Effect of thickness variation in high-efficiency InGaN/GaN light-emitting diodes

${\mathrm{In}}_x{\mathrm{Ga}}_{\text{(1−x)}}\mathrm{N/GaN}$ multiquantum-well light-emitting diodes (LEDs) having periodic thickness variations (TVs) in ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{(1−x)}}\mathrm{N}$ active layers exhibit substantially higher optical efficiency than LEDs with uniform ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{(1−x)}}\mathrm{N}$ layers. In these nanostructured LEDs, the thickness variation of the active layers is found to be more important than the In composition fluctuation in quantum confinement of excitons (carriers). Detailed scanning transmission electron microscopy-atomic number Z contrast analysis, where image contrast is proportional to $Z^2$ (Z being the atomic number), was carried out to investigate the variation in thickness as well as the spatial distribution of In. In the nanostructured LEDs, there are short-range thickness variations (SR-TVs) (3–4 nm) and long-range thickness variations (LR-TVs) (50–100 nm) in ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{(1−x)}}\mathrm{N}$ layers. It is envisaged that LR-TV is key to quantum confinement of the carriers and enhancement of the optical efficiency. We propose that the LR-TV is caused by two-dimensional strain in the ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{(1−x)}}\mathrm{N}$ layer below its critical thickness. The SR-TV may be caused by In composition fluctuation.