Hot-Wire Chemical Vapor Deposition Of Polycrystalline Silicon : From Gas Molecule To Solar Cell

Abstract

Although the effort to investigate the use of renewable energy sources, such as wind and solar energy, has increased, their contribution to the total energy consumption remains insignificant. The conversion of solar energy into electricity through solar cells is one of the most promising techniques, but the use of these cells is limited by the high cost of electricity. The major contributions to these costs are the material and manufacturing costs. Over the past decades, the development of silicon based thin film solar cells has received much attention, because the fabrication costs are low. A promising material for use in thin film solar cells is polycrystalline silicon (poly-Si:H). A relatively new technique to deposit poly-Si:H is Hot-Wire Chemical Vapor Deposition (Hot-Wire CVD), in which the reactant gases are catalytically decomposed at the surface of a hot filament, mainly tungsten and tantalum. The main advantages of Hot-Wire CVD over PE-CVD are absence of ion bombardment, high deposition rate, low equipment cost and high gas utilization. This thesis deals with the full spectrum of deposition, characterization and application of poly-Si:H thin films, i.e. from gas molecule to solar cell. Studies on the decomposition of silane on the filament showed that the process is catalytic of nature and that silane is decomposed into Si and 4H. The dominant gas phase reaction is the reaction of Si and H with silane, resulting in SiH3, Si2H6, Si3H6 and H2SiSiH2. The film growth precursors are Si, SiH3 and Si2H4. Also, XPS results on used tantalum and tungsten filaments are discussed. The position dependent measurements show larger silicon contents at the ends of the tungsten filament, as compared to the middle, due to a lower filament temperature. This effect is insignificant for a tantalum filament. Deposition time dependent measurements show an increase in silicon content of the tungsten filament with time, while the silicon content on the tantalum filament saturates rather quickly. The deposition of poly-Si:H layers using tantalum as filament material has been investigated by ellipsometry. Spectroscopic ellipsometry studies on a series of films deposited at increasing hydrogen dilution revealed that the crystallinity of the layers increased. The deposition of profiled layers, using a highly crystalline seed layer was studied by both spectroscopic and kinetic ellipsometry. Both studies showed that by using a seed layer, more crystalline layers could be deposited using higher silane flows at which normally amorphous silicon layers are grown. Poly-Si:H thin films deposited using tungsten, tantalum and rhenium as filament material have been characterized. "Device-quality" poly-Si:H layers have been deposited using tungsten as filament material. The materials deposited using tantalum and rhenium showed properties that are less than "device-quality". Finally, some solar cell results are given. The highest efficiency obtained was 4.41% for a n-i-p solar cell with a thickness <1.5 mu-m without the use of a back reflector. An increase in solar cell performance could be achieved by (i) using a lower substrate temperature, (ii) incorporating a textured back reflector to increase the effective pathway of the light and (iii) by using a more stable n-layer, possibly deposited by Hot-Wire CVD.