High electron mobility AlGaN/GaN heterostructures grown on sapphire substrates by molecular-beam epitaxy

High-quality AlGaN/GaN heterostructures have been grown by ammonia gas-source molecular-beam epitaxy on sapphire substrates. Incorporation of a low-temperature-grown AlN interlayer during the growth of a thick GaN buffer is shown to substantially increase the mobility of the piezoelectrically induced two-dimensional electron gas (2DEG) in unintentionally doped AlGaN/GaN heterostructures. For an optimized AlN interlayer thickness of 30 nm, electron mobilities as high as 1500 cm(2)/V s at room temperature, 10 310 cm(2)/V s at 77 K, and 12 000 cm(2)/V s at 0.3 K were obtained with sheet densities of 9x10(12) cm(-2) and 6x10(12) cm(-2) at room temperature and 77 K, respectively. The 2DEG was confirmed by strong and well-resolved Shubnikov-de Haas oscillations starting at 3.0 T. Photoluminescence measurements and atomic force microscopy revealed that the densities of native donors and grain boundaries were effectively reduced in the AlGaN/GaN heterostructures incorporating low-temperature-grown AlN interlayers. (C) 2000 American Institute of Physics. [S0003-6951(00)02506-7].