Role of oxygen bubble generation in anodic film growth on InP

Abstract

The morphologies of barrier-type anodic films grown on InP have been investigated using atomic force microscopy, scanning electron microscopy and transmission electron microscopy in order to gain further understanding of the non-linear voltage–time behaviour and the development of blisters in the films. The films, which are two layered, with an outer layer of In 2O 3 and an inner phosphorus-rich layer, containing also indium species, were formed at 50 A m −2 in aqueous 0.1 M sodium tungstate electrolyte at 298 K. During the early stages of anodizing, i.e. within the initial region of linear voltage–time response, a relatively compact and uniform film forms, with a comparatively featureless surface. However, as the voltage increases, cavities develop within the film, and in places the anodic film material detaches partially from the substrate. The film surface then discloses fine-textured roughness and coarse protuberances associated with cavity formation and detachment respectively. Such transformation of the film morphology correlates with a decrease in slope of the voltage–time response, which commences at a formation voltage of approximately 17 V, with a film thickness of about 25 nm. The behaviour is explained by generation of oxygen within the anodic film material, which forms high pressure bubbles of gas. Such gas formation is related to the presence of an In 2O 3 outer layer of the film and the presence of units of In 2O 3 in the inner layer of the film, which facilitate oxidation of O 2− ions of the film material.