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°C, and the second process consisted of an annealing at 530°C. The pressure variations in the UHV chambers recorded during both thermal treatments showed a behavior related to the removal of As- and Ga-oxides from the substrate surface. The thermally treated GaAs (100) substrates were analyzed by in-situ reflection high-energy electron diffraction (RHEED), Auger electron spectroscopy, and ex-situ atomic force microscopy (AFM). The samples before 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 oxide desorption process. The pits have a density in the order of 109/cm2, and some are as deep as 120 Å. We explain the pits formation mechanism 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°C, which coincides with the desorption of the most unstable oxides, presumably As-oxides.

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