Production and evolution of composition, morphology, and luminescence of microcrystalline arsenic oxides produced during the anodic processing of (100) GaAs

GaAs when exposed to a 7 V anodic bias in aqueous HCl, forms pitted structures from which visible photoluminescence has been observed. Previous work in our laboratory identified the source of the luminescence to be arsenic oxide microcrystals, formed during the electrochemical oxidation, which evolve in composition and morphology during the course of the anodic processing. The density and size of pits is dependent on the solution conditions as well as the applied potential program (stepped vs swept to 7 V. At early times (t < 30 min) the pits are composed of a mixture of small (similar to 1 mu m) and larger (10's of mu m) faceted microcrystals identified by x-ray photoelectron spectroscopy and energy-dispersive x-ray analysis to be a combination of As(III) and As(V) oxides. At longer times As(III) oxides predominate, and the smaller microcrystals are no longer observed, suggesting that the smaller microcrystals are As(V) oxide, and that they evolve chemically into As(III) oxide over the course of the oxidation. A suitable reaction pathway which explains the observed predominance of As(III) species is suggested. The luminescence properties of these electrochemically produced structures were investigated by near-field excitation. The luminescent properties evolve in parallel with the morphology and composition although the form of the spectrum is invariant in time and consistent with that obtained from bulk As2O3 and As2O5. The similarity of emission obtained from the two types of microcrystals suggests an impurity-related origin. (C) 1999 American Institute of Physics. [S0021-8979(99)04221-8].