Coverage dependence ofSb∕Si(111)adsorption and desorption modes: Interplay between chemical interactions and site transitions

Abstract

The kinetic and thermodynamic properties of adsorption and desorption of Sb onto a Si(111) surface are analyzed by mass spectrometry and ab initio calculations. Two domains of temperature are evidenced. At $T<600\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ there is an irreversible adsorption involving a subtle competition between sticking, adsorption, then dissociation of ${\mathrm{Sb}}_{4}$ tetramers associated to a partial reflection of ${\mathrm{Sb}}_{4}$ molecules on the Sb-covered surface. At $T>800\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, ${\mathrm{Sb}}_{4}$ molecules are dissociated close to the surface leading to a simple reversible adsorption/desorption of Sb monomers. In this temperature range, adsorption/desorption isotherms can be recorded for various temperatures, which reveal a surprising behavior since quasi-Langmuir isotherms appear to be the consequence of a two-dimensional (2D) phase transition. More precisely, as shown by ab initio calculations, during the submonolayer adsorption process, the adsorption site evolves (as a function of coverage) from ternary towards on-top position and the character of the Sb-Sb effective interactions changes from repulsive towards attractive. The 2D phase transition close to $\ensuremath{\theta}\ensuremath{\approx}0.7$ ML seems to be associated to characteristic signatures in many other experiments. Then for high enough supersaturations, it is possible to overpass $\ensuremath{\theta}=1$ ML by the formation of dimers, first partially located on top sites which repel one another, up to a second phase transition around $\ensuremath{\theta}=1.3$ ML in which dimers leave the on top sites and stand up to occupy almost vertically the hollow ones for building the second Sb layer. Lastly surfactant effect is quantified.