Anodic film growth on InP in sodium tungstate

Anodic film growth on InP and the wider implications to alloys have been examined, with the aim of probing film composition to gain insight into the ionic transport processes responsible for duplex film formation. For anodizing at relatively high efficiency, the anodic film reveals an outer indium-rich layer, essentially free of phosphorus species, and an inner layer containing both indium and phosphorus species. Growth of such films, on a co-operative transport basis, leading to amorphous film formation, can be explained by the faster outward migration of indium species relative to phosphorus species. The inner layer would then be composed of appropriately arranged In–O and P–O units. However, such an arrangement is difficult to reconcile with the X-ray photoelectron spectroscopy data, which suggest the presence of phosphorus in the form of P 2O 5, PO 4 3−, PO 3 − and polyphosphate species. In order to explain fully the transport process for film growth and to understand film composition, further studies using marker and tracer experiments are essential.