Optical properties of direct bandgap semiconductors : from bulk to low dimensional structures

Abstract

Optical and electrical properties of solids have a close relationship with their dimensionalities. In this thesis, optical properties of several direct bandgap materials with different dimensionalities were investigated in detail. In three dimensional bulk zinc oxide (ZnO) single crystal, two electron satellite transition (TES) was examined in terms of a radiative Auger effect. On the basis of experimental spectral data, a model was proposed to interpret temperature dependence of its integrated intensity. Meanwhile, the phonon coupling with various excitonic transitions were discussed as a function of temperature in bulk ZnO single crystals.

Being as a new family of two dimensional materials, monolayers of transition metal dichalcogenides (TMDCs) have received an increasing research interest in the past few years because they have been proved to be direct bandgap semiconductors. Light emission properties of tungsten sulfide (WS2) monolayers were characterized by using excitation power dependent photoluminescence (PL) technique. Two kinds of emission mechanisms, namely band-edge free excitonic transition and localized-states ensemble emission, were revealed in WS2 monolayers. Meanwhile, PL and Raman mapping for WS2 monolayers were conducted. It was found the relative intensity of inter-plane vibration mode with respect to in-plane mode is critical for monolayer identification with Raman spectroscopic technique and PL mapping can yield more information on the uniformity and quality of the samples.

In the third part of this degree research, various low dimensional nanostructures of WS2, ZnO and GaN were studied. WS2 nanotubes were found to show interesting Raman light scattering features, while ZnO nanorods prepared by vapor phase transport method were revealed to have distinctive light emission properties. Finally, by focused ion beam milling, GaN and ZnO nano-array were fabricated. Surface vibration mode was firmly demonstrated to exist in these nanostructures with optical studies and theoretical analysis.