Optical second-harmonic generation from GaAs(100) surfaces: The influence of H₂

The interaction of H₂ with GaAs(100) surfaces has been monitored using optical second-harmonic generation (SHG) excited using an Nd:YAG laser operating on the sub-bandgap fundamental line at 1064 nm. The SHG response has been recorded both before and after treatment of the substrate in H₂ to remove contamination and under a high overpressure of H₂ (10 mbar). It is found that the polar SHG response from a GaAs(100) surface contaminated with carbon and oxygen is essentially structureless apart from the appearance of a peak in SHG intensity when the incident electric vector is aligned with a (110) azimuth. After cleaning in H₂, the polar SHG intensity response is both more intense and more structured, showing maxima corresponding to the situation in which the incident electric vector is aligned with both the (110) and (-110) azimuths. No further changes are observed in the polar SHG response when measurements are made with the GaAs(100) surface under 10 mbar H₂. While these data are preliminary and may not be directly interpreted in terms of a particular two-dimensional surface overlayer, they illustrate the high surface SHG response obtainable from GaAs(100), which is discussed in terms of the presence of a highly non-linear surface layer.