a Modulation depth spectra of the GeSn devices measured at different electric fields. Clear variations in transmittance are observed, providing strong evidence of the Franz–Keldysh effect. The lowest direct transition energies are indicated by the solid black arrows. b Transmittance spectra for the GeSn devices and c absorption spectra under zero-bias conditions for the GeSn active layers. d Applied AC bias voltage (upper panel) and AC optical modulation signal (lower panel) at 100 kHz at λ = 2000 nm for device N1. e Normalized radio frequency response of device N1 (D = 250 µm) at λ = 2000 nm showing a 3-dB bandwidth of 19.06 MHz.