Study of the GaAs surface oxide desorption process by annealing in ultra high vacuum conditions

We have studied the desorption mechanism of Ga- and As-oxides on GaAs (100) by subjecting the substrates to two thermal processes in ultra high vacuum (UHV) conditions. The first process was an outgassing at 350 degrees C, an d the second process consisted of an annealing at 530 degrees C. The pressu re variations in the UHV chambers recorded during both thel mal treatments showed a behavior related to the removal of As- and Ga-oxides from the subs trate surface. The thermally treated GaAs(100) substrates were analyzed by in-situ reflection high-energy electron diffract ion (RHEED), Auger electro n spectroscopy, and ex-situ atomic force microscopy (AFM). The samples befo re the oxide removal showed a diffuse RHEED image with intense background, characteristic of an amorphous material. In contrast the RHEED patterns of substrates after the oxide desorption process were streaky and well defined indicating a high quality surface. AFM images clearly showed the presence of surface pits on the GaAs(100) samples exposed to the high temperature ox ide desorption process. The pits have a density in the order of 10(9)/cm(2) , and some are as deep as 120 Angstrom. We explain the pits formation mecha nism in terms of chemical reactions of the surface oxides with the elements of the substrate. We found that pits are generated at temperatures as low as 350 degrees C, which coincides with the desorption of the most unstable oxides, presumably As-oxides.