Under-surface observation of thin-film alumina on NiAl(100) with scanning tunneling microscopy

Scanning tunneling microscopy (STM) was used to investigate the oxide structures underlying the surface of alumina thin-film grown on NiAl(100). At a bias voltage (on the sample) below 2.0 V, STM topography images of the alumina layer beneath the surface were obtained. A probe with depth of 2–8 Å was readily attained. The under-surface observation shows that the film consists of stacked layers of alumina whereas the layered alumina unnecessarily comprises entire θ-Al2O3 unit cells. The lattice orientation of the upper alumina layer differs from that of the lower one by 90° — the newly grown oxide structurally matching the horizontal oxide rather than the lower oxide. The results indicate a growth process competing with the typical mode of epitaxial growth for the growth of alumina film.

► STM images of alumina layers beneath the surface of alumina film/NiAl(100) obtained.
► A probe depth of 0.2–0.8 nm is readily attained.
► The orientation of upper layer of alumina may differ from that of lower ones by 90°.