Excitons and Phonons in GaN. Magnetooptical and Spatially Resolved Investigations

A comprehensive study of the direct photoluminescence from free-exciton states in GaN using polarization-dependent and magnetooptical measurements is presented. We measured and identified fine-structure splittings 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 μm width and covered by six {11̲01} side facets were investigated by spatially resolved cathodoluminescence and micro-Raman spectroscopy. Beside a narrow luminescence peak at 355 nm, originating from the 2 μm 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 found along the {11̲01} pyramidal facets and directly visualized by monochromatic cathodoluminescence images and line scans. In GaN epilayers grown on GaAs substrates a series of sharp modes in the range between 60 cm$\text{}^{-1}$ and 250 cm$\text{}^{-1}$ for temperatures below 100 K was found. The intensities of these modes increased drastically with decreasing temperature. Raman excitation spectra showed a maximum between 514.5 nm and 568 nm. A comparison of spatially resolved investigations with that of intentionally doped GaN epilayers showed that the in-diffusion of As from the substrate plays an important role. Raman spectra as a function of external fields, like magnetic field and hydrostatic pressure, gave additional information about the defect type and the underlying scattering mechanism.