Oxygen-Induced Franz-Keldysh Effect and Surface States on GaAs (110) Surfaces in Ellipsometry

On (110) GaAs surfaces cleaved in ultrahigh vacuum the changes of the ellipsometric angles $\delta \psi$ and $\delta \Delta$ induced by oxygen adsorption have been measured for photon energies $1.5<\mathrm{}\hslash \omega <3.5\mathrm{}$ eV. The main spectral structure is explained as due to the Franz-Keldysh effect at critical points of the bulk band structure induced by a change of the band bending. From a comparison with electroreflectance data information can be obtained about band-bending changes and about surface states.