Figure 2: Silicon vertical junction microdisk modulator design and characterization.
(a) A three-dimensional sketch of the electro-optic silicon microdisk modulator, showing the cross-section, size, metal connections and the optical mode overlapped with the vertical p–n junction. The grey regions illustrate the depletion region in the vertical p–n junction within the microdisk and the undoped bus waveguide adjacent to the microdisk. The full doping profile, including the n+ and p+ regions as well as all of the wiring shown in the diagram was included in all calculations. The electrical equivalent diagram of the circularly contacted microdisk is also overlaid, illustrating n+, p+, p, n-doped region and vertical via resistances (Rn+, Rp+, Rp, Rn andRvia) as well as the junction capacitance (Cj). Owing to its cylindrical symmetry, the device has only vertical and radial resistance terms and no azimuthal resistance term, which significantly reduces the contact and device resistance. (b) A scanning electron microscopy image of the silicon microdisk modulator, revealed by dry etching the SiO2 around the modulator to show the metal interconnect, circular contact, silicon bus waveguide and the microdisk. The signal pad, connected by short (~10–100μm) wires, is shown on the left side of the image. (c) The simulated finite element model (FEM) vertical junction carrier distribution profile is plotted as a function of applied voltage and depth. (d) The measured and simulated (green dashed curves) transmission spectra of the resonator is plotted, at ~26.5 °C and with applied DC bias voltages of 0.25, 0 and −0.25 V, respectively.
