Photoconductivity of the Si(111)-7 × 7 and √3 × √3-Ag surfaces

The photoconductivity of a Si(111) wafer was found to strongly depend on the surface superstructures on it. Positive photoconductivities were measured on the clean 7 × 7 surface with photon energies larger than the band gap energy, which was attributed to the creation of excess electron-hole pairs in the bulk. On the √3 × √3-Ag surface, however, negative photoconductivities were measured with light of the same energy range. This phenomenon was qualitatively explained as a result of a reduction of the conduction electrons in a surface-state band, which was due to an enhanced recombination rate between the surface-state electrons and the excess holes created in the surface space-charge layer by illumination.