Chemical and photochemical processes in sulfide passivation of GaAs(100): In situ optical study and photoemission analysis

We demonstrate a new approach to study semiconductor surface passivation. This approach, which we have applied to the case of GaAs(100) passivation by sodium sulfide water solutions, consists of using both reflectance anisotropy spectroscopy during passivation and ultraviolet photoemission spectroscopy after removing the sample from the solution. We find that (i) complete chemical passivation requires a treatment duration significantly longer than what was used in previous works; in our case, the oxygen contamination is strongly reduced; (ii) photochemical processes play an important role for improving the surface electronic properties. As a result, photoemission spectroscopy, which uses powerful light excitation, may strongly alter the chemical bonds. From experimental evidence and using a chemical model based on first principles estimates, we propose a detailed description both of the surface chemical reactions that occur during passivation and of the chemical bonds that are formed at the semiconductor surface and in the passivating overlayer.