Optimisation of Microcrystaline Silicon Deposited by Expanding Thermal Plasma Chemical Vapor Deposition for Solar-Cell A

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Optimisation of Microcrystaline Silicon Deposited by Expanding Thermal Plasma Chemical Vapor Deposition for Solar-Cell Application R. Jimenez Zambrano1, R.A.C.M.M. van Swaaij1, and M.C.M. van de Sanden2 1 Department of Micro-electronics, DIMES-ECTM, Delft University of Technology, P.O. Box 5053, Delft, NL-2600 GB, Netherlands 2 Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, Eindhoven, NL-5600 MB, Netherlands

ABSTRACT Microcrystalline silicon (µc-Si:H) deposited with ETP-CVD shows a high degree of porosity. In this paper the underlying reasons for this porosity are investigated, using infrared absorption measurements. The results suggest that this porosity is due to the relatively low surface mobility of radicals on the surface and the incorporation of clusters in the material. The porosity of the material could be influenced by applying RF biasing on the substrate. In that case material with a higher density is obtained, which is ascribed to a higher surface mobility of radicals on the film surface during growth as a result of ion bombardment and to modifications in the mechanism of cluster incorporation..

INTRODUCTION Amorphous and microcrystalline silicon (a-Si:H and µc-Si:H) are materials with demonstrated properties for photovoltaic application. These materials are expected to be used for mass production of cheap and high efficient solar cells. Due to the low absorption of µc-Si:H a thick layer (> 1 µm) is required, which renders the deposition rate a crucial parameter. Several techniques are used to grow µc-Si:H at high deposition rates. Promising results have been achieved by very high frequency (VHF) plasma-enhanced chemical vapor deposition (PECVD) in the high-pressure depletion (HPD) regime [1,2] and VHF-PECVD in a triode-reactor configuration [1]. The combination of VHF-PECVD and HPD achieves high deposition rates due to an efficient dissociation of feed gases together with a decrease in ion energy and an increase in ion flux at higher frequencies. With this technique µc-Si:H solar cells with 9.9% efficiency at 0.45 nm/s and 6.4% at 4.5 nm/s have been obtained [2]. Using the triode reactor configuration µc-Si:H solar cells with 3.4% efficiency at 6 nm/s have been deposited [3]. Expanding thermal plasma chemical vapor deposition (ETP-CVD) is a remote plasma technique characterized by high deposition rates and low-energy ion bombardment. In this technique a plasma is created in a cascaded arc at high pressure and this plasma then expands into the reactor that is maintained at lower pressure. It was reported earlier that with this technique µc-Si:H could be grown at 3.7 nm/s [4]. Also it was shown that the material properties of µc-Si:H could be influenced significantly by moving the injection ring towards the substrate [5], though the results suggested that the mix of depositing radicals did not change when moving the injection ring [6]. The initial implementation of µc-Si:H in solar cells shows that porous

material with crack-like voids is deposited with ET