From two- to three-dimensional alumina: Interface templated films and formation of γ−Al2O3 (111) nuclei

Oxide thin films are of fundamental importance due to their applications in materials science, optics, corrosion protection, catalysis, and microelectronics. A multistep oxidation procedure is employed to precisely tune the alumina (${\mathrm{Al}}₂{\mathrm{O}}₃$) thickness on a NiAl(110) alloy from two atomic bilayers to $1.5\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$. Structural changes are analyzed with x-ray photoelectron diffraction and low-energy electron diffraction. The long-range order does not relate to any bulk structure and is imposed by the crystallized interface. The large unit cell formed at the interface persists in thicker films. In contrast, the local order changes at a thickness above $0.5\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ from the complex structure of this prelayer under the formation of subnanometer-sized $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Al}}₂{\mathrm{O}}₃$(111)-type nuclei. The band structure is monitored with angle-resolved photoelectron spectroscopy. Increasing film thickness results in a slight decrease of the work function, but does not lead to significant changes of the electronic band structure. The presented multistep procedure opens a route for the synthesis of thin film structures in general and in particular provides fundamental insight in the surface structure of spinel-based transition ${\mathrm{Al}}₂{\mathrm{O}}₃$.