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Timestamp: 2019-04-22 08:35:21+00:00

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Fig. 1 The calculated free carrier absorption for different levels of n and p-type doping concentrations (units of cm−3) in Ge. Data from [22–25]. The inset shows the secondary ion mass spectrometry measurement of the phosphorous doping concentration in the 2 µm thick Ge epitaxial layers.
Fig. 2 (a) A scanning electron microscope image showing the sidewall of a 1 µm deep etched Ge-on-Si rib-waveguide. (b) The modeled TE mode profile for a Ge-on-Si rib-waveguide at 8 µm wavelength.
Fig. 3 (a) A schematic diagram showing the optical characterization setup. (b) The Fabry-Perot fringes observed from transmission through a 22.5 mm long Ge-on-Si rib waveguide, as the sample is thermally tuned. Thermal expansion of the stage causes a drift in the alignment, producing a slope in the raw data, which is subsequently corrected.
Fig. 4 (a) - The propagation losses from a 4 µm wide Ge-on-Si rib waveguide for both transverse electric (TE) and transverse magnetic (TM) polarizations. The error bars denote the standard deviation of the losses calculated by successive Fabry-Perot fringes in the transmission measurement. (b) - The Si substrate absorption measured by Fourier transform infrared spectroscopy. The corresponding waveguide loss caused by the optical overlap of the TE and TM modes in the Ge-on-Si rib waveguide with the Si substrate as modeled by Lumerical Mode Solutions.
Fig. 5 (a) and (b) The measured waveguide loss (solid lines) and the calculated loss following the subtraction of the Si absorption (grey dashed lines), for TM and TE modes respectively. The data points are fitted with a second order polynomial. (c) The scattering losses as calculated by an analytical scattering model (Payne and Lacey) for a 4 µm wide fully etched ridge Ge-on-Si waveguide for three different rms roughness amplitude values. A correlation length of 150 nm was used.

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