Reshaping of 3D GaN Structures during Annealing: Phenomenological Description and Mathematical Model

In this work, the thermal decomposition of GaN three-dimensional (3D) microstructures was studied in an attempt to better understand the peculiarities of the complex growth process. Microfins with nonpolar a-plane sidewall facets have been investigated as model structures, and the influence of initial geometry and different annealing conditions in a metal–organic chemical vapor deposition reactor on the thermal reshaping has been investigated. We demonstrate that the annealing of these structures in an ammonia-containing atmosphere, which is comparable to that used during overgrowth, results in the decomposition of the c-plane top surfaces, diffusion of gallium adatoms to the a-plane sidewalls, and their reincorporation on these facets. Such behavior is attributed to the differences in thermal stability of the polar and nonpolar surfaces. By studying the thermal decomposition of microfins with varying geometry, we show that this phenomenon depends mainly on the aspect ratio of these structures. We attribute the strongly enhanced decomposition rate of high-aspect-ratio microstructures to the depletion of the gallium adlayer by adjacent crystal facets, which is the main factor discerning the growth of 3D microstructures from the planar case. Additionally, a simplified model for thermal decomposition involving the main physical mechanisms was proposed and applied to reproduce the experimentally observed behavior. The modeling procedure allows the experimental determination of the diffusion coefficients of gallium adatoms on the a-plane surface under the growth conditions applied. This work highlights the high sensitivity of 3D microstructures to thermal decomposition and offers a toolbox to understand their dependence on the geometry. A proper comprehension of thermal decomposition enables superior control of the GaN overgrowth on 3D topographies, which is a crucial approach for realizing novel device architectures relevant for power electronics or microLED displays.