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 s urface morphology of photoluminescent porous silicon (PS) layers and w e demonstrate the feasibility of modifying these films on a nanometer scale. For surface imaging, very thin porous coatings (similar to 20 n m in thickness) were used to allow a sufficient current flow through t he material. The samples were prepared by anodic etching at low curren t densities (0.3-3 mA/cm(2)), followed by a post-chemical treatment to yield appreciable photoluminescence in the visible range. The diamete r of the detected surface features is typically well below 5 nm, in ac cordance with the quantum confinement approach for describing the lumi nescent 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 ele ctroluminescence signal.