Optical and magneto-optical characterization of heteroepitaxial gallium nitride

We describe some recent work on the characterization of strain effects on excitons and band structure in heteroepitaxial GaN, as well as magnetoluminescence studies of excitons and donors in this material. Epilayer strains were measured directly from wafer curvature, which avoids the need to know the lattice constants of unstrained GaN in advance. The change in strain between room temperature (where it is usually measured) down to low temperature, where detailed optical spectroscopy is usually performed, was estimated and found to be significant. Low temperature photoluminescence and reflectance were used to determine the exciton energies as a function of measured strain, yielding some of the deformation potentials and band structure parameters of the material. Magnetospectroscopy and reflectance were used to identify excited states of the excitons, yielding an exciton binding energy around 26.4 meV. Residual donors and their binding energies were studied from two-electron transitions, which have been identified for the first time by their Zeeman splittings in high magnetic fields. Resonant electronic Raman scattering was also used to study excited states of shallow donors, and yielded improved resolution compared to photoluminescence.