INFLUENCE OF THE FORMATION CONDITIONS ON THE MICROSTRUCTURE OF POROUSSILICON LAYERS STUDIED BY SPECTROSCOPIC ELLIPSOMETRY

The influence of various parameters of the electrochemical process on the resulting microstructure of porous silicon layers was studied by s pectroscopic ellipsometry. The first parameter, the etching time, is k nown to determine the layer thickness. In addition, although the curre nt density is kept constant, the microstructure of the layers changes, at least in the top part probed by the light, with increasing etching time. A change in the microstructure was also found upon leaving the samples in the electrolyte for a certain time interval after the end o f the electrochemical process. The main effect in both situations seem s to be an increase in porosity of the layers. For long etching times or applied illumination with short wavelength light during the etching process the spectra indicate that the diameters of the crystals compr ising the silicon skeleton are reduced.