Fundamental properties of excitons and phonons in ZnO: A spectroscopic study of the dynamics, polarity, and effects of external fields

The work in hand presents a spectroscopic study of optical transitions and lattice dynamics of ZnO under the influence of external fields. A comparative study of different ZnO single crystals and homoepitaxial ZnO films reveals pronounced differences in the free and bound exciton luminescence which can be related to different impurity centers and strain levels. The charge state and donor or acceptor character of all observed shallow bound exciton centers is identified by photoluminescence spectroscopy in external magnetic fields. A correlation between the localization energies of excitons bound to the same chemical element in the neutral and ionized charge state is reported. Furthermore, different excited states of the free and bound excitons are investigated and identified by photoluminescence excitation spectroscopy and temperature dependent photoluminescence spectroscopy. The properties of the shallow impurity bound excitons are compared to defect related deeply bound excitons. Not much was known about the defect centers of these deeply bound excitons. By the combination of different experimental techniques, a multitude of new information is obtained including the defect type and charge state of the binding centers, the temperature dependence of the localization energies, the strength of the exciton-phonon coupling of the deeply bound excitons, the recombination dynamics of shallow and deeply bound excitons, the influence of uniaxial pressure on the electronic energy levels, and the spatial distribution of the luminescence centers. Based on these data, a defect model for deeply bound excitons in ZnO is developed which is capable of explaining all experimental results and provides a theoretical estimation of the extend of the defect complexes. Another part of this work is dedicated to the study of the valence band ordering in ZnO which has been debated controversially for almost half a century. High resolution magnetoPL measurements of bound excitons are presented which allow to resolve this long lasting disagreement in the literature. A previously undetected hole fine splitting could be resolved which provides, in combination with polarization and angular resolved measurements, conclusive evidence for a reversed valence band ordering in ZnO compared to other wurtzite II-VI and III-V compound semiconductors. The lattice dynamics of ZnO crystals are studied by Raman spectroscopy under the influence of external pressure. A variety of important material parameters is derived including high precision values of the hydrostatic pressure coefficients and Grüneisen parameters of all Raman active modes. The accuracy of the here reported values exceeds those of previously published values in the literature. For Born’s transverse effective charge, an incorrect pressure dependence in the literature is discovered and revised. Raman measurements of ZnO single crystals under uniaxial pressure are reported for the first time. In combination with the hydrostatic pressure measurements on the same samples, the first experimental determination of the phonon deformation potentials of all Raman active modes in ZnO is achieved. Furthermore, a study of the resonant Raman scattering at bound exciton states is presented. The influence of the excitation energy and temperature on the decay dynamics of phonon replicas and Raman modes is investigated. The dissociation of the bound excitons at elevated temperatures is found to alter the lifetimes of the LO modes under resonant excitation which are directly determined by time resolved measurements on the picosecond time scale.