Correlation of electronic and structural properties of MBE-grown AlGaN/GaN heterostructures to Al/N flux ratio during nucleation layer growth

We have demonstrated that carrier mobility and buffer leakage in MBE-grown AlGaN/GaN high electron mobility transistor structures grown on SiC depend sensitively on the Al/N flux ratio during the growth of the AlN nucleation layer (NL). In particular, as the Al/N flux ratio increases from slightly below to slightly above unity, the Hall mobility increases from 1050 cm ²/V·s to 1450 cmVV-s and the buffer leakage increases by three orders of magnitude. Observations by transmission electron microscopy indicate that in structures in which the AlN nucleation layer growth was Al-rich the AlN and GaN layers contain stacking faults, the interface between them is rough, and the near-interface region of the nominally wurtzite-GaN buffer contains regions of cubic material. By contrast, structures in which the AlN NL was grown Al-lean show sharp interfaces between the AlN and GaN layers, and neither stacking faults nor cubic material were observed.