Surface photovoltage spectra and surface states of GaAs(100) with submonolayers of Cu and Pd

Surface photovoltage spectra of GaAs(100) were measured after thermal cleaning of the semiconductor surface in ultrahigh vacuum and after the deposition of thin (d = 0.05−4 A) layers of Pd and Cu. The long-wavelength part (hv < Eg) of the photovoltage spectra was found to be determined by photo-ionization of surface states. The deposition of metal atoms onto the clean GaAs surface greatly increased the photoresponse in this part of the photovoltage curves. This pointed out the appearance of new surface states caused by the adsorption of Cu and Pd. The ionization energy of these surface states and its dependence on the metal coverage were determined.