High-resolution photoemission yield and surface states in semiconductors

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Accurate measurements of the total photoemission yield, down to the 10−11 electron per incident photon range with a resolution in energy better than 20 meV, bring new information, compared to other methods, on surface states in the gap and the upper part (∼1 eV) of the valence band of semiconductors. It is shown that the first derivative, with respect to energy, of the yield curves gives a good picture of the density of states and that experiments on differently doped samples allow us to distinguish between surface and bulk state effects. Some results obtained on cleaved Si(111), Ge(111) and GaAs(110) surfaces are discussed. In particular, on the clean surface, detailed information is obtained on intrinsic surface states and on the effect of linear defects such as steps and even of surface point defects. In the case of GaAs(110), it is experimentally demonstrated that the changes as a function of energy of the escape function for electrons excited from surface states can be neglected. Interaction with gases such as O2 or H2 brings information on sticking coefficients and allows the discussion of the influence of transition matrix elements.