Dynamic interfacial processes at the amorphous silicon/aqueous electrolyte boundary

Abstract

The photoelectrochemical behaviour of amorphous hydrogenated silicon (a-Si:H) in aqueous electrolytes has been investigated and the light-induced formation of interfacial oxide films and their dissolution in fluoride-containing solutions have been studied in detail. Three distinctively different surface conditions can be prepared: (i) reduced surfaces which are free of oxidized silicon; (ii) surfaces with natural oxides (d = 1.5 run); and (iii) passivated surfaces exhibiting thick electrochemical oxide films. Photocurrent transients exhibiting a photocurrent doubling, a film formation and a passivation regime are presented. The electrodes under different surface conditions have been analysed by X-ray photoelectron spectroscopy (XPS). Evidence for a non-stoichiometric silicon-rich SiO x, layer, as observed on thermal oxides on silicon, was found. The photoelectrosynthesized oxide films incorporate substantial amounts of water and/or hydroxyl groups as demonstrated by the Ol s binding energy at E B= 533 eV. The photocurrent transients of a-Si:H are compared with those of crystalline silicon. Differences in the temporal evolution of the currents are interpreted assuming a different film growth mechanism. The electronic properties of the samples with differently conditioned surfaces have been investigated. Under reducing conditions at negative potential, a photoconductive gain effect leading to quantum yields of Q > 10 is observed. Schottky barriers have been fabricated and analysed. Generally, oxide-free, slightly fluoride-covered surfaces (≈10% of an atomic layer) show superior electronic behaviour compared to oxidized samples.