MECHANISM OF PORE FORMATION ON N-TYPE SILICON

The morphology of porous silicon formed on n-type silicon by anodic dissolution in HF solution and its relation to the current-potential characteristics are reported. An individual pore generally consists of three sections: a thin surface layer with pore size much smaller than in the bulk; an equidiameter main body; and a curved pore bottom, of which the radius of curvature is the smallest at the pore tip and increases from the tip to the wall. The i-V characteristics and the calculation of the field distribution at the interface suggest that current flow during anodic polarization is caused by interface tunneling. The electric field at the curved pore bottom, which determines the tunneling current, is greatly increased due to the effect of curvature. Due to the difference in the radius of curvature along a pore bottom, the kind of reactions and the current density there also change. The distribution of the current density along the pore bottom then determines the pore diameter as well as the spacing between the pores. The effect of potential, pH, and dopant concentration on the morphology of porous silicon are also discussed.