Anomalous structural evolution and 3×3 reconstruction of a clean Si(111) surface observed after thermal desorption of thallium

Abstract

The authors have observed anomalous structural evolution of a clean Si(111) surface, which was prepared first by thallium (Tl) deposition on a Si(111)7 × 7 surface, leading to incomplete formation of a Tl/Si(111)1 × 1 structure at 300 °C and subsequent desorption of Tl at 350 °C. Thus prepared clean Si(111) surfaces exhibit structural changes with increase of temperature: 2×1→(3×3)R30°→2×1→5×5→7×7, as observed by low-energy electron diffraction and scanning tunneling microscopy. Among the above structures, the (3×3)R30° formed at 450–550 °C is found not to be a simple adatom structure. Instead, by means of ab-initio calculations a new metastable (3×3)R30° reconstruction was found, having analogous structural features and formation origin to a 2 × 1 reconstruction known as the Pandey chain model. This new (3×3)R30° model is 0.09 eV per surface atom less favorable than the 2 × 1 reconstruction. A reason for the anomalous occurrence of the (3×3)R30° structure is explored with an aid of Monte Carlo simulations.