Sub-bandgap optical properties of wide bandgap semiconductors

Abstract

Optical phenomena play a very important role in semiconductor material analysis and optoelectronic device applications. In this thesis we focus on two phenomena: phonon-assisted excitonic luminescence and Franz-Keldysh effect in electroabsorption. For the phonon-assisted luminescence, we employed O’Rourke’s method to derive a general algebraic expression of phonon-assisted excitonic luminescence, and then analyzed the influence of various factors such as temperature phonon mode, and Huang-Rhys factor on luminescence spectrum. We proved that the derived luminescence spectral shape expression can be reduced to the Huang-Rhys’s one for low temperature, only one phonon mode and small Huang-Rhys factor. We finally applied the theory to explain the basic characteristics of phonon-assisted luminescence spectrum of bound excitons in GaN at low temperatures. In the second part of this thesis study, we theoretically investigated the Franz-Keldysh effect in electroabsorption spectra of GaN-based quantum structures under the action of electric fields. It was found that a general red shift and oscillatory structures generally appear in theoretical electroabsorption spectra of various GaN quantum structures. The former can be attributed to the quantum-confinement Stark effect, while the latter is ascribed to the Franz-Keldysh effect. Moreover, it was concluded that the smaller the size, the more significant both the effects. When exciton effect was taken into consideration, both the effects tend to weaken due to the Coulomb attractive coupling between electron and hole.