Recrystallization of epitaxially deposited germanium films on low cost metal substrates for multijunction III-V solar cell applications



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tarix27.02.2018
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RECRYSTALLIZATION OF EPITAXIALLY DEPOSITED GERMANIUM FILMS ON LOW COST METAL SUBSTRATES FOR MULTIJUNCTION III-V SOLAR CELL APPLICATIONS. M. Kassis, C. Plourde, R. Raffaelle*, NanoPower Research Laboratories msk6556@rit.edu, rprsps@rit.edu.

High efficiency III-V multi-junction solar cells are traditionally grown by OrganoMetallic Vapor-Phase Epitaxy (OMVPE) on single crystalline germanium (Ge) substrates. Germanium single crystal ingot growth is expensive to manufacture and the functional contribution of the bulk substrate to device performance is minimal compared to the electrically active area. To mitigate this issue, thin amorphous Ge films can be deposited on robust, lightweight metal foils suitable for OMVPE III-V growth. The focus of this study concentrated on transforming amorphous Ge films into a polycrystalline structure through thermal annealing. Once recrystallized, the Ge is proposed to act as the lower junction in a triple-junction solar cell reducing the overall cost and maximizing the electrically active area to film thickness ratio. Molybdenum substrates were prepared with various wetting layer materials at different thicknesses via chemical vapor deposition (CVD) and physical vapor deposition (PVD) to promote germanium adhesion. A germanium thin film was deposited via electron beam evaporation followed by a variety of capping layers prepared by either of these deposition methods. It has been shown that capping layers facilitate Ge recrystallization both by exploiting the melting point of the eutectic alloy and by affixing the germanium during the anneal process. Samples were annealed in a furnace under nitrogen ambient at temperatures ranging from 410C to 910C for four hours. These material dependent temperatures were found to be appropriate for Ge grain nucleation. Depending on the capping material used, the layers were removed either by wet chemical etching or reactive ion etching. The remaining germanium films were analyzed by Nomarski Microscopy, Scanning Electron Microscopy (SEM), and Energy-Dispersive X-Ray Spectroscopy (EDS). Applications of polycrystalline germanium multi-junction solar cells will be discussed and resulting grain growth will be presented.
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