Structure of an ultrathin aluminum oxide layer grown on a NiAl(110) substrate

Ultrathin Al-oxide layers were grown on clean NiAl(110) substrates by a usual two-step technique and analyzed in situ by high-resolution medium energy ion scattering (MEIS) combined with photoelectron spectroscopy using synchrotron radiation light. It is found that the oxide layer comprises four atomic layers of alternating O/Al planes and the surface is terminated unambiguously with an oxygen layer. The MEIS spectra observed show that the oxide layer takes a four-layered structure of O(1.35 +/- 0.07)/Al(1.2 +/- 0.05)/O(1.25 +/- 0.1)/Al(0.95 +/- 0.1)/NiAl(110) in unit of 10(15) at./cm(2). This supports a reduced corundum structure rather than stoichiometric gamma-Al(2)O(3)(111) and k- Al(2)O(3)(001). The observed Al 2p(1/2,3/2) core level spectra consist of three components with binding energies (Al 2p(3/2)) of 72.53, 73.55, and 74.75 eV, which are assigned to the bulk NiAl, the fourth layer Al on top of the bulk NiAl(110), and the second layer Al sandwiched between the top and third layers oxygen, respectively. The component with the highest binding energy for the as-grown (oxidized at 400 degrees C) film shifts to lower energy side by 0.4 eV after subsequent annealing at 900 degrees C, indicating that the annealed surface is reduced and the Al-O bond length is extended significantly.