Determination of thickness and lattice distortion for the individual layer of strained Al_0.14Ga_0.86N/GaN superlattice by high-angle annular dark-field scanning transmission electron microscopy

Al_0.14Ga_0.86N/GaN and GaN layers in the strained-layer superlattice (SLS) in GaN-based laser diodes were distinguished as dark and bright bands, respectively, in a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image. From the HAADF-STEM images the thickness of the AlGaN layers was determined to be 2.24±0.09 nm and that of GaN layer 2.34±0.15 nm, which corresponds to nine atom planes in the [0001] direction. The parameters of the distorted AlGaN and GaN lattices were evaluated to be a=0.32, c=0.50 nm and a=0.32, c=0.52 nm, respectively. This shows that the resultant good lattice matching on the (0001) AlGaN/GaN interfaces suppressed the generation of misfit dislocation in the SLS cladding.