VIBRATIONAL AND COLLECTIVE EXCITATIONS OF THE CS/GAAS(110) INTERFACE

We present a high-resolution electron-energy-loss analysis of the interface system Cs/GaAs(110) grown at room temperature. We study the development of the GaAs collective excitation features (Fuchs-Kliewer surface phonon and plasmon of the dopant-induced free carriers) as a function of the alkali-metal deposition. An energy-loss structure at about 8 meV is inferred by a detailed data analysis, that we attribute to a Cs-induced excitation. By using an appropriate dielectric model applied to our data, Cs does not form a uniform metallic layer at saturation coverage, while it cannot be ruled out that it forms metallic clusters of a few tens of angstrom in size. Cs deposition is known to produce a band bending at the (I 10) surface of GaAs. Using a semiclassical model for interpreting the energy and intensity modifications of the plasmon and phonon modes, we also deduced the widening of the depletion layer due to the adsorbate. We then obtained a band bending of 0.73 eV for the n-type doped substrate, in good agreement with previous results. We remark that this indirect method for determining the band bending is not affected by any surface photovoltage effects.