The image surface reconstruction of alkali/Si(1 1 1):B semiconducting surfaces

The surface structure of alkali doped Si(1 1 1):B ultra-thin films has been studied by low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM). A comparative study of K/Si(1 1 1)-3 × 1 and K/Si(1 1 1):B-image interfaces allowed us to determine the saturation coverage to be 0.5 monolayer in the later case. The image-surface reconstruction is shown to be a common property of pure K, Rb, Cs materials and K0.4Rb0.6 alloys but progressively disappears if Rb is replaced by Ca. Taking into account the existence of two distinct boron sites in the ratio 1/3 as seen from B-1s core levels spectra, LAPW-DFT calculations have been carried out in order to optimize the atomic structure. As a result, alkali adatoms are shown to form trimers leading to a large modulation of the Sisingle bondB bonds accompanied by an inhomogeneous doping of the dangling bonds in agreement with voltage dependent STM images.