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 ∼50 nm thickness represents the kinetic stabilization of LT growth. LT-AlN layers with thicknesses of ∼20 nm provide excellent surface smoothness of <0.9 nm and large relaxation, ∼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 ∼50 nm thickness.