Structural and compositional analyses of a strained AlGaN∕GaN superlattice

Abstract

We investigated the nanostructure of AlGaN∕GaN strained-layer superlattice (SLS) cladding in a GaN-based violet laser diode (LD) using a scanning-transmission electron microscope (STEM). Metal-organic vapor-phase epitaxy was used to grow 200 pairs of n-Al0.14Ga0.86N∕n-GaN layers directly on the n-GaN:Si contact layer that was deposited on a (0001) sapphire substrate. The Al0.14Ga0.86N and GaN layers were distinguished as dark and bright bands in the high-angle annular dark-field (HAADF) images taken in the [1¯21¯0] zone axis. The widths of the Al0.14Ga0.86N and GaN layers were determined to be 2.24±0.09 and 2.34±0.15nm, respectively. The lattice parameters of the Al0.14Ga0.86N were measured to be a=0.32±0.01nm and c=0.50±0.02nm, and those of the GaN, a=0.32±0.02nm and c=0.52±0.03nm. This is a direct illustration of the SLSs, where a good lattice matching in the basal plane caused by shrinkage of the Al0.14Ga0.86N lattice normal to the basal plane suppresses the generation of misfit dislocations. Dislocations, appearing as dark contours in bright-field STEM images and as bright contours in HAADF images, run either parallel or perpendicular to the c direction. Another mechanism of the SLS to suppress lattice defects in the LDs has also been disclosed.