High-quality and crack-free Al_xGa_1-xN (x∼0.2) grown on sapphire by a two-step growth method

We report a two-step growth of Al_xGa_1-xN (x∼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 500 arcsec for the 1.7 μm Al _0.2Ga_0.8N grown. The Hall measurement showed a low background carrier concentration of 7×10¹⁷ cm^-3. The electron mobility was 150 cm² V^-1 s^-1 for the Al_0.2Ga_0.8N film doped to 2×10¹⁸ 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.