On the specific features of the plasma-assisted mbe synthesis of $n^{+}$-GaN layers on GaN/$c$-Al$_{2}$O$_{3}$ templates

The results obtained in a study of the synthesis of $n^{+}$-GaN layers by plasma-assisted molecular-beam epitaxy on GaN/$c$-Al$_{2}$O$_{3}$ templates are reported. In particular, a method is developed for the pre-epitaxial cleaning of the GaN surfaces of templates to remove foreign atoms. It is shown that, to form GaN layers of comparatively good quality, including those doped with silicon up to $\sim$4.6 $\times$ 10$^{19}$ cm$^{-3}$, GaN template surfaces should be pre-epitaxially cleaned in a flow of activated nitrogen particles, with the substrate temperature increased from $T_S$ = 400 to 600$^{\circ}$C and the substrate surface subsequently exposed to a flow of activated nitrogen at a fixed value of $T_S$ = 600$^{\circ}$C for 1 h. After that the substrate temperature should be raised to $T_S$ = 700$^{\circ}$C and the GaN surface finally cleaned by means of a procedure for gallium deposition/desorption.