Title: Structure of Native Two-dimensional Oxides on III--Nitride Surfaces,
Abstract: When pristine material surfaces are exposed to air, highly reactive broken
bonds can promote the formation of surface oxides with structures and
properties differing greatly from bulk. Determination of the oxide structure,
however, is often elusive through the use of indirect diffraction methods or
techniques that probe only the outer most layer. As a result, surface oxides
forming on widely used materials, such as group III-nitrides, have not been
unambiguously resolved, even though critical properties can depend sensitively
on their presence. In this work, aberration corrected scanning transmission
electron microscopy reveals directly, and with depth dependence, the structure
of native two--dimensional oxides that form on AlN and GaN surfaces. Through
atomic resolution imaging and spectroscopy, we show that the oxide layers are
comprised of tetrahedra--octahedra cation--oxygen units, similar to bulk
$\theta$--Al$_2$O$_3$ and $\beta$--Ga$_2$O$_3$. By applying density functional
theory, we show that the observed structures are more stable than previously
proposed surface oxide models. We place the impact of these observations in the
context of key III-nitride growth, device issues, and the recent discovery of
two-dimnesional nitrides.