Microstructure Effects in Hot-wire Deposited Undoped Microcrystalline Silicon Films
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1066-A06-12
Microstructure Effects in Hot-wire Deposited Undoped Microcrystalline Silicon Films Wolfhard Beyer, Reinhard Carius, Dorothea Lennartz, Lars Niessen, and Frank Pennartz IEF5 Photovoltaik, Forschungszentrum Jülich GmbH, Leo Brandt Strasse, Jülich, 52425, Germany ABSTRACT The microstructure of hot-wire microcrystalline silicon films prepared at a wide range of deposition conditions was characterized by both the microstructure parameter from infrared absorption data (analyzing the Si-H stretching modes) and the effusion spectra of (low dose) implanted He and Ne. Parameter ranges leading to the growth of a dense material are identified. A (relatively) high silane flow at rather high filament temperature is found to result in a dense material at high deposition rate. The microstructure data obtained by the two microstructure characterization methods are found to be largely correlated. INTRODUCTION With the success of plasma-grown thin film silicon for application in solar cells and other large area devices, the interest in alternative deposition methods for the base material is rising. Of particular interest is the hot-wire (HW) deposition method with high solar cell efficiencies achieved on small area [1], good potential for upscaling to large areas [2] and with no ion bombardment involved. On the other hand, it has been found that HW a-Si:H films, compared to plasma grown material, have a different and more pronounced void-related microstructure [3]. In this work, we focus on the (void-related) microstructure of undoped microcrystalline silicon using the microstructure parameter from IR absorption measurements [4] and the effusion spectra of (low dose) implanted helium and neon [5,6] for characterization. Aim is, to identify deposition conditions leading to the growth of a dense material since a void related microstructure may be detrimental for the long term material stability required in solar cells. The second aim of this work is a comparison of microstructure data of the two microstructure characterization methods for a large set of different Si:H samples. EXPERIMENTAL DETAILS The hot wire microcrystalline silicon films (HW µc-Si:H) were grown in a stainless steel reactor, using two tantalum filaments (length 38 mm, diameter 1 mm) at a distance of about 3 cm from the temperature-stabilized substrate holder. The quartz /crystalline silicon substrates were fixed to the substrate holder by silver paste ensuring a good thermal contact. The filament temperature was measured by a pyrometer. The gases silane (SiH4) and hydrogen were fed into the reactor by mass flow controllers and pumped (via a butterfly valve) by a turbomolecular/ roughing pump system. Various series of films were deposited with one deposition parameter (filament temperature Tfil, substrate temperature TS, pressure p, silane and hydrogen flows) varying while other parameters were fixed. Film thickness determined by a step profiling instrument was typically 1 µm. Infrared absorption was measured using a Fourier transform spectrometer. The Si
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