Photoemission and STM study of an In nanocluster array on the Si(111)-7x7 surface

The formation of nanoclusters for submonolayer indium on the Si(111)-7x7 surface was investigated by scanning tunneling microscopy (STM) and high-resolution photoelectron spectroscopy. The In 4d spectra indicate distinct bonding configurations for the well-known nanoclusters formed at 420-550 K and for the initial adsorbates at lower temperature. The STM imaging reveals a different type of clusters, with uniform shape and size, formed by the initial adsorbates. The spectral evolution of Si 2p indicates the importance of Si restatom sites for the formation of both initial clusters and nanoclusters in contrast to Si adatoms. The surface becomes semiconducting after the formation of the nanocluster array. For the well-developed nanoclusters, we found a surface state in valence bands at a binding energy of 0.6 eV. The origin of this surface state is discussed in comparison with a recent theoretical calculation.