Influence of Ga flux on the growth and electron transport properties of AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy

The growth and electrical properties of the Al0.15Ga0.85N/GaN two-dimensional electron gas (2DEG) were studied for a range of III/V ratios during growth. All films were grown by RF-plasma assisted molecular beam epitaxy. Al0.15Ga0.85N growth in the Ga-rich regime has a surface structure that is controlled by the Ga flux, ranging from morphologies indicative of statistical roughening effects to step-flow growth. The 2DEG sheet carrier concentration was insensitive to Ga flux-this is because the saturated 2DEG charge is approximately equal to the fixed polarization charge at the AlGaN/GaN interface. By increasing the Ga flux during growth, the measured low temperature 2DEG mobility was found to increase, reaching a maximum just before Ga droplets began to form on the surface. In the Ga droplet growth regime, the mobility decreased and was insensitive to further increase in Ga flux. The highest quality surface structure and maximum mobility were realized for structures grown with Ga fluxes just below those necessary to form droplets. (C) 2001 Elsevier Science B.V. All rights reserved.