Time-resolved micro-photoluminescence of epitaxial laterally overgrown GaN

Abstract

Epitaxial laterally overgrown GaN (ELOG) structures are microscopically characterized using cathodoluminescence (CL), micro-Raman spectroscopy and time-resolved micro-photoluminescence. Two ELOG samples consisting of a 3 μm thick GaN buffer layer on (0 0 0 1) sapphire masked with SiO 2 stripes parallel to 〈1 1 0 0〉 and 〈1 1 2 0〉 direction, respectively, were investigated. Beside their technological relevance ELOG samples are a superior subject of investigations due to internal gradients in strain and free carrier concentration. To study the influence of the different lateral growth mechanisms on the optical properties of the GaN in the coherently grown and in the overgrown region, we correlate the temporal behavior of the near band gap luminescence with the local free carrier concentration as determined by the peak position of the coupled phonon plasmon modes (LPP). The spatial resolution of both of the microscopic methods is about 1 μm. As a result we observe distinct 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. The observed behavior is explained by the fact that extrinsic or intrinsic defects give rise to shallow donor levels, causing a gradient in free carrier concentration. Therefore the near-band-gap emission changes from excitonic luminescence to band-to-band recombination depending on the distance from the interface as evidenced by micro-photoluminescence. Beside these drastic changes we also observe a change in decay times in the region of excitonic emission correlated with the donor concentration which leads to a different influence of the coulomb screening effect.