Specific features of NH$_3$ and plasma-assisted MBE in the fabrication of III-N HEMT heterostructures

The specific features of how nitride HEMT heterostructures are produced by NH$_3$ and plasma-assisted (PA) molecular-beam epitaxy (MBE) are considered. It is shown that the use of high-temperature AlN/AlGaN buffer layers grown with ammonia at extremely high temperatures (up to 1150$^\circ$C) can drastically improve the structural perfection of the active GaN layers and reduce the dislocation density in these layers to values of 9$\times$10$^8$–1$\times$10$^9$ cm$^{-2}$. The use of buffer layers of this kind makes it possible to obtain high-quality GaN/AlGaN heterostructures by both methods. At the same time, in contrast to ammonia MBE which is difficult to apply at $T<$ 500$^\circ$C (because of the low efficiency of ammonia decomposition), PA MBE is rather effective at low temperatures, e.g., for the growth of InAlN layers lattice-matched with GaN. The results obtained in the MBE growth of AlN/AlGaN/GaN/InAlN heterostructures by both PA-MBE and NH$_3$-MBE with an extremely high ammonia flux are demonstrated.