Benjamin Strahm
The constant energy consumption and world-wide demography expansion add to the potential risks of ecological and human disaster associated with global warming. This makes it a necessity to develop renewable energy technologies such as photovoltaic energy. These technologies already exist, but their cost has to be reduced in order to compete with well-established energies based on natural resources such as oil, coal or natural gas. A first step has been performed successfully in recent years in the field of low cost photovoltaic (PV) solar cells. Manufacturing equipment for large area (> 1 m2) amorphous silicon thin film solar cell production is now available. Even if this type of PV cell has lower energy conversion efficiencies (≈ 9 – 10 %) than other types of cells such as crystalline silicon cells (≈ 25 %), it has lower financial and ecological costs. Nevertheless, the next generation of large area silicon thin film PV cells promise higher conversion efficiencies (≈ 12 %) and stability under light exposure. This new type of PV cell is based on microcrystalline silicon grown on large glass substrates by plasma-enhanced chemical vapor deposition from silane (SiH4) and hydrogen (H2) gas, as for the previous amorphous silicon generation. Amorphous/microcrystalline silicon PV multi-junction cells require a thick (≈ 2 µm) microcrystalline intrinsic light absorber layer because of the need to fit the photo-generated current of the two stacked cells. This increases the cost of the final product since the deposition rate of microcrystalline silicon achieved nowadays is limited to a few Å/s, making the processing time very long. The enhancement of the deposition rate while maintaining a good material quality, i.e. at the boundary between amorphous and microcrystalline growth, is difficult because the phenomena involved in the deposition of microcrystalline silicon are not well understood, and the optimization is then generally performed empirically. The plasma composition is measured using Fourier transform infrared absorption spectroscopy and optical emission spectroscopy. It is shown that the deposited films can be classified into three categories (amorphous, transitional and microcrystalline) as a function of silane concentration in the plasma, while it is impossible to do so as a function of all other process parameters (silane input concentration, RF power, pressure, etc...) if they are all varied simultaneously. This means that the common way to deposit microcrystalline silicon by strongly diluting the silane with hydrogen (< 5 %) is not unique. This is because the plasma composition does not depend only on the gas composition, but also on the fraction of silane depleted in the plasma. Analytical and numerical plasma chemistry modeling show that this is because the silane concentration in the plasma determines the species flux towards the growing film surface. Hence, in agreement with existing phenomenological models of microcrystalline growth, the lower the silane concentration in the plasma, the higher the H to SiHx flux ratio towards the surface and the higher the crystallinity. This is used to demonstrate the feasibility of the growth of microcrystalline silicon in large area reactors using radio-frequency excitation (40 MHz) even from pure silane gas. Moreover, it is shown that the optimum in terms of deposition rate and deposition efficiency tends towards pure silane and not to the common H2-diluted regime. Following this conclusion, an optimization strategy is constructed by varying only the hydrogen flow rate and the working pressure. Guidelines for the selection of the initial process parameters are given in order to achieve high deposition rate (> 10 Å/s) and high gas utilization efficiency (> 80 %) of good quality microcrystalline silicon with a high input silane concentration (> 10 – 20 %) by using the optimization strategy. Furthermore, it is shown by using time-resolved plasma composition measurement and modeling that the time necessary to reach chemical steady-state is about 1 second in large area Plasma-Box™ reactors. This time is much shorter than times reported for small laboratory reactors, which are typically of about 1 minute. It is demonstrated by using two-dimensional modeling that this is due to back-diffusion of the silane molecules from the vacuum chamber to the plasma zone in laboratory reactors, whereas the plasma fills the whole volume in large area Plasma-Box™ reactors, making the plasma composition quasi-instantaneously uniform at plasma ignition. This is of importance for the film microstructure uniformity across the thickness of the layer. In large area Plasma-Box™ reactors, it is not necessary to use strategies such as H2-dilution profiling in order to eliminate the plasma-induced amorphous incubation layer at plasma ignition, because plasma composition suitable to grow microcrystalline silicon is reached directly from ignition. Finally, it is shown that the Telegraph effect and the standing-wave, which are the two necessary and sufficient distinct electromagnetic modes to determine the electromagnetic fields in a RF reactor, affects the film thickness and microstructure uniformity. It is shown that the film thickness variation at the reactor edges due to the Telegraph effect can be eliminated by symmetrizing the two electrodes. It is also shown that the standin-gwave affects significantly the film microstructure of microcrystalline silicon deposited from discharges with a silane input concentration about 10 %, whereas for low input silane concentrations (< 4 – 5 %) the microstructure is more or less uniform. This means that the higher the silane concentration, the better has to be the design of the lens-shaped electrode to compensate for the standing-wave in large area reactors.
EPFL2007
