Scanning tunnelling microscopy and spectroscopy of nanocrystalline silicon films

Scanning tunnelling microscopy (STM) has been sometimes applied in recent years to characterize porous silicon. In contrast, other forms of light emitting Si, such as nanocrystalline silicon films, prepared by different methods, have not been, or are only occasionally, studied by STM related techniques. In this paper STM and spectroscopy measurements have been performed on nanocrystalline silicon films obtained by low pressure chemical vapour deposition followed by boron implantation. Subsequent annealing of the samples caused an increase of the crystallites size. Scanning tunnelling spectroscopy 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.