Well-width dependence of the emission linewidth in ZnO/MgZnO quantum wells

Photoluminescence (PL) spectra were measured as a function of well width (L_W) and temperature in ZnO/ Mg_0.1Zn_0.9O single quantum wells (QWs) with graded thickness. The emission linewidth (full width at half maximum) was extracted from the emission spectra, and its variation as a function of L_W was studied. The inhomogeneous linewidth obtained at 5 K was found to decrease with increasing L_W from 1.8 to 3.3 nm due to the reduced potential variation caused by the L_W fluctuation. Above 3.3 nm, however, the linewidth became larger with increasing L_W, which was explained by the effect related with defect generation due to strain relaxation and exciton expansion in the QW. For the homogenous linewidth broadening, longitudinal optical (LO) phonon scattering and impurity scattering were taken into account. The LO phonon scattering coefficient Γ_LO and impurity scattering coefficient Γimp were deduced from the temperature dependence of the linewidth of the PL spectra. Evident reduction of Γ_LO with decreasing L_W was observed, which was ascribed to the confinement-induced enhancement of the exciton binding energy. Different from Γ_LO, a monotonic increase in Γ_imp was observed with decreasing L_W, which was attributed to the enhanced penetration of the exciton wave function into the barrier layers.