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, which is responsible for the visible luminescence, is selectively dissolved by KOH solution exposing the macroporous layer. The dependence of the morphology on doping density and charge passed through the electrode 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 studied using scanning electron microscope (SEM) and atomic force microscope (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 particular morphology before 1 C/cm(2). The AFM, which in general has larger 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 compounds is presented. A model is proposed which attempts to address the initiation of the macroporous morphology through nonuniformities in the photoetching current. Using this model a breakdown field for photocurrent multiplication near surface dopant atoms is found for N-d > 10(16)/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.