Photoluminescence efficiency of Al-rich AlGaN heterostructures in a wide range of photoexcitation densities over temperatures up to 550 K

Abstract

Time-resolved photoluminescence and light-induced transient grating techniques were applied for the comparative investigation of the evolution of the internal quantum efficiency and of the carrier diffusion in an AlxGa1-xN silicon-doped epilayer and AlxGa1-xN MQWs (x>0.6). The experiments were performed between 80 and 550 K under various densities of excitation from 1μJcm-2 to 1mJcm-2. The decrease of the photoluminescence efficiency measured at high excitations is quantitatively correlated to the increase of the diffusion coefficients of carriers and to the increase of their nonradiative recombination rate. We evidence the reduction of the density of localized excitons with increasing both excitation density and temperature. The decrease of the excitation-dependent lifetime is less pronounced than the corresponding drop of the time-integrated photoluminescence efficiency. This is dominated by the thermal dissociation of excitons. At the lowest excitation densities, the excitons are captured to vacancy complexes. When increasing the excitation density, the ionization of excitons is produced, which leads to enhancing the nonradiative recombination of holes to aluminum vacancies and this simultaneously quenches the PL efficiency, due to the saturation of the bimolecular free-carrier-plasma recombination. At the initial recombination stages and under high excitation conditions we reveal diffusive recombination on dislocations. After a careful and sophisticated modeling, we establish relevant numbers for the following: (i) the free exciton binding energies, with values of 104 and 140 meV in the MQWs and in the layer respectively; (ii) the exciton localization energies, which are framed in the 12-35-meV range; (iii) the lifetimes of the localized excitons, which sit in the 2-4-ns range; (iv) the free exciton and carrier radiative recombination rate coefficients that are rex=(0.6±0.2)×(T/300)-1×109s-1 and Brad=(7±1)×(T/300)-3/2×10-10cm3s-1, respectively; (v) the capture cross section for excitons to the vacancy complex σ=(2±1)×(300/T)2×10-16cm2. Regarding the electron and hole capture cross sections to aluminum vacancy we found values of (1.5±1)×10-13cm2 and (7±1)×10-13cm2, respectively. The value of the Coulomb dislocation radius for the free-carrier recombination is established to be 12-15 nm.