OPTICAL 2ND-HARMONIC GENERATION STUDIES OF THE NATURE OF THE OXIDE-COVERED AND CLEAN C(4X4) AND (2X4) RECONSTRUCTED GAAS(001) SURFACES

We report the application of the ''epioptic'' technique of optical sec ond harmonic generation (SHG) to the study of surface chemical process es occurring at the technologically important (001) surfaces of the no n-centrosymmetric compound semiconductor GaAs. SHG has been used to mo nitor the thermal removal of surface contamination from an GaAs(001) s ubstrate mounted in a conventional molecular beam epitaxy (MBE) chambe r. Rotational anisotropy plots of the reflected SH signal recorded at near-normal incidence indicate that the oxide layer is not fully disor dered and possesses some structural anisotropy, i.e. some preferred ne t surface orientation. In addition the rotational anisotropy plots wer e observed to change in both intensity and apparent phase between the oxide-covered surface and the clean reconstructed surfaces. Following heating to 850 K under the background As2 flux present in the chamber, the SH signal from the clean, reconstructed surface was approximately 5 x weaker than that observed for the contaminated surface and was su bsequently found to be insensitive to the reconstruction change betwee n the (2 x 4) and c(4 x 4) surfaces, in agreement with theoretical pre dictions for surfaces having 2mm and 4mm symmetry, respectively. These data are discussed in terms of the difficulties encountered when atte mpting to observe a surface SH response from a non-centrosymmetric sub strate. The sensitivity of the technique towards the thin oxide overla yer suggests that for overlayers which are likely to possess net orien ted in-plane dipoles due to the structural mismatch with the substrate , an SH response may be observed in preference to a bulk response when a normal incidence geometry is employed.