The magneto-optical properties of semiconductors and the band structure of gallium nitride

This thesis has applied magneto-optical techniques to enable a better understanding of the band structure of gallium nitride, particularly the complex behaviour expected as a result of strong valence band mixing. Effective hole masses are deduced from free excitonic-like transitions observed in magneto-reflectivity, to give a heavy A hole mass of 1.3 mo and the first experimental suggestion of a light B band mass. From the Landau-level-like transitions, once the phonon correction to the electron dispersion is considered, the deduced A valence band dispersion confirms the heavy mass, but also shows a large non-parabolicity. The effect of strain on the valence band is seen through different hole contributions to the g factor in the donor-bound exciton luminescence from heteroepitaxial compared with homoepitaxial material. Not only are the g-factors sensitive to strain, but they also are sensitive to the state of binding, being very different for the B 1s and 2s excitonic states. Despite this, in lower dimensional structures the observation of an enhanced g-factor suggests a re-ordered valence band in GaN/AlxGa1-xN quantum wells with respect to GaN epilayers, caused by a reversal of the states in the strained AlxGa1-xN barriers. The role of spontaneous and piezoelectric polarisation was found to be important in both quantitatively understanding the increased exciton binding energy with the reduction of the dimensionality in the wells, and observing a resonant phenomenon from an undoped GaN/AlxGa1-xN single heterojunction. The oscillating peak intensity was understood in terms of the changing screening efficiency of the two-dimensional electron gas. Finally, many-body effects within an electron-hole plasma in lnxAlyGa1-x-yAs quantum wells have been explored to study the effects of mass renormalisation at carrier densities typically present in laser devices. A peak in the mass renormalisation confirms the importance of excitonic correlations in the excited plasma. (author)