Desorption kinetics of indium adlayers on GaN(0001): Fractional order and non-monotonic behavior

Epitaxial growth is a dynamic process and, to the first order, is governed by the nature and the rates of elementary surface kinetic mechanisms, such as adatom desorption. In compound-type growing surfaces, particularly in III-nitride molecular beam epitaxy, where the presence of a metallic surface bilayer has a catalytic role, desorption also affects the rates of other elementary mechanisms. In this study, we investigated the desorption of an indium (In) adlayer from GaN(0001) surfaces, a critical kinetic process in the epitaxy of In-containing alloys, using reflection high-energy electron diffraction, density functional theory calculations, and quasi-continuous modeling methods to reveal the underlying physical mechanisms. Our results demonstrate that while the indium bilayer desorbs in a layer-by-layer mode, the desorption mechanisms from the bottom and top monolayers differ significantly. The bottom follows a 3/4 order Polanyi–Wigner relation, attributed to contributions from two different adlayer phases. The top monolayer desorption exhibits a non-monotonic dependence on coverage. This is associated with the liquidus status of this monolayer and its continuous restructuring during desorption. These findings clarify and quantify indium desorption processes from GaN(0001) surfaces, offering insights into analogous mechanisms in other compound-type material systems.