We report a two-step growth of AlxGa1-xN (x similar to 0.2) by metal-organic vapor-phase epitaxy without a pretreatment of the substrate. High-quality, crack-free and near GaN-free AlGaN was achieved with a thickness much exceeding the theoretical critical thickness of the growth of AlGaN on GaN. The method was compared with the conventional ones such as AlGaN on AlN nucleation layer and AlGaN on GaN buffer layer. In situ reflectometry monitoring indicated different behaviors of the high temperature grown AlGaN over different underlayers. The double crystal X-ray rocking curve showed a full-width at half-maximum of only 500arcsec for the 1.7 mu m Al0.2Ga0.8N grown. The Hall measurement showed a low background carrier concentration of 7 x 10(17) cm(-3). The electron mobility was 150 cm(2)V(-1) S-1 for the Al0.2Ga0.8N film doped to 2 x 10(8) cm(-3). The surface morphology and crystal quality were systematically examined at different growth stages. The likely mechanism for the strain relaxation and high-quality AlGaN films grown were also suggested. (c) 2005 Elsevier B.V. All rights reserved.