IN-SITU OPTICAL SPECTROSCOPY OF GA DIMERS ON GAP, GAAS, AND GASB BY SURFACE CHEMICAL MODULATION

The optical characterization of the surface anisotropy of three Ga-bas ed semiconductor compounds using an in situ technique based on the che mical modulation of the surface is described. In this technique, the a nisotropic optical reflectivity is modulated by a periodic variation o f the surface stoichiometry using valved pulsed cells for group-V elem ents (As, P, Sb). The substrate is maintained at sufficiently high tem perature in order to obtain rapid desorption of group-V molecules from the surface during flux interruptions. Linearly polarized light, refl ected at near normal incidence by the sample, is collected separately along one of the two principal axes of the crystal. [110] and [110]. T he change in the surface coverage induces a change in the intensity of the reflected light, and the normalized variation Delta R/R is record ed as a function of wavelength. Spectra for (001) surfaces in the 1-3 eV range have been obtained with this method for a set of Ga-based bin ary III-V compounds GaP, GaAs, and GaSb, showing well-defined features for light polarized along the [110] direction, parallel to Ga dimers. These observed maxima are attributed to transitions between the occup ied Ga dimer and the unoccupied dangling bond bands, at characteristic energies in each material.