Influence of Current Density on the Composition of GaAs Anodic Oxide Films

Abstract

By using various current densities from ∼0.1 to ∼1 mA.cm−2, oxide films, up to 210 nm thick, were grown by anodic oxidation of substrates in a basic organic electrolyte consisting of diethylene glycol, sodium triphosphate, and water. The composition of such films was analyzed by low energy noble gas ion scattering spectrometry (ISS). It is shown that bulk oxide composition and interface width depend on current densities and thicknesses. For oxidations at ∼1 mA.cm−2 the surface of the oxide is Ga enriched and As depleted. The interface has a graded composition; its width (up to 40 nm for oxide 210 nm thick) depends on oxide thickness. When low current densities (∼0.1 mA.cm−2) are used, the whole oxide is homogeneous and the interface is abrupt (5 nm for oxide 100 nm thick). Independent of current density, uniform and constant bulk As concentration is found. Variation of Ga concentration as a function of oxide thickness and current density has been observed. A model for oxidation in terms of probability of movement for As, Ga, and O ions is suggested and discussed. Depth resolution of ISS analysis method is estimated (5 nm) and secondary effects such as ion milling inhomogeneities, preferential sputtering, surface contamination, and incorporation are discussed.