Microwave PECVD of Micro-Crystalline Silicon

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Microwave PECVD of Micro-Crystalline Silicon Wim Soppe1, Camile Devilee1, Sacha Schiermeier1, Harry Donker2, J.K. Rath3 1 ECN Solar Energy, P.O. Box 1, 1755 ZG Petten, The Netherlands. 2 Laboratory for Inorganic Chemistry, Delft University of Technology, Delft, The Netherlands 3 Debye Institute, Utrecht University, Utrecht, The Netherlands

ABSTRACT The deposition of micro-crystalline silicon by means of PECVD with a new linear microwave plasma source is investigated. This plasma source has successfully been introduced in the large scale production of multi-crystalline Si solar cells for the deposition of passivating silicon nitride layers. Advantages of this linear plasma source are the high deposition rates and the large area (up to 80 cm width, no length limitations) on which a homogeneous deposition can be achieved. Since this source has not been applied for deposition of micro-crystalline silicon before, we explored a large parameter space (substrate temperature, pressure, MW-power, gas flow rates), in order to find optimum growth conditions. It is observed that with this microwave source it is possible to grow micro-crystalline layers at significantly higher silane/hydrogen ratios and higher deposition rates than for conventional RF PECVD. In this paper, structural properties of the silicon layers, as investigated by Raman and FTIR spectroscopy, XRD and SEM measurements are discussed. INTRODUCTION Electricity generated by photovoltaic devices can acquire a significant share of the total energy production only if it’s costs are reduced drastically with respect to present price levels. One important factor for the high costs of PV devices today is the usage of relatively large amounts of ultra-pure semiconductor material (mainly silicon). Several options are investigated world wide to tackle this problem and one very promising option is the usage of thin film silicon layers, grown at low temperatures on cheap substrates. These layers are typically 1 µm thick, whereas the silicon wafers, which are commonly used in the PV industry have a thickness of about 300 µm. A well known example of thin film silicon grown at low temperatures is amorphous silicon. This material is applied in the PV industry for several decades but it suffers from some serious disadvantages, blocking its application for PV devices with moderate to high conversion efficiencies, namely light-induced degradation and low absorption at longer wavelengths. Recently, however, it has been discovered that it is possible to grow a crystalline form of silicon too at low temperatures, in the range of 150 – 300 °C. This material, commonly called microcrystalline (µc) silicon [1], does not suffer from light induced degradation. Micro-crystalline silicon has been applied in single junction solar cells, for which efficiencies of 10 % have been achieved [2]. Another very interesting application of µc-Si is its usage as a low band gap material in a multi-junction cell. Such multi-junction cells in which µc and amorphous silicon layers are combined have a perspe