Transmission electron microscopy investigations of the formation of macropores in n- and p-Si(001)/(111)

Morphology and interfaces of macropores in (001)- and (111)-oriented n- and p-type silicon were studied by analytical and high-resolution transmission electron microscopy (TEM) for different stages of their evolution during electrochemical etching with oxidizing and non-oxidizing electrolytes. In n-type Si(001) etched under oxidizing conditions macropores along 〈100〉 (diameters ∼1 μm) connected with dendritic pores (diameters ∼0.25 μm) are formed. The dendritic pores consist of periodic arrangements of truncated octahedral voids and oxide interface inclusions preferentially at the pore tips, indicating that pore formation proceeds in an oscillatory mode and that the pore nucleation stage is governed by oxidation and subsequent oxide dissolution at the reactive Si-electrolyte interface. At later stages of their formation the interfaces of macropores in n- and p-type Si(001) possess {111} facets, and oxide interface inclusions are absent thus indicating that {111} facets are stabilized against further dissolution. In p-Si(001) etched by non-oxidizing organic electrolytes dense arrays of Si fibers and pores with lateral dimensions of a few nanometers form at the surface and inside macropores with preferential [100] pore axes orientation. The presence of this mesoporous Si indicates strongly localized collective direct dissolution processes and a predominance of a direct dissolution mechanism for p-Si etched with a non-oxidizing electrolyte. In (111)-oriented Si wafers the macropores grow along 〈113〉 directions indicating an orientation-dependent growth mechanism.