Morphological properties of porous-Si layers for nq-emitter applications

Porous silicon layers prepared on nq emitters are investigated by a combination of spectroscopic ellipsometry, transmission electron microscopy and sheet resistance measurements. The porous silicon is formed by an anodic surface treatment of uniformly doped emitters prepared by chemical vapour deposition and of diffused emitters. The results show a good correspondence between the thicknesses obtained with the different techniques. The porosity of the layers increases for increasing doping level and saturates at a doping above ;5=1019rcm3. The formation rate shows an inverse doping dependence. The top surface layer of the porous silicon is in all cases more dense than the bulk, which is related to an initially faster formation rate. q1999 Elsevier Science B.V. All rights reserved.