The application of porous silicon to optical waveguiding technology

The porosification of silicon can be achieved by the partial electrochemica l dissolution (anodization) of the surface of a silicon wafer. The degree o f porosity is dependent on the anodization parameters and can generally be controlled within the constraints imposed by substrate dopant type and conc entration. Control of porosity leads to control of refractive index, and th erein lies the concept of using porous silicon as an optical waveguide. We discuss porous silicon waveguides, for the visible to the infrared, prod uced by a number of approaches: 1) epitaxial growth onto porous silicon (wh ere the porous layer acts as a substrate for a higher refractive index wave guide epilayer); 2) ion implantation (where either selective areas of high electrical resistivity can be produced, which act as a barrier against poro sification, or where the surface of a porosified layer is amorphised to for m a waveguide; 3) porous silicon multilayers (where the anodization paramet ers are periodically varied to produce alternate layers of different porosi ty and thus refractive index); and 4) oxidation of porous silicon (where a porosified layer is oxidized to form a graded-index, dense or porous, oxide waveguide).