SURFACE-STRUCTURE IMAGING, ELECTRONICALLY INDUCED MODIFICATIONS AND ELECTROLUMINESCENCE OF POROUS SILICON BY SCANNING-TUNNELING-MICROSCOPY

By means of scanning tunneling microscopy (STM) we have examined the surface morphology of photoluminescent porous silicon (PS) layers and we demonstrate the feasibility of modifying these films on a nanometer scale. For surface imaging, very thin porous coatings (similar to 20 nm in thickness) were used to allow a sufficient current flow through the material. The samples were prepared by anodic etching at low current densities (0.3-3 mA/cm(2)), followed by a post-chemical treatment to yield appreciable photoluminescence in the visible range. The diameter of the detected surface features is typically well below 5 nm, in accordance with the quantum confinement approach for describing the luminescent properties in this material. By increasing the tunnel voltage and current beyond imaging conditions, electronic modifications of the PS films could be induced, which were subsequently detected with the same instrument. Operating the STM in the point-contact regime, local regions of PS layers 5-75 mu m thick could be excited entailing an electroluminescence signal.