. Within the thermal model, the data could be fitted assuming that heat diffusion is reduced in the solid near the surface during the time of photoexcitation. The interpretation is the following: the normal thermal parameters of Si describe ground state Si where the lattice exchanges energy with a (small) density of conduction band electrons that depends on tem-