Crucial role of vibrational entropy in the Si(111)-7$\times$7 surface structure stability

We investigate the relative thermodynamic stability of the $3\times 3$, $5\times 5$, $7\times 7$, $9\times 9$ and infinitely large structures related to the dimers-adatoms-stacking faults family of Si$(111)$ surface reconstructions by means of first-principles calculations. Upon accounting for the vibrational contribution to the surface free energy, we find that the $5\times 5$ structure is more stable than the $7\times 7$ at low temperatures. While a phase transition is anticipated to occur at around room temperature, the $7\times 7\to 5\times 5$ transformation upon cooling is hindered by the limited mobility of Si atoms. The results not only flag a crucial role of vibrational entropy in the formation of the $7\times 7$ structure at elevated temperatures, but also point for its metastable nature below room temperature.