A high-resolution, analytical study of the anodic film formed on GaAs in a tungstate electrolyte

The anodic film formed in aqueous tungstate electrolyte at 2.5 A m(-2), to about 295 nm thickness, on n(+)-type GaAs at high faradaic efficiency, about 94%, has been examined by analytical transmission electron microscopy, using ultramicrotomed film sections, Rutherford backscattering spectroscopy, x-ray photoelectron spectroscopy, electron probe micro-analysis and scanning electron microscopy. The film is revealed to be amorphous and to comprise a uniform distribution of units of Ga2O3 and As2O3 across the main film thickness, with possible gallium enrichment in the outermost 10 nm or so of the film. Gallium and arsenic are incorporated into the anodic film at the alloy/film interface in the substrate proportions, without development of a layer enriched either in gallium or in arsenic just beneath the anodic film. The formation ratio for the film is about 2.01 nm V-1. The film, formed by migration both of cations and of anions across its thickness, is enriched in arsenic relative to the substrate composition, the level of enrichment suggesting that Ga3+ ions migrate outwards in the film about 2.4 times faster than do As3+ ions, based on a cation transport number of 0.2. The Ga3+ ions may be ejected, to the electrolyte, under the electric field, on reaching the film/electrolyte interface, with limited formation of an outer layer of essentially Ga2O3 at the film/electrolyte interface, or form a layer of Ga2O3, up to about 10% of the total film thickness, which is thinned after anodizing by exposure to the electrolyte and the rinse water. Significantly, the outer layer of film material developed by the faster migrating Ga3+ ions prevents loss of As3+ ions from the film during film growth. However, during prolonged exposure to aqueous conditions in the absence of the field, the film becomes enriched in gallium species due to preferential dissolution of arsenic species. The high faradaic efficiency of film growth is a consequence of the presence of a tungsten-enriched layer, probably a gel composed of hydrated tungsten oxide, which develops at the film/electrolyte interface during film growth. No significant presence of tungsten species within the bulk of the anodic film is detected.