Formation of aluminum oxide thin films on FeAl(1 1 0) studied by STM

The surface morphology and atomic structure of clean and oxidized FeAl(1 1 0) surfaces have been investigated with scanning tunneling microscopy (STM). An incommensurate reconstructed structure, having FeAl2 stoichiometry confined to the outmost layer, is observed on the clean surface due to preferential Al segregation upon annealing to 1125 K. When the reconstructed clean surface is exposed to oxygen at elevated temperatures, an ordered ultra-thin aluminum oxide film is formed. Based on STM data, a structural model of the oxide film is proposed, which exhibits a quasi-hexagonal oxygen layer and accommodates an even mix of octahedral and tetrahedral occupancy of Al ions arranged in an alternating zigzag–stripe structure. STM imaging with tunnel voltages in the range of the bulk band gap implies that the thin film oxide electronic structure differs substantially from the bulk oxide, and indicates a local density of states around the oxide constituents within the bulk band gap.