Growth of AlN/GaN layered structures by gas source molecular-beam epitaxy

Abstract

AlN/GaN layered structures with layer periods between 1.5 and 40 nm have been grown on (0001) oriented sapphire and α(6H)-SiC substrates. The growth was performed using a modified gas source molecular-beam epitaxy (MBE) technique. Standard effusion cells were used as sources of Al and Ga, and a small, MBE compatible, electron cyclotron resonance (ECR) plasma source was used to activate nitrogen gas prior to deposition. Chemical analysis of the layers was conducted using Auger spectrometry. X-ray diffractometry, transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM) were employed for the structural and microstructural studies. Coherent interfaces (no relaxation by misfit dislocations) were observed for bilayer periods smaller than 6 nm. By contrast, completely relaxed individual layers of GaN and AlN with respect to each other were present for bilayer periods above 20 nm. Cathodoluminescence showed a shift in the emission peak from 3.42 eV for the sample with individual 10-nm thick GaN wells and AlN barriers to 4.11 eV for the sample having 1-nm thick individual layers.