THE ELECTROCHEMICAL NATURE OF GAAS IN ACIDS - A COMPARISON WITH INP

The chemical composition of surface oxides grown on GaAs was investigated using x-ray diffraction techniques. Native oxides were prepared by exposing the GaAs to HCl and HNO3 etchants at concentrations ranging from pH 5 to the maximum acidities available. A mixture of crystalline stoichiometric phases were formed. The chemical oxides appear to be more complex than indicated by previous studies by x-ray photoelectron spectroscopy, but were independent of the type of etchant or its acidity. Only constituent oxides, rather than mixed oxides, were found on the oxidized GaAs surface. The thermodynamics may play a key role in the reaction pathway. In the HCl etch, Ga(I) and Ga(I)-Ga(III) chloro compounds are not formed, and the dissolution of GaAs is severely inhibited even in the most acidic solutions under illumination. This complete absence of nucleophilic attack of the Ga-As bond by a Cl- ion is reasonably well explained by the fact that the intermediate, GaCl, is not thermodynamically stable.