THE BEWILDERING SURFACE ATTRIBUTES OF GALLIUM-ARSENIDE A CATHODOLUMINESCENCE PERSPECTIVE

We report on a surface related study of the cathodoluminescence (CL) p roperties of high purity epitaxial GaAs. The nature of the surface has been modified by chemical passivation with (NH4)2S and by low-voltage electron beam charging. We have investigated low temperature excitoni c behaviour and compared the CL response with photoluminescence (PL). CL exhibits a greater sensitivity and diversity in behaviour to surfac e treatment and excitation than does PL. The sulphidation treatment is found to increase the dark surface electric field (0.1 eV shift in th e Fermi level toward the valence band) and decrease the surface recomb ination velocity. Exciton-polariton notches correlate with the presenc e of surface oxide and level of crystal excitation. We discuss the pos sible role of microscopic roughness (scattering derived from spatial f luctuations in the dielectric constant) in this anomalous behaviour. D ifferences observed in exciton injection-level behaviour also point to a fundamental distinction between PL and CL. This distinction relates to the response of the luminescence dead layer to excitation. The rol e of voltage dependent electron yield (cross-over phenomenon) is discu ssed in this matter. The problematic nature of these optical character ization techniques are exacerbated when dealing with surface related p henomena.