ROLE OF PHONON ENTROPY IN FORMATION OF THE 7×7 STRUCTURE ON THE SI(111) SURFACE

The relative thermodynamic stability of 3×3, 5×5, 7×7, and 9×9 structures belonging to the DAS (dimeradatom-stacking fault) family of structures on the Si(111) surface has been investigated using first-principles calculations based on density functional theory. Taking into account the phonon contribution to the surface free energy, the 5×5 structure was found to be more stable than 7×7 at low temperatures. The phase transition 7×7 → 5×5 should occur at temperatures close to the room temperature, but such a surface structure transformation during sample cooling is hindered due to the limited mobility of Si atoms at low temperatures. The results show the crucial role of phonon entropy in the formation of the 7×7 structure at elevated temperatures and the metastable nature of this structure at temperatures below room temperature.