Evolution of surface morphology and strain in low-temperature AlN grown by plasma-assisted molecular beam epitaxy

The evolution of stress-driven surface roughening in low-temperature (LT) grown AlN has been investigated in a wide range of film thicknesses using plasma assisted molecular beam epitaxy and atomic force microscopy analysis. The relaxation of residual strain causing morphological instability after approximately 50 nm thickness represents the kinetic stabilization of LT growth. LT-AlN layers with thicknesses of approximately 20 nm provide excellent surface smoothness of <0.9 nm and large relaxation, approximately 94% of the lattice mismatch strain. AlN films thicker than 50 nm, for which the scaling exponents are greater than 1, revealed stress-driven surface roughening with coherent islands. The implementation of thick LT-AlN buffer layers is limited by the stress-driven surface roughening above approximately 50 nm thickness.