Scanning tunnelling microscopy and spectroscopy of nanocrystalline siliconfilms

Scanning tunnelling microscopy (STM) has been sometimes applied in recent y ears to characterize porous silicon. In contrast, other forms of light emit ting Si, such as nanocrystalline silicon films, prepared by different metho ds, have not been, or are only occasionally, studied by STM related techniq ues. In this paper STM and spectroscopy measurements have been performed on nanocrystalline silicon films obtained by low pressure chemical vapour dep osition followed by boron implantation. Subsequent annealing of the samples caused an increase of the crystallites size. Scanning tunnelling spectrosc opy enabled us to determine the surface band gap in films. In all annealed nanocrystalline films the value of this gap is similar to the value in bulk Si. However, a large value of the gap, of about 4.5 eV, is measured in as- implanted films. The different behaviour is explained in terms of a quantum confinement effect related to the nanocrystal's size.