Growth instability of N-polar GaN on vicinal SiC substrate using plasma-assisted molecular beam epitaxy

The growth instabilities of N-polar GaN on vicinal SiC substrates with an offcut angle of 4° towards the m-plane using plasma-assisted molecular beam epitaxy (PA-MBE) were systematically studied. The morphology with the coexistence of step bunching and step meandering was demonstrated experimentally for N-polar GaN grown on vicinal SiC substrates. The morphology evolution of N-polar GaN as a function of time reveals that the step bunching instability occurred first in the initial stage, followed by the step meandering. The step bunching instability is attributed to the negative Ehrlich-Schwoebel barrier (ESB), which dominates the initial growth. On the other hand, step meandering is explained by the higher positive ESB along the edges of macrosteps. Additionally, step meandering of N-polar GaN was enhanced for samples grown with lower Ga flux, while step bunching was found to be alleviated. On the other hand, its Ga-polar counterpart only demonstrated step bunching features. In addition, N-polar GaN grown on vicinal SiC under optimal conditions still exhibited a much rougher surface than the N-polar GaN grown on on-axis substrates. These results indicate that PA-MBE grown N-polar GaN surface may not be improved by using vicinal SiC substrates when compared with that grown on on-axis SiC substrates.