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

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 x 7 surface, leading to incomplete formation of a Tl/Si(111)1 x 1 structure at 300 degrees C and subsequent desorption of Tl at 350 degrees C. Thus prepared clean Si(111) surfaces exhibit structural changes with increase of temperature: 2 x 1 -> (root 3 x root 3)R30 degrees -> 2 x 1 -> 5 x 5 -> 7 x 7, as observed by low-energy electron diffraction and scanning tunneling microscopy. Among the above structures, the (root 3 x root 3)R30 degrees formed at 450-550 degrees C is found not to be a simple adatom structure. Instead, by means of ab-initio calculations a new metastable (root 3 x root 3)R30 degrees reconstruction was found, having analogous structural features and formation origin to a 2 x 1 reconstruction known as the Pandey chain model. This new (root 3 x root 3)R30 degrees model is 0.09 eV per surface atom less favorable than the 2 x 1 reconstruction. A reason for the anomalous occurrence of the (root 3 x root 3)R30 degrees structure is explored with an aid of Monte Carlo simulations. (C) 2015 American Vacuum Society.