  Atomic layer deposition (ALD) is an essential tool in semiconductor device
fabrication that allows the growth of ultrathin and conformal films to
precisely form heterostructures and tune interface properties. The
self-limiting nature of the chemical reactions during ALD provides excellent
control over the layer thickness. However, in contrast to idealized growth
models, it is experimentally challenging to create continuous monolayers by ALD
because surface inhomogeneities and precursor steric interactions result in
island growth during film nucleation. Thus, the ability to create pin-hole free
monolayers by ALD would offer new opportunities for controlling interfacial
charge and mass transport in semiconductor devices, as well as for tailoring
surface chemistry. Here, we report full encapsulation of c-plane gallium
nitride (GaN) with an ultimately thin (~3 {\AA}) aluminum oxide (AlOx)
monolayer, which is enabled by the partial conversion of the GaN surface oxide
into AlOx using a combination of trimethylaluminum deposition and hydrogen
plasma exposure. Introduction of monolayer AlOx significantly modifies the
physical and chemical properties of the surface, decreasing the work function
and introducing new chemical reactivity to the GaN surface. This tunable
interfacial chemistry is highlighted by the reactivity of the modified surface
with phosphonic acids under standard conditions, which results in
self-assembled monolayers with densities approaching the theoretical limit.
More broadly, the presented monolayer AlOx deposition scheme can be extended to
other dielectrics and III-V-based semiconductors, with significant relevance
for applications in optoelectronics, chemical sensing, and
(photo)electrocatalysis.
