Measurement and modeling of ultrafast carrier dynamics and transport in germanium/silicon-germanium quantum wells

Edward L Ginzton Laboratory, Stanford University, 348 Via Pueblo Mall, Stanford, CA 94305, USA.
Optics Express (Impact Factor: 3.49). 12/2010; 18(25):25596-607. DOI: 10.1364/OE.18.025596
Source: PubMed


We measure the intervalley scattering time of electrons in the conduction band of Ge quantum wells from the direct Γ valley to the indirect L valley to be ~185 fs using a pump-probe setup at 1570 nm. We relate this to the width of the exciton peak seen in the absorption spectra of this material, and show that these quantum wells could be used as a fast saturable absorber with a saturation fluence between 0.11 and 0.27 pJ/μm. We also observe field screening by photogenerated carriers in the material on longer timescales. We model this field screening by incorporating carrier escape from the quantum wells, drift across the intrinsic region, and recovery of the applied voltage through diffusive conduction.

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    • "The speed of MQW electroabsorption modulators is typically RC limited [39]. Ge/SiGe QW devices in particular should be relatively unaffected by saturation effects at higher optical intensities , due to the fast rate of carrier scattering out of the conduction band Γ valley [40]. A 3 dB bandwidth of 37 GHz has been previously reported in III–V AFPMs, and similar performance is likely attainable with optimization of the current Ge/SiGe devices. "
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    • "For the line shape, we have empirically chosen to use a hyperbolic secant (sech) function because of its relatively good agreement with experimental data. The FWHM includes homogeneous (e.g., from ultrafast carrier lifetimes [27]) and non-homogeneous broadening contributions (e.g., from well width fluctuations [19]). Characteristic exciton absorption peaks calculated this way are shown in Fig. 1 with the solid black line. "
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