EXCITONS AND PHONONS IN GAN - MAGNETOOPTICAL AND SPATIALLY-RESOLVED INVESTIGATIONS

A. comprehensive study of the direct photoluminescence from free-excit on states in GaN using polarization-dependent and magnetooptical measu rements is presented. We measured and identified fine-structure splitt ings in the n = 1 state of the A-exciton. From the magnetoluminescence data obtained in fields up to 15 T we determined the g-values of the conduction and valence bands parallel and perpendicular to the c-axis. Self-organized hexagonal GaN pyramids of 5 mu m width and covered by six {1 (1) under bar 01} side facets were investigated by spatially re solved cathodoluminescence and micro-Raman spectroscopy. Beside a narr ow luminescence peak at 355 nm, originating from the 2 mu m thick GaN layer, an additional broad luminescence band was observed from the GaN pyramids around a wavelength of 357 nm. A strong energy shift is foun d along the {1 (1) under bar 01} pyramidal facets and directly visuali zed by monochromatic cathodoluminescence images and line scans. In GaN epilayers grown on GaAs substrates a series of sharp modes in the ran ge between 60 cm(-1) and 250 cm(-1) for temperatures below 100 K was f ound. The intensities of these modes increased drastically with decrea sing temperature. Raman excitation spectra showed a maximum between 51 4.5 nm and 568 nm. A comparison of spatially resolved investigations w ith that of intentionally doped GaN epilayers showed that the in-diffu sion of As from the substrate plays an important role. Raman spectra a s a function of external fields, like magnetic field and hydrostatic p ressure, gave additional information about the defect type and the und erlying scattering mechanism. PACS numbers: 71.35.-y, 78.40.Fy, 78.45. +h.