Origin of a parasitic surface film on p(+) type porous silicon

The presence of a parasitic surface film of 80 nm thickness has been observ ed by x-ray reflectivity on the top of some p(+) type porous silicon layers , related to a contamination of the substrate. After testing several method s to clean the substrate and to avoid this film, it was found that a 300 de grees C thermal annealing of the substrate is sufficient to obtain a homoge neous porous layer. The thickness of the perturbed surface layer is determi ned by anodic oxidation experiments and the effect of the parasitic surface film on the porous silicon formation is studied by comparing porous layers formed on untreated and on annealed substrates. The hypothesis of a passiv ation of the boron doping atoms by hydrogen is discussed and we review the observations of nonhomogeneous porous layers which could be related to such a contamination problem. (C) 1998 American Institute of Physics. [S0021-89 79(98)02124-0].