Dislocation and morphology control during molecular-beam epitaxy of AlGaN/GaN heterostructures directly on sapphire substrates

We report on the growth and transport characteristics of high-density (similar to10(13) cm(-2)) two-dimensional electron gases confined at the AlGaN/GaN interface grown by plasma-assisted molecular-beam epitaxy on sapphire substrates. For structures consisting of a 25 nm Al0.30Ga0.70N barrier deposited on a 2 mum insulating GaN buffer, room-temperature mobilities averaging 1400 cm(2)/V s at a sheet charge density of 1.0x10(13) cm(-2) are consistently achieved. Central to our approach is a sequence of two Ga/N ratios during the growth of the insulating GaN buffer layer. The two-step buffer layer allows us to simultaneously optimize the reduction of threading dislocations and surface morphology. Our measured sheet resistivities as low as 350Omega/square compare favorably with those achieved on sapphire or SiC by any growth method. Representative current-voltage characteristics of high-electron-mobility transistors fabricated from this material are presented. (C) 2002 American Institute of Physics.