High-rate deposition of a-SiN x :H films for photovoltaic applications
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High-rate deposition of a-SiNx:H films for photovoltaic applications W.M.M. Kessels, F.J.H. van Assche, J. Hong, J.D. Moschner,1 T. Lauinger,1 D.C. Schram, and M.C.M. van de Sanden Dept. op Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands 1 Institut für Solarenergieforschung Hameln-Emmerthal (ISFH), Am Ohrberg 1, D-31860, Emmerthal, Germany ABSTRACT The feasibility of the new ‘Expanding Thermal Plasma’ technique for the deposition of a-SiNx:H at high deposition rates (typically ~200 Å/s) has been investigated. The structural film properties of the a-SiNx:H are reported for various process conditions and the application of the material as antireflection coating on (multi)crystalline silicon solar cells is studied. Furthermore, the performance of the material for surface and bulk passivation is investigated.
INTRODUCTION Hydrogenated amorphous silicon nitride (a-SiNx:H), besides a variety of applications in microelectronics, also has an important application in photovoltaics. The optical properties of a-SiNx:H make the material very well suited for antireflection coatings on crystalline Si (c-Si) solar cells, while a-SiNx:H can simultaneously yield a reduction of the surface and bulk recombination of the minority charge carriers in the cells [1]. Surface passivation is induced by band bending at the Si–a-SiNx:H interface due to fixed insulator charges and charged interface states and is important for very-high efficiency cells. The bulk passivation properties play a significant role in solar cells made from defect-rich multi-crystalline silicon (mc-Si) wafers or ribbons, in which hydrogen can passivate bulk defects and grain boundaries. The hydrogen is known to be released from the a-SiNx:H film during the anneal required for the formation of the screen-printed metal contacts. The application of a-SiNx:H on solar cells requires therefore the presence of (sufficient) hydrogen in the film while optimum antireflection coating and surface passivation performance require generally Si-rich a-SiNx:H. This is contrary to the main applications of silicon nitride in the microelectronics industry in which mostly N-rich material is used [2]. Because the deposition of a-SiNx:H is an additional process step in the crystalline Si solar cell technology, it has to satisfy very strict cost requirements. A general industrial concern is therefore the a-SiNx:H deposition rate. The deposition rates that have been reported in literature are generally lower than 10 Å/s [2]. Therefore this paper will deal with a study on the feasibility of very high-rate deposited a-SiNx:H (up to 200 Å/s). The experiments have been carried out with the Expanding Thermal Plasma (ETP) which, in recent years, has proven to be able to deposit solar grade a-Si:H at rates up to 100 Å/s [3].
THE EXPANDING THERMAL PLASMA FOR a-SiNx:H DEPOSITION The Expanding Thermal Plasma (ETP) technique for a-SiNx:H deposition is schematically presented in Figure 1 and the operating conditions are listed in Table I. Basically,
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