Direct correlations between XPS analyses and growth film by chronopotentiometry on InP in liquid ammonia (−55 °C)

This paper is based on the understanding of the formation of a reproducible polyphosphazene-like film ([(H2N)PN]n) obtained on InP by anodic treatment in liquid ammonia. The approach is innovative as it combines indications from the coulometric charges and the related chemical information from XPS analyses. Anodic charges are accurately monitored by galvanostatic treatment between 0.05 mC cm−2 and 12.5 mC cm−2. XPS investigation of the treated surfaces demonstrates the presence of an anodic film on InP. Whatever the spent charge, the specific P2p and N1s signals agree with the growth of an ultrathin phosphazene layer. From 0.25 mC cm−2 to 12.5 mC cm−2, a quasi constant XPS response is revealed without thickening of the film. However a gradual chemical evolution of the modified surface is clearly observed for the lower anodic charges (from 0.04 mC cm−2 to 0.5 mC cm−2). In this case, the surface is entirely recovered by the film as soon as 0.25 mC cm−2 is consumed at the interface. Same atomic surface ratios are indeed revealed indicating that a constant chemical composition is consistent with a polyphosphazene film. On the basis of atomic surface ratios evolutions determined by XPS, a mechanism of the film growth is deduced. It requires a nucleation step which is followed by a phosphazene coalescence phenomenon in the two dimensions of the surface. A final phosphazene monolayer film is suggested if a sufficient anodic charge spent at the interface is considered, allowing a quantitative discussion related to electrochemical and XPS data.