Time-resolved micro-photoluminescence of epitaxial laterally overgrown GaN

Epitaxial laterally overgrown GaN (ELOG) structures are microscopically cha racterized using cathodoluminescence (CL), micro-Raman spectroscopy and tim e-resolved micro-photoluminescence. Two ELOG samples consisting of a 3 mu m thick GaN buffer layer on (0001) sapphire masked with SiO2 stripes paralle l to [1100] an [1120] direction, respectively, were investigated. Beside th eir technological relevance ELOG samples are a superior subject of investig ations due to internal gradients in strain and free carrier concentration. To study the influence of the different lateral growth mechanisms on the op tical properties of the GaN in the coherently grown and in the overgrown re gion, we correlate the temporal behavior of the near band gap luminescence with the local free carrier concentration as determined by the peak positio n of the coupled phonon plasmon modes (LPP). The spatial resolution of both of the microscopic methods is about 1 mu m. As a result we observe distinc t differences in the decay times of the near band gap luminescence for the two different samples as well as for different regions within one sample. T he observed behavior is explained by the fact that extrinsic or intrinsic d efects give rise to shallow donor levels, causing a gradient in free carrie r concentration. Therefore the near-band-gap emission changes from excitoni c luminescence to band-to-band recombination depending on the distance from the interface as evidenced by micro-photoluminescence. Beside these drasti c changes we also observe a change in decay times in the region of excitoni c emission correlated with the donor concentration which leads to a differe nt influence of the coulomb screening effect. (C) 2000 Elsevier Science B.V . All rights reserved.