Mechanism of initial structure formation on highly doped n-Si (111)

Present models for porous silicon formation require valence band holes to start the Si dissolution. They do not explain porous Si formation on highly n-doped Si in the dark where the hole concentration is negligibly small. To enlighten the corresponding mechanism, dark current–voltage characteristics for n-Si (111) in diluted NH4F solution were recorded for different dopant densities ND. The onset potentials for porous silicon formation are shifted from −0.34 to +4.37 V versus SCE if ND is decreased from 2×1019 to 2×1017 cm−3. The relative potential drop in the Helmholtz layer amounts to 0.52±0.05 V in all cases, however. Hence, the existence of an interface state 0.3–0.5 eV below the conduction band edge is concluded which penetrates some Å into the Helmholtz layer. If the potential drop in the Helmholtz layer is sufficiently high the state is shifted above the conduction band edge and facilitates electron injection into the conduction band. This step is suggested to initiate the corrosion reaction. The findings are supported by in-situ AFM investigations of a moderately and highly doped Si samples.