Preparation of a well ordered iron oxide on Cu(1 1 0)

Abstract

We have grown an ultra-thin film of iron oxide by oxidising a smooth epitaxial 5 ML Fe film which was deposited on Cu(1 1 0). In order to prevent roughening of the Fe film, the oxidation of the iron took place at 130 K. At low oxygen exposures, <2×10 −6 mbar s, a surface oxide forms. After completion of the surface oxide layer a disordered Fe 2O 3 film grows. Upon annealing, ordering of the oxide film is observed at temperatures above 400 K. A Fe 2O 3 film with a very well ordered surface could be produced by heating the disordered oxide film. The ordered oxide surface shows a (21×6) reconstruction, which is interpreted as a coincidence lattice of the Cu(1 1 0) substrate and a Fe 2O 3(1 1 1) lattice which is slightly distorted due to a misfit with the substrate (0.7–2%) and to a misalignment between the major crystallographic axes of the lattices. When the oxide film is slowly heated, decomposition of the oxide takes place in two stages: Between 550 and 700 K, the Fe 2O 3 film decomposes due to a reaction of this oxide with a buried layer of Fe or FeO. Between 700 and 800 K a further decomposition of the Fe 2O 3 is found, but now accompanied by desorption of oxygen and solution of Fe in the Cu substrate. The amount of oxygen and iron on the surface keeps on decreasing after the almost complete decomposition of Fe 2O 3 (at 800 K) until a new phase is obtained which is stable between 950 and 1100 K. This high temperature phase ( T>950 K) shows a ( n×8) reconstruction with n=17–19. This reconstruction is interpreted as the result of a coincidence lattice of the Cu(1 1 0) substrate with a distorted FeO(1 1 1) lattice.