A HIGH-RESOLUTION, ANALYTICAL STUDY OF THE ANODIC FILM FORMED ON GAASIN 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 effici ency, about 94%, has been examined by analytical transmission electron microscopy, using ultramicrotomed film sections, Rutherford backscatt ering spectroscopy, x-ray photoelectron spectroscopy, electron probe m icro-analysis and scanning electron microscopy. The film is revealed t o be amorphous and to comprise a uniform distribution of units of Ga2O 3 and As2O3 across the main film thickness, with possible gallium enri chment in the outermost 10 nm or so of the film. Gallium and arsenic a re incorporated into the anodic film at the alloy/film interface in th e substrate proportions, without development of a layer enriched eithe r 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 migratio n both of cations and of anions across its thickness, is enriched in a rsenic 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 f ield, on reaching the film/electrolyte interface, with limited formati on of an outer layer of essentially Ga2O3 at the film/electrolyte inte rface, or form a layer of Ga2O3, up to about 10% of the total film thi ckness, which is thinned after anodizing by exposure to the electrolyt e and the rinse water. Significantly, the outer layer of film material developed by the faster migrating Ga3+ ions prevents loss of As3+ ion s from the film during film growth. However, during prolonged exposure to aqueous conditions in the absence of the field, the film becomes e nriched 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 int erface during film growth. No significant presence of tungsten species within the bulk of the anodic film is detected.