Measurement of Phonon Anharmonicity in Zinc Oxide Using Temperature Dependent Raman Spectroscopy

Abstract

The aim of this thesis is to use temperature-dependent Raman spectroscopy to study the effect of defects on the anharmonicity of optical phonons in Wurtzite zinc oxide (ZnO) nanoparticles, which is unexplained by existing theories. Due to its high piezoelectric properties and exciton binding energy, wurtzite zinc oxide (ZnO) has long been regarded as a material of choice for piezoelectric transducers and ultraviolet emitters at 300 K. Nevertheless, native defects in ZnO often lead to undesirable defect levels in the electronic bandgap and phonon bands, which hinder the commercial applications of ZnO. Therefore, the origin of the native defects in ZnO has been the subject of much discussion, and many conclusions about their nature and physical properties have been drawn. Nevertheless, the effect of defects on phonon dynamics, which have a crucial role in determining the efficiency of piezoelectricity and even the emission of the material, is not yet understood. In this thesis, we tackle this issue by using temperature-dependent Raman spectroscopy. Room temperature measurements are used to determine first-order and second-order Raman modes. Temperature dependent Raman spectroscopy carried out on heated and cooled samples are used to investigate the anharmonicity of optical phonon of the center of the Brillouin zone. Raman line-shape measured in the temperature range 300-1000 K is fitted to a theoretical model derived on the basis of perturbation theory to retrieve the two-phonon density of states. The results show that upon heating, the defect complexes change their configuration in an irreversible process, resulting in a strong effect on the two-phonon density of states.