Intrinsic and Defect-Induced Surface States of Cleaved GaAs(110)

Photoemission-yield-spectroscopy measurements are reported for a set of cleaved $n$-GaAs(110) surfaces, clean or gradually oxygen covered. Depending on the cleavage, the Fermi level at the clean surface is observed at the bulk position or toward midgap. For the first time, the existence of surface states in the gap, close to the conduction band is directly demonstrated when the density of the defect-induced surface states found near the valence-band maximum is small.