Abstract
Germanium has been an enabler of the information age. Ge on Si nucleates Si photonics as well as high-speed CMOS electronics. Recently, Ge has played a significant role in integrating materials such as III–Vs on Si. The structure of GaAs on a thick Ge layer on Si has been studied for many years to expand its device application menu such as lasers, high-performance transistors, and tandem solar cells on Si. However, an ultra-thin Ge buffer layer (referred to as (Ge) hereafter) technology described in this chapter has created new fields for applications. One of the emerging fields is the structure and properties of AlGaAs/GaAs/(Ge)/Si/Ge, which has been impossible to create previously using the thick Ge buffer on Si technology. Here, we demonstrate an application as a new green power generation platform, i.e., high-efficiency cost-effective tandem solar cells using Si as a cell as well as the mechanical substrate. The (Ge) thickness has not been fully optimized yet, but is in the range 10–20 nm. Our design for a tandem solar cell shows that its theoretical efficiency reaches 43%. The key attributes are the crystalline quality and surface roughness of ultrathin (Ge). We have experimentally optimized the (Ge) buffer thickness to achieve both requirements and prototyped Ge solar cells on Si. The Ge solar cells have successfully reproduced their ideal external quantum efficiency. This is the proof of concept of the success of the Ge challenge as the material enabler to integrate Si and GaAs.
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Acknowledgments
The basic concept was created at the Massachusetts Institute of Technology(MIT) and submitted as the MIT case number 10445 while one of the authors (KW) was at the MIT Microphotonics Center. We are grateful to our collaborators, D.T. Danielson, T. Montalbo,J. Graham, J. Michel, and L.C. Kimerling at MIT, and Y. Okita and J. Osaka at the University of Tokyo.
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Ichikawa, R., Takita, S., Ishikawa, Y., Wada, K. (2011). Germanium as a Material to Enable Silicon Photonics. In: Lockwood, D., Pavesi, L. (eds) Silicon Photonics II. Topics in Applied Physics, vol 119. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-10506-7_5
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DOI: https://doi.org/10.1007/978-3-642-10506-7_5
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