Competitive Formation of Ultra-thin Alumina Films at the Fe_0.85Al_0.15(110) Surface

Using surface sensitive techniques (photoemission, low-energy electron diffraction, and scanning tunneling microscopy), the present work reveals the competitive formation upon oxidation of two epitaxial oxide bilayer films of self-limited thickness on the surface of the ferritic random A2 body-centered alloy Fe_0.85Al_0.15(110). When oxidizing the substrate at 1193 K, a film (herein labeled oxide-A) similar to that studied in depth at NiAl(110) [Kresse et al., Science 2005, 308, 1440] is found. At a slightly lower annealing temperature (1073 K), alumina patches segregated from the bulk act as seeds for the growth of a new long-range ordered alumina film (oxide-B) with two domains having a ∼(23 × 23) Ųhexagonal rotated unit cell. While showing different anion/cation chemical environments, the two films have a Al₂O_2.5±0.2stoichiometry and stand on an Al-enriched subsurface with a similar ∼3 nm deep segregation profile. Most importantly, thermal treatments show that the new structure B is more stable than A. This finding conflicts with the apparent ubiquity of oxide-A that has been observed on many substrates of various symmetries and compositions. This competitive formation of ultra-thin alumina oxides questions the origin of their structural (di)similarity and the actual role of these seeds in the transition toward thicker alumina films at higher pressure.