Abstract
The spectral response of photoconductivity of clean, cleaved silicon surfaces shows at 80°K a broad shoulder for photon energies lower than the band gap. This shoulder exhibits a sequence of minima, equally spaced with 59 meV. From the spectral dependence of photoresponse and the energetic position of the minima, it is concluded that the photoconductance of the clean, cleaved surface is caused by indirect transitions from the valence band to surface states 0.5 eV above the valence-band edge.