Aluminum Phase Segregation Effect in Growing an AlGaN Nanorod with the Self-catalytic Vapor-liquid-solid Mode

By using the self-catalytic pulsed growth technique of MOCVD, we grow an AlGaN/GaN multi-shell structure on a GaN NR. Thin AlGaN shells separated by GaN layers are formed. Ga source is supplied in the first half-period of a growth cycle for forming a Ga droplet at the top of a GaN NR. N and Al sources are then supplied in the second half-period. Although Al can be absorbed by the melted Ga droplet for precipitation, a GaN layer is first deposited because of the lower chemical potential for GaN nucleation, when compared with AIN. AlGaN is not precipitated until the chemical potential for AlN nucleation reaches an equilibrium condition when the Ga droplet is almost used out. Because the Al adatoms on the NR sidewalls slow down the upward migration of Ga adatoms, the Ga droplet becomes smaller along growth cycle leading to the decreasing GaN shell-layer thickness until a steady state is reached. As the coverage of the Ga droplet on a slant facet becomes smaller, the area of (1-102)-plane decreases and eventually the whole slant facet turns into the (1-101)-plane. Formulations are built for theoretically modelling and numerically evaluating the Al phase segregation behaviors.