High-quality GaN layers on c-plane sapphire substrates by plasma-assisted molecular-beam epitaxy using double-step AlN buffer process

High-structural-quality c-axis GaN layers were grown on (0001) sapphire substrates by N-2-plasma-assisted molecular-beam epitaxy (MBE) using a double-step AlN buffer layer process. The introduction of double-step AlN buffer layers grown at two different high temperatures improved the quality of the GaN layers dramatically. The full-width at half maximum (FWHM) (omega scan) of GaN layers along the symmetric (0002) and asymmetric (10 (1) over bar1) diffraction planes were 104 and 848 arcsec, respectively, whereas they were 532 and 1335 arcsec for the GaN layers grown by a single-step AlN buffer process. Double-step AlN buffers improve the structural quality of the layers and decrease the X-ray dislocation density. We have shown that the quality of the underlying AlN buffer layer is the determining factor in the successful growth of high-quality GaN epitaxial layers.