MORPHOLOGY OF N-TYPE MACROPOROUS SILICON - DOPING DENSITY-DEPENDENCE

Macroporous silicon film was prepared on n-type Si (100) substrates by anodizing n-type silicon under illumination. The nanoporous film, whi ch is responsible for the visible luminescence, is selectively dissolv ed by KOH solution exposing the macroporous layer. The dependence of t he morphology on doping density and charge passed through the electrod e of the macroporous layer is reported here. The doping densities were varied from N-d = 10(13) to 4 x 10(18)/cm(3). The morphology was stud ied using scanning electron microscope (SEM) and atomic force microsco pe (AFM). A dense superficial pattern of submicron size etch pits was found for the heavily doped samples after less than or equal to 1 C/cm (2). The lightly doped samples (N-d < 10(16)/cm(3)) do not show any pa rticular morphology before 1 C/cm(2). The AFM, which in general has la rger resolution than the scanning electron microscope, does not reveal further (secondary) morphological pattern, beneath the ones resolved by the SEM. A comparison with the morphology of photoetched II-VI comp ounds is presented. A model is proposed which attempts to address the initiation of the macroporous morphology through nonuniformities in th e photoetching current. Using this model a breakdown field for photocu rrent multiplication near surface dopant atoms is found for N-d > 10(1 6)/cm(3). Furthermore, a saturation of the etch pits density for N-d > 10(17)/cm(3) is predicted. Both results are supported by the present experiments.