Chemical preparation of GaAs surfaces and their characterization by Auger electron and x-ray photoemission spectroscopies

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Abstract
Surface regions of GaAs crystals treated by removal of anodically grown oxide layers with aqueous HCl and NH4OH solutions were investigated with Auger electron and x-ray photoemission spectroscopies. After treatment and air exposure, the surfaces were covered with about 10 Å of oxide consisting of Ga2O3 and As2O3. Semiquantitative estimates of the surface compositions revealed that after removal of the anodic oxide with HCl etch, the surfaces contained excess As, up to 10 Å of carbon contamination, and small quantities of S, Cl, Ca, and N. The excess As originates from a pileup of elemental As at the oxide/GaAs interface during oxide growth. The As-rich surface is highly reactive and readsorbs contaminants from the air and/or the etch-rinsing solutions. By contrast, after removal of the anodic oxide with NH4OH etch, the surfaces contained much less excess As, carbon, and contaminants. The most reproducible results were obtained by following a procedure developed in this work which produced surfaces containing only about 0.5 Å carbon and no detectable excess As or contaminants. The procedure involves a thick anodic oxide growth and NH4OH etch to remove contaminants, followed by a light anodic oxidation and NH4OH etch for eliminating the excess As. A charged vacancy model for anodic oxide growth is advanced which accounts for the chemical composition throughout the anodic oxide. The model for the accumulation of excess As at the oxide/GaAs interface indicates that this pileup is difficult to avoid and may contribute to the band bending and stabilization of the GaAs surface potential near midgap.

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