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.