Strain-relaxation in NH ₃-source molecular beam epitaxy of AlN epilayers on GaN epitaxial templates

Temporal evolution of surface morphology in AlN epilayers grown by NH ₃-source molecular beam epitaxy on the GaN/(0001) Al ₂O ₃ epitaxial templates was correlated with changes in the degree of the residual strain and the layer thickness. They began to crack for the thickness as thin as 10 nm. However, atomic-layer step-and-terrace surface structures were maintained for the thickness up to 32 nm. Tensile biaxial stress decreased with further increase in the thickness due to the lattice relaxation, which caused surface roughening. An 1580-nm-thick, nearly strain-compensated AlN epilayer, of which threading dislocation density was reduced down to 6 × 10 ⁹ cm ^-2, exhibited excitonic photoluminescence peaks at 6.002 and 6.023 eV at 9 K and a near-band-edge peak at 5.872 eV at 293 K.