Charge transfer and structure of K/Si(111)-2~(1/2)3 x 2~(1/2)3-B surface studied by reflection high-energy positron diffraction

摘要

The atomic coordinates and charge transfer of the K/Si(l11)-2~(1/2)3 × 2~(1/2)5-B surface were investigated using reflection high-energy positron diffraction (RHEPD). Upon the adsorption of K onto the Si( 111 )-~(1/2)3 × ~(1/2)3-B surface, the position of the 111 Bragg reflection is shifted to a lower angle, thus suggesting that a change in the inner potential arises from the charge transfer of 1.Oe~- per K atom to the surface. From RHEPD rocking curves recorded at 113 K, we determined that the top K atoms and the Si adatoms are adsorbed 2.96 and 2.51 A from the first Si layer, respectively. The Si adatoms are significantly displaced upward compared to that found for the Si(l 11)-~(1/2)3 x ~(1/2)3-B surface. Furthermore, we found that the K atoms are adsorbed between the H_3 and T_4' sites. These are consistent with the recent theoretical prediction that the Si adatoms are upwardly displaced and the K atoms form a trimer structure in the 2-~(1/2)3 x 2~(1/2)3 unit cell at low temperature [Phys. Rev. Lett. 107, 187603 (2011)], although the flat model where all the Si adatoms have the same height is also acceptable in the rocking curve analyses. However, at room temperature, which is above the surface phase-transition temperature for 2~(1/2)3 x 2 ~(1/2)3 to~(1/2)3 x~(1/2)3, the K atoms are randomly located in the 2~(1/2)3 x 2~(1/2)5 unit cell. This phase transition is accompanied by the disordering of K atoms.