Surface morphology evolution of N-polar GaN on SiC for HEMT heterostructures grown by plasma-assisted molecular beam epitaxy

Abstract

The surface morphology evolution of N-polar GaN with growth time was investigated and compared with Ga-polar GaN. N-polar GaN directly grown on SiC substrates was found to have slower three-dimensional (3D) to two-dimensional (2D) growth transformation and lesser coalescence than the Ga-polar counterpart, resulting in rougher surface morphology, whereas the AlN nucleation layer (NL) accelerated 3D to 2D transformation, resulting in smoother surface morphology. N-polar GaN was found to have mound-type surface morphology with clustered atomic steps, unlike the regular screw-type dislocation-mediated step-flow growth observed for Ga-polar GaN. This was explained by the lower diffusion of adatoms on the N-polar surface due to its higher surface energy and higher Ehrlich-Schwoebel barrier (ESB). In addition, the increased III/V ratio in N-polar GaN growth was found to reduce the surface roughness from 2.4 nm to 1 nm. Without Si doping, the N-polar GaN HEMT heterostructures grown under optimized conditions with smoother surface morphologies exhibited sheet carrier density of 0.91 × 1013 cm-2 and mobility of 1220 cm2/V∙s. With Si δ-doping, the sheet carrier density was increased to 1.28 × 1013 cm-2 while the mobility was reduced to 1030 cm2/V∙s. These results are comparable to the state-of-the-art data of PA-MBE-grown N-polar GaN HEMT heterostructures on SiC substrates.