In situ optical spectroscopy of Ga dimers on GaP, GaAs and GaSb by surface chemical modulation

Abstract

The optical characterization of the surface anisotropy of three Ga-based semiconductor compounds using an in situ technique based on the chemical modulation of the surface is described. In this technique, the anisotropic optical reflectivity is modulated by a periodic variation of the surface stoichiometry using valved pulsed cells for group-V elements (As, P, Sb). The substrate is maintained at sufficiently high temperature in order to obtain rapid desorption of group-V molecules from the surface during flux interruptions. Linearly polarized light, reflected at near normal incidence by the sample, is collected separately along one of the two principal axes of the crystal, [110] and [11¯0]. The change in the surface coverage induces a change in the intensity of the reflected light, and the normalized variation ΔR/R is recorded 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 binary 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 occupied Ga dimer and the unoccupied dangling bond bands, at characteristic energies in each material.