Comparison of APCVD to LPCVD Processes in the Manufacturing of ZnO TCO for Solar Applications
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Comparison of APCVD to LPCVD Processes in the Manufacturing of ZnO TCO for Solar Applications Wei Zhang1, Tom Salagaj1, Jiuan Wei1, Christopher Jensen1 and Karlheinz Strobl1 1 CVD Equipment Corporation, 1860 Smithtown Ave., Ronkonkoma, NY 11779 ABSTRACT A FirstNano EasyTube 3000 CVD system with a 3” diameter horizontal tube furnace was used to investigate the optimization of both APCVD (Atmospheric Pressure Chemical Vapor Deposition) and LPCVD (Low Pressure Chemical Vapor Deposition) processes to grow both boron and fluorine doped ZnO films with a sheet resistance, slice resistance and haze suitable for their potential utilizations as TCO (Transparent Conductive Oxide) layers for photovoltaic applications. Growth rates as high as 100 nm per minute have been obtained in some parameter regions for both processes. In both cases the resulting material property parameters were the same or better than reported in the literature. Although the horizontal hot wall CVD R&D reactor is not optimum for uniform TCO thin film deposition it allowed us to investigate the interrelationship of the most critical parameters with the resulting material properties. The driving force for this work is the ultimate goal of demonstrating a process parameter solution suggesting that ZnO films (usable for either display system manufacturing and/or photovoltaic applications) can be deposited with optimized material properties that are comparable to LPCVD or sputtering processes, but that the APCVD solution could be more economical for large scale thin film ZnO coating implementation. Ultimately our desire is to transfer such a ZnO deposition process to our proprietary, APCVD CVDgCoat™ platform, which can coat up to 4 meter wide glass sheets and metal foils that move continuously. INTRODUCTION TCO thin films and/or materials typically have low electrical resistivity and high transmittance for visible light. Wide band gap semiconducting metal oxides such as Indium Oxide, Tin Oxide and Zinc Oxide can be doped with suitable amounts of Fluorine or Antimony to improve their conductivity while retaining high transparency. Among them Zinc Oxide has prominent advantages over the other TCOs. This is because, firstly, Zinc precursors are generally non-toxic. Secondly, Zinc materials are less expensive than Tin, Indium or Cadmium. Currently, fluorine doped Tin Oxide (FTO) is the most widely used front electrode for thin film based solar module production. However, FTO can be reduced in hydrogen plasma during Silicon deposition process. The resultant elemental Tin at the silicon interface will cause optical loss and unwanted diffusion, which will lower solar cell efficiency. Zinc Oxide, however, is typically more stable than Tin Oxide in a plasma reducing environment. In addition, it has a higher optical transmittance than the other TCOs. All of the above suggests Zinc Oxide a very promising candidate for solar cell applications [1]. Chemical Vapor Deposition (CVD) processes generally have better deposition rate and uniformity than the other dep
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