MESOSTRUCTURE OF PHOTOLUMINESCENT POROUS SILICON

Scanning electron microscopy, atomic force microscopy, and Raman spect roscopy were used to characterize the microstructure of photoluminesce nt porous silicon (PS) layers formed by the anodic etching (HF:H2O:eth anol), at various current densities, of p-type (100) silicon wafers po ssessing resistivity in the range 1-2 OMEGA cm. Existing models for th e origin of luminescence in PS are not supported by our observations. Cross-sectional as well as surface atomic force micrographs show the m aterial to be clumpy rather than columnar; rodlike structures are not observed down to a scale of 40 nm. A three-dimensional model of the me sostructure of porous silicon is discussed. Room-temperature Raman sca ttering measurements show no evidence for a-Si:H or polysilanes and th e material reported here is composed of 10 nm roughly spherical Si nan ocrytallites rather than 3 nm wires postulated in standard quantum con finement models.