Bipolaronic insulator on alkali/Si(111):B-2√3X2√3R30° interfaces

In this paper, we present a systematic study of the structural and electronic properties of the alkali/Si(111):B interface, combining low-energy electron diffraction and (angle-resolved) photoemission spectroscopy. We show that potassium (K), rubidium (Rb), and caesium (Cs) ultrathin films, including solid solutions K-Rb, share the same physics. First of all, the 2 root 3 X 2 root 3R30 degrees surface reconstruction, evidenced only very recently using potassium is shown to be a common feature at the saturation coverage. A 3 X 3 reconstruction is also observed for coverages just below saturation. From an electronic point of view, the band features found with K are generalized. As a distinct point, we evidence that the surface state is constituted by two subbands which have the reconstruction symmetry. At low temperature (55 K), a systematic shift of the surface state toward higher binding energies is observed together with a reduction of its linewidth. A close inspection of the data reveals fine, systematic effects depending on the alkali. Indeed as substituting from K to Cs, the surface-state binding energy is reduced continuously whereas the bandwidth increases. K differs from Rb and Cs, by a smaller reduction of the linewidth and a higher energy shift upon cooling. These features are interpreted consistently within the bipolaron model, which was invoked previously to explain the insulating nature of the K/Si:B surface state. Polaronic effects are shown to be enhanced with K, as already demonstrated on alkali/GaAs(110) interfaces.