Resonant photoemission from surface states in GaP

Enhanced electron-emission yield was observed in GaP(110)-cleaved crystals, by constant-final-state, constant-initial-state and energy-distribution-curve spectroscopies at photon energies which excite transitions from the Ga $3d$ levels to empty surface states. We prove that the resonance is mainly due to direct recombination involving filled surface states ≃ -2 eV below the top of the valence band. We observed an inversion of the intensity ratio between Ga $3d_{\frac{5}{2}}$ and $3d_{\frac{3}{2}}$ with respect to the statistical value due to the contribution of localized states to the optical transitions. In particular, different intensity ratios in constant-initial-state spectra at different initial-state energies have been interpreted as being due to different localization of the filled states participating to the resonant process.