Direct measure of band edge optical absorption of silicon nanostructures using photothermal deflection spectroscopy

Abstract

Quantum confined silicon nanoparticles (Si-NPs) are a promising material for optoelectronic applications. In this thesis, I studied the band edge optical absorption of free standing Si-NPs and Si-NPs in an amorphous silicon matrix, a composite material also known as nano-crystalline silicon (nc-Si:H). In general, it is difficult to directly observe the absorption threshold in these materials because of silicon’s low absorption coefficient. For this study, absorption was measured using photothermal deflection spectroscopy (PDS). PDS is a highly sensitive technique which can directly measure the optical absorption of materials through the generated heat. The first part of the thesis focuses on the structural and optical properties of size varied free standing Si-NPs. Si-NPs were plasma synthesized and the size control was achieved by using a silicon etchant gas. Both photoluminescence (PL) and PDS absorption spectra showed a blue shift with decreasing Si-NP size. An important outcome from this study is that a ~320 meV difference between the PL peak and absorption edge was observed which indicates that PL is a defect related process. The second part of the thesis presents the optical absorption of nc-Si:H as a function of crystal volume fraction (Xc). A low energy enhancement in absorption relative to amorphous silicon by itself is seen in the material with high Xc. In addition, two different bandgaps, one close to amorphous silicon and another close to bulk crystalline silicon, were extracted from this material which indicates that the enhancement in absorption is due to optical transitions directly involving Si-NPs. Based on the energy of the absorption, softening of the quantum confinement when Si-NPs are imbedded in an amorphous silicon matrix is confirmed.