Lowering Of Kinetic Energy Research Articles

Coverage dependence of the electronic structure of potassium adatoms on the Si(001)-(2 x 1) surface.

The electronic structure of potassium adatoms on the Si(001)-(2\ifmmode\times\else\texttimes\fi{}1) surface is studied by first-principles calculations within the local-density-functional theory for a wide range of the K coverage (\ensuremath{\Theta}) including low \ensuremath{\Theta} values with negligibly small direct interactions among adatoms. The symmetric dimer model is assumed for the substrate Si which is modeled by a ten-layer slab. As possible adsorption sites for K, a raised site on the Si dimer chain and/or a valley site between two dimer chains are considered. The calculations are performed for \ensuremath{\Theta} ranging from (1/6 to 1 in units of Si monolayers including the Levine model (\ensuremath{\Theta}=(1/2) and also recently proposed double-layer model (\ensuremath{\Theta}=1). No free-electron-like surface band characteristic of the K 4s state appears in the Si gap even for higher \ensuremath{\Theta}. However, the two gap states originating from the Si dangling bonds in the limit of \ensuremath{\Theta}\ensuremath{\rightarrow}0 shift to higher binding energies by \ensuremath{\sim}0.5 eV with increasing \ensuremath{\Theta} due to the kinetic energy lowering caused by the delocalization of their wave functions toward the overlayer. This is interpreted as a result of the Si-K hybridization which leads to the polarized covalent Si-K bond. The adatom region is essentially neutral even for lower \ensuremath{\Theta} if the charge density is averaged in a K sphere, which implies that the adatom-induced dipole moment to reduce the work function shold be attributed to the adatom polarization due to the Si-K orbital mixing rather than the conventional \ensuremath{\Theta}-dependent charge transfer.