Si-doped Al0.3Ga0.7N grown on (0 0 0 I)-oriented sapphire is optimized by using a superlattice (SL) layer. Atomic force microscopy (AFM), high-resolution X-ray diffraction (HRXRD), secondary ion mass spectrometry (SIMS), and Hall effect measurements show that n-type Al0.3Ga0.7N grown on a SL layer gives high-quality crystalline and electrical properties. A 1.8-mu m-thick crack-free n-type Al0.3Ga0.7N layer is demonstrated with a doping concentration of 3 x 1018CM-3, an excellent mobility of 80 cm(2)/(V s), and an RMS roughness of 0.40 nm. Using the SL layer also results in the absence of hexagonal hillocks on the AlGaN surface, which are indicative of a high defect density. The study of an identical n-type Al0.3Ga0.7N layer grown on a low-temperature AIN interlayer shows a lower carrier concentration, mobility, and crystalline quality. (c) 2006 Elsevier B.V. All rights reserved.