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 (∼10¹³cm^-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 Al _0.30Ga_0.70N barrier deposited on a 2 μm insulating GaN buffer, room-temperature mobilities averaging 1400cm²/Vs at a sheet charge density of 1.0×10¹³cm^-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 350/ 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.