Exciton-phonon interaction in InGaN/GaN and GaN/AlGaN multiple quantum wells

The exciton-phonon coupling has been studied in ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N/GaN}$ and ${\mathrm{GaN/Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ multiple quantum wells (MQWs) and compared with that in ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ and GaN epilayers. Phonon replicas with up to four phonons can be well resolved only in the alloy regions of the MQWs $({\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ or ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N)}$ and was independent of the structure (well or barrier), while no phonon replicas of the exciton transitions were observed for the free-exciton transitions in the GaN and the localized exciton transitions in the ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ epilayers. It thus suggests that the symmetry properties of MQWs, which modifies the phonon dispersion relation, together with alloy disorder are responsible for the enhanced exciton-phonon interaction in III-nitride MQW. The coupling constant $S$ of the exciton-phonon interaction is extracted for an ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N/GaN}$ and ${\mathrm{GaN/Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ MQW, and is found to be $\text{S=0.802}$ and 0.556, respectively. The implications of the modified exciton-phonon coupling in MQWs in terms of understanding the fundamental physics of this system as well as practical device applications are discussed.