Plasma assisted molecular beam epitaxy (PA MBE) is one of the main technologies for manufacturing AlGaN heterostructures with a high Al content, emitting within wide UV spectral range down to a wavelength of 210 nm. The structural quality of UV-emitting devices can be improved by a precise control of the stoichiometric conditions during PA MBE that allows one to grow AlGaN layers with either rough or atomically smooth droplet-free surface morphology at the nitrogen- or metall-rich conditions, respectively. The paper reports on growth kinetics of Al_xGa_1-xN films with high Al content (x>0.5) at the metal-rich conditions. Also we propose a new pulsed growth technique to achieve the atomically-smooth droplet-free film surface.
The Al_xGa_1-xN/AlN/c-Al₂O₃ (x=0.5-0.8) samples were grown at substrate temperature (T_S) gradually varied from 650 to 800ºC using the group-III-to-activated nitrogen flux ratio (III/N) in the 1.1-2.2 range. In situ laser reflectometry and pyrometry measurements were used to determine growth rate (or alloy composition) as well as to estimate the metal(Ga) desorption rate and group III excess on the growth surface. The AlGaN growth kinetics data were used to obtain droplet-free AlGaN layers under the strong Ga-rich conditions at T_S=700-730ºC by using the controlled short-term interruptions of either only metal or all growth fluxes. Although optical and scanning electron microscopes revealed the droplets-free surface for the films grown by both techniques, the cracking effect was observed for the growth with only metal flux interruptions. This is explained by the formation of strained GaN insertions due to Ga surface segregation, leading to the tensile stress in AlGaN layers. Uncracked atomically-smooth surface with RMS~0.4 nm over area of 2x2 μm² was achieved for the 600-nm-thick Al_0.8Ga_0.2N films grown at III/N=2.2 with the short-term interruptons of all fluxes.

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1. Plasma-assisted molecular beam epitaxy of AlGaN-based quantum well structures demonstrating spontaneous and stimulated emission in mid-ultraviolet spectral range