Cerium-Induced Surface Reconstructions and Phase Transition on Si（111）

Various Cerium-induced reconstructions on Si(111)and their phase transitions were investigated by scanning tunneling microscopy. A \sqrt 3 ×\sqrt 3 -R30° reconstruction was observed on the sample at the Ce cover-age of 1/3 ML and with sample annealing at 500℃. This surface structure evolved into a 2×3 reconstruction aftersample annealing at 850℃. With the annealing temperature rising to 950℃,we observed the coexistence of severalreconstructions,including 2×3,17×6,11×6 and 5×2. The pure 5×2 phase was obtained by further increasing theannealing temperature to 1150℃. The structural models of the 2×3,5×2 and(2 n+1)×6 phases composed of honey-comb chains and Seiwatz chains were proposed according to the electron counting rule. The atomic proportion of Siatoms consisting of the honeycomb chains in the 5×2 reconstruction is the largest among the observed reconstruc-tions. As the honeycomb chains are more stable than the Seiwatz chains,the 5×2 reconstruction is the energetically most favorable structure. The phase transition from the \sqrt 3 ×\sqrt 3 -R30° reconstruction to the 2×3 reconstruction isdriven by the formation of the Seiwatz chains,while the structural evolution to the 5×2 reconstruction is driven bythe formation of the honeycomb chains. Our study is of great significance for understanding the physical mechanismand phase transition of rare earth metals-induced reconstructions on the Si(111)surfaces.