MESOSTRUCTURE OF PHOTOLUMINESCENT POROUS SILICON

Scanning electron microscopy, atomic force microscopy, and Raman spectroscopy were used to characterize the microstructure of photoluminescent porous silicon (PS) layers formed by the anodic etching (HF:H2O:ethanol), at various current densities, of p-type (100) silicon wafers possessing resistivity in the range 1-2 OMEGA cm. Existing models for the origin of luminescence in PS are not supported by our observations. Cross-sectional as well as surface atomic force micrographs show the material to be clumpy rather than columnar; rodlike structures are not observed down to a scale of 40 nm. A three-dimensional model of the mesostructure of porous silicon is discussed. Room-temperature Raman scattering measurements show no evidence for a-Si:H or polysilanes and the material reported here is composed of 10 nm roughly spherical Si nanocrytallites rather than 3 nm wires postulated in standard quantum confinement models.