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Porosity superlattices have been investigated by transmission electron microscopy, photoluminescence and reflectance spectroscopy. The superlattices were formed on p-type doped Si using two different techniques. Firstly, for homogeneously doped substrates we have periodically varied the formation current density and thereby the porosity. Secondly, the current density was kept constant while etching was performed on periodically doped Si layers. For the first type of superlattices the layer thicknesses were determined by transmission electron microscopy. The results are in good agreement with the values calculated from the etching rate and time. For both types of superlattices, reflectance and photoluminescence spectra show strong modulation due to the periodicity of the superlattice.

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Lateral superlattices in porous silicon layers have been generated. Using the photosensitivity of the etching process, periodic stripes are formed not only on the surface but also in the depth of the layer. The modulation depth depends on the illumination wavelength. The periodicity is obtained from the interference pattern of two laser beams, and can be easily modified by changing the wavelength or the incidence angles of the beams. The samples formed by this procedure were characterized by light diffraction. Two-dimensional structures can also be obtained by rotating the sample or by interference of four laser beams. This kind of in-depth lithography and the resulting low-cost fabrication of gratings out of porous silicon offer a wide range of potential applications in integrated optics and photonics.