Decreasing density of grown-in dislocations in AlN/$c$-sapphire templates grown by plasma-activated molecular beam epitaxy

Multiple-crystal X-ray diffraction and a multi-beam optical stress sensor were used to study AlN/$c$-sapphire templates grown by plasma-assisted molecular beam epitaxy. The influence of the nucleation and buffer layers growth regimes, temperature, the ratio between Al and N* growth fluxes on the stress generation and the character of the dislocation structure were analyzed. Templates with the best crystal quality with screw and edge threading dislocation densities in a range of 4 $\cdot$ 10$^{8}$ и 8 $\cdot$ 10$^{9}$ cm$^{-2}$, respectively, were obtained at the flux ratio of Al to N$^*$ close to 1 by using two-stage temperature regimes.