Observation of structural depth profiles in porous silicon by atomic force microscopy

The morphology of porous silicon (PS) formed by wet etching of Si crystals in fluoride solutions was investigated by atomic force microscopy (AFM). The experiments were made in a liquid cell to allow the measurements to be made before the drying process caused restructuring of the surface porosity. We studied the surface roughness, showing experimentally that PS samples produced in high HF concentrations are smoother than PS samples produced in low HF concentrations. We also demonstrated that using the capillary forces produced by the AFM probe tip itself, it is possible to etch layers of the PS material, opening "windows" to observe the interior PS layers. We identified through Fourier transform analysis the most frequent dimensions of the pores, concluding that these pores in general do not suffer appreciable vertical narrowing and that high HF concentrations are favorable for the formation of more pores of smaller size.