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 25nm Al0.30Ga0.70N barrier deposited on a 2m insulating GaN buffer, room temperature mobilities averaging 1400cm2/Vs at a sheet charge density of 1.0x1013cm-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/ٱ 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.