The role of aluminum in the formation of macropores, microtubes of silicon and nickel made from porous silicon

Silicon tubes with small dimensions show to be a suitable material for sensors application as well as to other areas of nanotechnology, but its production is not trivial, because of the influence of different formation parameters and the most important of them could be pointed out to the substrate pre-conditioning. In this paper is reported the investigation about the influence of aluminum treatment of crystalline substrates upon the electrical and morphological features of macroporous silicon layers, as well as upon the microtubes yielded by selective corrosion in etchant based in ammonium fluoride solution mixed with nickel ions, as function of the sintering time. The results point out that solely for sample sintered for 4.0 hours nickel microtubes are formed, whereas for other sintered times, silicon microtubes formation are achieved. This fact is strongly linked to the amount of aluminum and oxygen diffused inside the crystalline silicon substrate, which remains even after pores formation. These diffused impurities promote strong chemical modification of the substrate, reason by which its open circuit potential is sharply shifted toward more negative regions. The Tafel plot shows a highest increment of the cathodic current of this sample, which is compatible with the sharp shift of the open circuit potential and enhancement of the exchange anodic current giving rise to the condition for the formation of nickel microtubes through the substitution of silicon atoms by nickel owing to the galvanic displacement effect. The effect of Al-treatment upon the pore's morphology of macroporous layers are shown, as well as the possible mechanism for the formation of nickel microtubes from these macroporous structures are also discussed in this paper.