Kinetic Model for Ternary III-Nitride Epitaxy: The Role of Vertical Segregation on Phase Separation

Phase separation of indium-bearing III-nitrides is difficult to control and is a major challenge in the development of these materials. On the other hand, this same process does not occur naturally for aluminum gallium nitride but has been leveraged to enable the synthesis of extremely coherent superlattice structures that may lead to interesting applications for AlGaN. This work proposes that phase separation in III-nitride alloys can be attributed to at least four distinct surface-driven mechanisms─thermal decomposition, lateral cation segregation, vertical cation segregation (VCS), and preferential cation incorporation─more than bulk-diffusion-enabled spinodal decomposition. An open source, self-consistent model has been developed to describe the evolution of surface adlayers during the growth of these III-nitride films. This model consists of a system of coupled differential equations which stochastically represent physical mechanisms acting on surface adatoms, and it is used to evaluate the influence of VCS and preferential cation incorporation on self-assembled superlattice formation in AlGaN to both further the understanding of phase separation and test the validity of surface-driven phase separation. Neither preferential incorporation nor VCS alone can explain the experimentally observed composition profiles. However, by combining these two effects, composition profiles similar to those of experimentally grown self-assembled superlattices have been obtained.