AlGaN/GaN high electron mobility transistor heterostructures grown by ammonia and combined plasma-assisted ammonia molecular beam epitaxy

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

Ultraviolet stimulated emission in AlGaN layers grown on sapphire substrates using ammonia and plasma-assisted molecular beam epitaxy

Ammonia and plasma‐assisted (PA) molecular beam epitaxy modes are used to grow AlN and AlGaN epitaxial layers on sapphire substrates. It is determined that the increase of thickness of AlN buffer layer grown by ammonia‐MBE from 0.32 μm to 1.25 μm results in the narrowing of 101 X‐Ray rocking curves whereas no clear effect on 002 X‐Ray rocking curve width is observed. It is shown that strong GaN decomposition during growth by ammonia‐MBE causes AlGaN surface roughening and compositional inhomogeneity, which leads to deterioration of its lasing properties. AlGaN layers grown by ammonia‐MBE at optimized temperature demonstrate stimulated emission (SE) peaked at λ = 330 nm, 323 nm, 303 nm and 297 nm with the SE threshold values of 0.7 MW cm−2, 1.1 MW cm−2, 1.4 MW cm−2 and 1.4 MW cm−2, respectively. In comparison to these, AlGaN layer grown using PA‐MBE pulsed modes (migration‐enhanced epitaxy, metal‐modulated epitaxy, and droplet elimination by thermal annealing) shows a SE with a relatively low threshold (0.8 MW cm−2) at the considerably shorter wavelength of λ = 267 nm