Origin of the broadening of surface optical transitions of As-rich and Ga-rich GaAs(001)

We have investigated the temperature dependence of the reflectance anisotropy spectra of clean GaAs(001), with a particular emphasis on the width of the spectral features related to surface optical transitions. Fundamental differences between the characteristic lines of the Ga-rich and As-rich surfaces have been found. For the gallium-rich surface, the dependence of width of the main negative line at 2.3 eV between 90 and 700 K can be explained by the electron-phonon coupling. This width is found to be reduced by adsorption of minute amounts of cesium. For the As-rich surface, the width of the line at 3 eV weakly depends on temperature, and its value can be interpreted by dispersion in k-space.