Comparison of the Photoelectric Properties of Cleaved, Heated, and Sputtered Silicon Surfaces

The properly cleaved silicon surface exhibits highly reproducible photoelectric properties, and is known from electron diffraction results to be of high structural perfection. The surface-atom rearrangement that occurs upon heating the cleaved (111) surface to 1000°K in vacuum causes marked changes in the electrical properties of the surface. The work function of a high-resistivity p-type sample which has the energy bands flat up to the surface in the cleaved state drops from 4.8 to 4.6 eV and the photoelectric threshold drops from ∼5.15 to ∼4.6 eV. The kinetic-energy distribution of emitted electrons and the yield spectrum indicate at least two distinct groups of photoelectrons in both the cleaved and heated samples. The principal group, of lower energy, is attributed to a direct optical transition in both cases. In the heated samples, the group of high-energy electrons is further separated from the low group than in the cleaved samples, and originates from just below the Fermi level at the surface. This group is tentatively ascribed to emission from surface states that have moved up to the Fermi level in the forbidden gap. The yield spectrum for a sputtered and annealed (111) silicon surface is closely similar to that for the cleaved and heated samples, as expected from the fact that in low-energy electron diffraction both types of surfaces have the same atomic arrangement. Changes in photoemission during cleaning show that ∼95% of the photoelectrons are trapped by an oxide layer ∼20 Å thick.