Detailed surface studies on the reduction of Al incorporation into AlGaN grown by molecular beam epitaxy in the Ga-droplet regime

A better understanding of the molecular beam epitaxy (MBE) grown AlGaN layers is crucial for the development of UV emitters based on III-nitrides. In this work, the growth kinetics of AlGaN structures deposited by plasma-assisted MBE under Ga-rich conditions on AlGaN/AlN/sapphire templates is studied in detail. The surface quality of the layers and the Al concentration was investigated in situ by reflection high-energy electron diffraction, atomic force microscopy and X-ray photoelectron spectroscopy. The Al content in the grown AlGaN layers was also determined by high-resolution X-ray diffraction, while optical properties were examined by photoluminescence and cathodoluminescence. Obtained results reveal that an increase of the Ga flux influences the Al incorporation probability only when stepping into the Ga-droplet regime. This leads to a decrease of the Al concentration in AlGaN layers, but to a different extent in the bulk and at the surface.