POROUS SILICON ANISOTROPY INVESTIGATED BY GUIDED LIGHT

We have studied light propagation in p-type waveguiding porous silicon layer. The planar waveguides were made by etching two layers with dif ferent porosities, the smaller one located on the top of the structure being the guiding layer. Light (a He-Ne laser beam lambda = 633 nm) i s injected in the guide by a prism coupler. Analysis of the different TE and TM guided modes allows to determine with high precision the ref ractive index of the two porous layers, for both electric field polari sations. For as-formed structures, self-supported or not, we measured a relatively strong index anisotropy which diminishes when the structu re is oxidised (anodically and thermally). The origin of the dielectri c constant anisotropy is attributed to a morphological anisotropy. We present a model which describes the porous layer as preferentially ori ented cylinders immersed in an isotropic medium composed of silicon sp heres. The important result of this oversimplified model is that a rel atively small proportion of silicon, less than 2% in columns, can dete rmine the optical anisotropy for a 65% porosity sample. The same model also permits to explain the optical anisotropy decrease when oxidised . (C) 1997 Elsevier Science S.A.