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 e lectrochemical etching with oxidizing and non-oxidizing electrolytes. In n- type Si(001) etched under oxidizing conditions macropores along (100) (diam eters similar to 1 mu m) connected with dendritic pores (diameters similar to 0.25 mu m) are formed. The dendritic pores consist of periodic arrangeme nts of truncated octahedral voids and oxide interface inclusions preferenti ally at the pore tips, indicating that pore formation proceeds in an oscill atory mode and that the pore nucleation stage is governed by oxidation and subsequent oxide dissolution at the reactive Si-electrolyte interface. At l ater stages of their formation the interfaces of macropores in n- and p-typ e Si(001) possess {111} facets, and oxide interface inclusions are absent t hus 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 th e surface and inside macropores with perferential [100] pore axes orientati on. The presence of this mesoporous Si indicates strongly localized collect ive direct dissolution processes and a predominance of a direct dissolution mechanism for p-Si etched with a non-oxidizing electrolyte. In (111)-orien ted Si wafers the macropores grow along [113] directions indicating an orie ntation-dependent growth mechanism. (C) 2000 Elsevier Science S.A. All righ ts reserved.