Study of GaAs(001) surfaces treated in aqueous HCl solutions

Chemically treated GaAs(001) surfaces in aqueous HCl (0.36≤ x ≤36 wt%) solutions at 20° C have been studied using spectroellipsometry (SE), ex situ atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and contact-angle measurement techniques. The SE data clearly indicate that the solutions cause the removal of the native oxide film. When the native oxide film is etch-removed, the resulting surface is still rough (∼8 Å). Longer exposure to the solutions with x ≤12 wt% results in surface roughening; the thickness of the roughened layer immersed in the solution with x =3.6 wt% for t =100 min, for example, is ∼60 Å, about twice as large as the AFM rms value (∼27 Å); the difference is due to the SE technique being sensitive to both the surface microroughness and the adsorbed chemical species. By contrast, the concentrated HCl (36 wt%) etching provides a nearly flat (AFM rms of ∼7 Å), Cl-terminated surface even after considerable etching. The XPS spectra clearly indicate the presence of chlorine on the HCl-treated surfaces. The HCl-cleaned GaAs surfaces are also found to be highly hydrophobic.