Plasma-assisted MBE growth of group-III nitrides: from basics to device applications

The growth of group-III nitrides by radio frequency plasma-assisted molecular beam epitaxy (RF-MBE) on a series of different substrates like Si(111), MOCVD-GaN templates and SiC/Si(111) is presented. A careful study of the growth conditions points to an effective group III/group V flux ratio at the substrate surface, during growth, as the most critical parameter controlling the properties of the grown layers, independent of the substrate. N-rich conditions lead to the formation of high crystal quality nanocolumns with very intense and narrow (1–2 meV FWHM) photoluminescence spectra, while compact material with smooth surface is obtained when lightly Ga-rich conditions are employed. The growth of III-nitrides on Si(111) substrates needs an initial deposition of Al, followed by a high-temperature AlN buffer layer. The quality of the material is assessed by the fabrication of a UV-LED and a UV-photodetector. The properties of the material clearly improve when high-quality GaN templates grown by MOCVD are used. MBE-grown layers on top of these templates replicate or even enhance the properties of the initial template, while exploiting the advantages of MBE versus MOCVD. Better control of the interfaces and lower growth-temperature, as compared with MOCVD techniques, allow to obtain high-quality 10-period AlGaN/GaN Bragg mirrors and InGaN-based multi-quantum well LEDs. Finally, the synthesis of 6H-SiC layers in Si(111) substrates and the subsequent growth of GaN layers are analyzed. Despite the polycrystalline nature of the SiC/Si(111), the luminescence intensity of the GaN grown on it is as high or even better than that from GaN/Si(111).