Optical Detection of Surface States in Ge

Measurements of field-effect-modulated optical absorption are presented for real surfaces of germanium. The spectra are taken for various values of the surface potential. They show a narrow peak at 0.438 eV (half-width{~}0.01 eV) having maximum amplitude for a surface reduced potential uₛ₀1.8 and a phase (with respect to the electric field) appropriate to transitions from a surface state to the conduction band. Evidence is given that such a transition is from a level $1.8kT$ above the mid-gap to the state Γ_2^' of the conduction band. A much broader band is also found at smaller energies (0.39 eV); bands due to free-hole and free-electron absorption are also evident.