Optical polarization anisotropy of a-plane GaN/AlGaN multiple quantum well structures grown on r-plane sapphire substrates

摘要

A series of nonpolar a-plane GaN/AlGaN multiple quantum well structures of varying quantum well width have been studied by polarization resolved photoluminescence and photoluminescence excitation spectroscopy at low temperature. The photoluminescence spectra from all the structures show two features that are observed to blueshift with reducing well width. The lower energy feature is associated with the recombination of carriers in regions of the wells intersected by basal-plane stacking faults, while the higher energy line is attributed to localized exciton recombination involving only the quantum wells. Using excitation spectroscopy with polarized light, we were able to resolve exciton features associated with both the ∣Y) and ∣Z) valence sub-bands. The observed polarization dependence of the transitions is consistent with a modification to the valence band-edge states due to anisotropic biaxial compressive strain in the quantum well. We were also able to determine the exciton binding energies directly from the photoluminescence excitation spectra, which were found to increase from 36 to 76 meV as the quantum well width reduced from 60 to 35 A.