Temperature-Induced Four-Fold-on-Six-Fold Symmetric Heteroepitaxy, Rocksalt SmN on Hexagonal AlN

We report on the occurrence of rotational domains involving 4-fold symmetric epitaxy on a hexagonal net, a rare case in the literature. A temperature-driven crossover from fully (111)- to (001)-oriented samarium nitride grown by molecular beam epitaxy on hexagonal (0001) aluminum nitride is observed by means of in situ reflection high energy electron diffraction, scanning tunnelling microscopy studies, and ex situ X-ray diffraction. Using an especially rich set of growth conditions and nitrogen precursors, we observe the key role played by the growth kinetics, with a strong thermal cracking of the ammonia source and a re-evaporation of Sm adatoms occurring in the same temperature range of the orientation crossover.