New silicion nanocrystal materials for photovoltaic applications
This Ph.D. thesis addresses the deposition, characterization and improvement of Si nanocrystal layers embedded in a SiC matrix (Si NC/SiC) as future absorber material for top cells of an all-Si tandem solar cell. The required enlargement of the Si NC bandgap is aimed to achieve with the help of quantum confinement (QC). No unambiguous evidence of QC in Si NC/SiC samples were presented in the literature up to now. Even though this also was not possible during this Ph.D. thesis, some necessary preconditions for QC were successfully complied, which are a better understanding and control of the crystallization mechanism, size control of the Si NC and defect passivation. The deposition of amorphous SixC1-x:H layers was conducted by plasma-enhanced chemical vapor deposition. During the annealing step subsequent to layer deposition, H effusion, formation of Si NC, and crystallization of the SiC matrix occurs. All Si NC/SiC samples were subjected to a comprehensive structural and optical characterization. The application of grazing incidence X-ray diffraction, Fourier transform infrared and Raman measurements to Si NC/SiC thin films were significantly improved during this work. The Si NC size control is an indispensable precondition for quantum confinement. For this purpose, two routes were proposed in this work. The Si NC size is shown to increase with increasing Si content of the SixC1-x:H layers. Thus, the Si NC size can be adjusted by adjusting the Si content. The advantage of this method is the fact that singlelayers can be deposited instead of multilayers (ML). The disadvantage is the uncontrollability of the Si NC spacing and the wide size distribution. The second route is with the help of a ML structure. Usually, Si NC/SiC ML structures show strong intermixing during the annealing step and are therefore not suitable for Si NC size control. In this work it is shown that the ML structure can be hindered from intermixing by oxygen incorporation during deposition.