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978-3-8439-0116-1, Reihe Werkstoffwissenschaften
Silicon carbide single and multilayer thin films for photovoltaic applications
220 Seiten, Dissertation Eberhard-Karls-Universität Tübingen (2011), Softcover, A5
In this thesis, the deposition and annealing of non-stoichiometric silicon carbide (Si1-xCx) single and Si1-xCx/SiC multilayers has been studied into detail. Furthermore, the integration of Si1-xCx layer stacks into recrystallised wafer equivalents (RexWE) and the application of Si-rich Si1-xCx single and Si1-xCx/SiC multilayers as precursor layers for Si quantum dots in a SiC matrix has been investigated.
An important precondition for the implementation of Si1-xCx single and Si1-xCx/SiC multilayers into photovoltaic devices is the accurate control of the the film thickness and stoichiometry. Amorphous hydrogenated Si1-xCx (a-Si1-xCx:H) thin films were prepared by plasma enhanced chemical vapour deposition (PECVD) using the decomposition of silane (SiH4) and methane (CH4). In the silane-starving low power plasma density regime, homogeneous and nanovoid-free films can be deposited with an accurate control of the film thickness. The composition of the Si1-xCx films can be varied from x = 0.15 to x = 55 solely by changing the SiH4/CH4 gas flow ratio. A higher plasma power density during the deposition favours the decomposition of SiH4 and CH4 in the plasma and increases the C incorporation and the deposition rate. Si-rich a-Si1-xCx:H film consist of Si-C, Si-Si and Si-Hn bonds, whereas C-rich a- Si1-xCx:H film are mostly composed of Si-C, C-C and C-Hn bonds. Due to the variation of the stoichiometry the optical and electronic properties of the films can be varied in a wide range. The refractive index is 2.4 for C-rich films and can be increased up to 3.5 for Si-rich a-Si1-xCx:H films. The optical bandgap shifts due to the increased incorporation of C into the amorphous network of a- Si1-xCx:H films and reaches values from 2.1 eV to 2.6 eV. Moreover, the influence of the deposition temperature on the properties of Si1-xCx films was studied. The standard depositions were done at a substrate temperature of 280°C. An increased substrate temperature during deposition resulted in a reduced growth rate and a reduced C incorporation into the Si1-xCx film. Fourier transformed infrared (FTIR) spectroscopy implied a higher density of the films deposited at higher temperatures. Due to the higher density of the films, the refractive index is also increased.