Macropore Formation on p-Type Silicon

Electrochemical anodisation of p-silicon of different crystal orientations and doping densities has been investigated in acetonitrile, propylene carbonate and dimethylformamide solutions containing hydrofluoric acid. The formation of a macroporous layer in acetonitrile and propylene carbonate is observed only if the resistivity of the silicon exceeds ∼10 Ω·cm for both (1 0 0) and (1 1 1) crystal orientations whereas in dimethylformamide the macroporous layer can be formed on ∼1 Ω·cm substrates. The influence of water concentration in the electrolyte on the morphology of the macroporous layer has also been studied. The resistivity of the used electrolyte solutions was measured and compared to that of silicon. Formation of a macroporous layer is explained by the effective collection of holes at the tip of the growing macropores.