Ultra-high vacuum deposition of sodium on indium monoselenide: intercalation or chemical reaction?

A X-ray photoelectron spectroscopy (XPS) study of the behaviour of sodium films deposited in ultra-high vacuum on InSe single-crystal or thin epitaxial films at room temperature and upon moderate annealings (up to 475 K) is reported. Major changes affect the substrate spectra, in particular indium, and clearly show the influence of the substrate morphology. For the InSe single-crystal substrate, annealing tends to restore the initial InSe surface as shown by XPS and RHEED, while for InSe films, no restoration occurs. In both cases, sodium has a partial ionic character and two phases are evidenced: one, [NaIn⁰Se_β], where indium is close to metal or covalent indium, and the other, Na_'[InSe], where indium is more ionic close to InSe. The results emphasize that interdiffusion and fast chemical reaction predominate for the thin film substrate; for the bulk substrate, interfacial reaction and surface diffusion are competitive.