| In this thesis, two thin film approaches have been explored and demonstrated to fabricate nanocrystals of the leading semiconductor material Si. These approaches derive from the two well-established techniques developed within the Si thin film technology over the last three decades: solid phase crystallization and chemical vapor deposition. This thesis focuses on synthesizing nanocrystalline Si (nc-Si) films with control over the average crystallite size and morphology attained by adjusting process parameters. Physical properties of Si nanocrystals including their variation with confinement size are then investigated. The emphasis has been on modeling and understanding the physical processes underlying such properties in light of the experimental data obtained. In the first approach to synthesize Si nanocrystals, strong dependence of crystallite size on the metal catalyst type is established and nanocrystals of ∼100 Å diameter have been demonstrated by use of Pd. These nanocrystals have been found to exhibit enhanced optical absorption. At this confinement size the extent of electron confinement is expected to be too insignificant to alter the nature of optical transitions from indirect to direct. This is also inferred from the absorption characteristics of these nanocrystals. On the other hand, for these nanocrystals measurable phonon confinement effects have been observed, such as the broadening of the first order Raman scattering peak. Accordingly, a model has been proposed which attributes the observed enhancement in optical absorption to enhanced oscillator strength of indirect optical transitions due to phonon confinement. In the second approach to obtain nc-Si films, a high-density plasma-assisted chemical vapor deposition technique is employed. This approach brings together two extreme plasma deposition conditions: (a) very low adatom mobility (obtained by low substrate temperature and reduced ion bombardment) and (b) highly reactive plasma with a high density of H radicals. Under these conditions, a high crystallinity can be achieved despite the low thermal energy. However the building blocks of these films is found to be nanometer-sized crystallites owing to very low adatom mobility. In one embodiment, the characteristic subgap photoluminescence in Si nanocrystals (i.e., peaking at 0.8 to 1.0 eV) is investigated. It is shown that this luminescence is consistent with its arising from transitions between band-tail states in the energy gap of the crystallites. (Abstract shortened by UMI.)... |
