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Strain-induced enhancement of near-infrared absorption in Ge epitaxial layers grown on Si substrate

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7 Author(s)
Ishikawa, Yasuhiko ; Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 ; Wada, Kazumi ; Jifeng Liu ; Cannon, Douglas D.
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Epitaxially grown Ge layers on Si substrate are shown to reveal an enhanced absorption of near-infrared light, which is effective for the photodiode application in Si-based photonics. Ge layers as thick as 1 μm were grown on Si substrate by ultrahigh-vacuum chemical-vapor deposition with a low-temperature buffer layer technique. X-ray-diffraction measurements showed that the Ge layer possesses a tensile strain as large as 0.2%, which is generated during the cooling from the high growth temperature due to the thermal-expansion mismatch between Ge and Si. Photoreflectance measurements showed that the tensile strain reduces the direct band-gap energy to 0.77 eV (c.f. 0.80 eV for unstrained Ge), as expected from the theory. Reflecting the band-gap narrowing, photodiodes fabricated using the Ge layer revealed an enhanced absorption of near-infrared light with the photon energy below 0.80 eV, i.e., with the wavelength above 1.55 μm. This property is effective to apply the photodiodes to the L band (1.56–1.62 μm) in the optical communications as well as the C band (1.53–1.56 μm). It is shown that the experimental absorption spectrum agrees with the theoretical one taking into account the splitting of light-hole and heavy-hole valence bands accompanied by the band-gap narrowing. Based on the calculation, the performance of the photodiode using the tensile-strained Ge is discussed.

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Journal of Applied Physics  (Volume:98 ,  Issue: 1 )