Growth mode modulation and crystalline quality improvement of highly relaxed n-Al0.6Ga0.4N on high-temperature-annealing AlN/sapphire template via SiH4-pretreatment

•The effect of SiH4-pretreatment on growing n-Al0.6Ga0.4N is investigated.•The 3D to 2D growth model transformation and strain relaxation during initial growth is facilitated with broken coherency and fresh-born dislocations.•The strain relaxation is almost completely contributed by dislocation inclination.•The growth of thick n-Al0.6Ga0.4N on HTA-AlN regrown layer is improved by SiH4-pretreatment.