A graded buffer layer is needed in order to grow III-V/SiGe on silicon substrate because of the lattice mismatch. In this work, optical absorption in the graded buffer layer is quantified by measuring the transmittances. We demonstrate a 7 μm graded buffer layer with Ge composition from 0% to 82% is equivalent to a 2 μm SiGe film with 82% Ge composition in terms of optical absorption. A SiGe solar cell was fabricated and characterized. Quantum efficiency measurements show that around 1 mA/cm<sup>2</sup> short circuit current is generated from the graded buffer layer in the fabricated solar cell.
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