Franz-Keldysh electro-absorption modulation in germanium-tin alloys

The direct-bandgap Franz-Keldysh theory has been applied to relaxed crystalline Ge1-xSnx alloys for 0 ≤ x ≤ 0.1 taking into account the light- and heavy-hole contributions. For electro-absorption modulation, the change-in-absorption spectrum Δα(ω, ɛ) was calculated for an applied electric field ɛ of 33 to 100 kV/cm using a B parameter determined by curve-fitting the zero-field absorption spectrum α(ω, ɛ) for each alloy. At a photon energy ∼10 meV below the gap, it was found that the figure of merit Δα(ω, ɛ)/α(ω, 0) ranged from 0.4 to 1.0 for 2% Sn, 0.5 to 1.2 for 5% Sn, 0.6 to 1.4 for 7.5% Sn, and 0.7 to 1.6 for 10% Sn. For a figure of merit ∼1, the waveguided resonant low-Q GeSn microdisk-and-bus modulator is expected to give a higher on/off contrast ratio than the non-resonant GeSn channel waveguide modulator.