The structural properties and surface morphology of AlN epitaxial layers grown by ammonia (NH3) and plasma-assisted (PA) molecular beam epitaxy (MBE) at different growth conditions on (0001) sapphire were investigated. The lowest RMS roughness of ∼0.7 nm was achieved for the sample grown by NH3 MBE at a substrate temperature of 1085 °C and NH3 flow of 100 standard cm3 min−1. Atomic force microscopy measurements demonstrated a terrace-monolayer step-like surface morphology. Furthermore, the optimal substrate temperature for growth of GaN and AlGaN layers was determined from analysis of the GaN thermal decomposition rate. Using the optimized growth conditions, high electron mobility transistor heterostructures were grown by NH3 MBE on different types of AlN nucleation layer deposited by NH3 MBE or PA MBE. The grown heterostructures demonstrated comparable two-dimensional electron gas (2DEG) properties. The maximum 2DEG mobility of ∼2000  cm2 V–1 s–1) at a 2DEG density of ∼1.17 × 1013 cm−2 was achieved for the heterostructure with a PA MBE-grown AlN nucleation layer. The obtained results demonstrate the possibility of successful combination of different epitaxial approaches within a single growth process, which will contribute to the development of a new type of hybrid epitaxy that exploits the advantages of several technologies.

A series of double heterojunction AlGaN/GaN/AlGaN high electron mobility transistor (HEMT) heterostructures are grown by ammonia MBE on sapphire substrates using AlN buffer layers with varying thicknesses and growth conditions. The grown heterostructures are investigated by atomic force microscopy as well as photoluminescence (PL), stimulated emission (SE), and photoreflectance (PR) measurements. It is shown that PL intensity cannot be used as a reliable criterion for quality control of HEMT heterostructures because of an influence of surface roughness. A new method of quality control of the active GaN layer in HEMTs is proposed based on determination of the SE threshold excitation level. An advantage of the method is that no reference sample is needed for comparison of different HEMT structures. Internal electric field strength values are determined from PR spectra and their correlation with 2DEG density is demonstrated.

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The growth rate evolution versus V/III ratio and substrate temperature was studied by means of optical reflectivity during MBE of GaN layers using NH 3 as nitrogen source. The GaN desorption becomes observable at temperatures above 800 ° C and causes the reduction of growth rate accompanied with the surface roughening at temperatures above 850-870 ° C. Unlike GaAs, which evaporates in accordance with the action mass law, the desorption rate of GaN is found to be almost independent of V/III ratio within the N-rich growth conditions. The activation energy for GaN desorption during the growth is found to be (3.2 ± 0.1) eV. This value is very close to the activation energy for free evaporation. At V/III ratio values exceeding 200 the GaN growth rate reduction caused by violation of the molecular flow regime is observed. The Mg-doped samples grown under these extreme conditions tend to have improved acceptor activation and thus p-type conductivity.