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(0.1)Zn(0.9)O 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 Γ(imp) 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.