Molecular-beam epitaxial growth of high electron mobility AlGaN/GaN heterostructures

We report the growth of high quality AlGaN/GaN heterostructures on sapphire by ammonia gas-source molecular-beam epitaxy. By inserting a low-temperature-grown AlN interlayer during the growth of the thick GaN buffer, the piezoelectrically-induced two-dimensional electron gas (2DEG) in the unintentional doped AlGaN/GaN heterostructures exhibited substantially increased mobility. With the optimized AlN interlayer thickness of 30 nm, electron mobilities as high as 1500, 10 310, and 12 000 cm2/V s were obtained at room temperature, 77 and 0.3 K respectively. Sheet densities of 9×1012 and 6×1012 cm−2 were measured at room temperature and 77 K, respectively. The 2DEG was confirmed by strong and well resolved Shubnikov–de Haas oscillations starting at 2.8 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 with AlN interlayers.