Abstract
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.