(A) study of design of compact silicon waveguide-integrated plasmonic Schottky sub-bandgap photodetectors

Abstract

Development of the silicon CMOS fabrication technology allows realizing the optical or optic-electric hybrid systems on chip scale. Silicon photonics gets much attention because the integration of optical systems can be achieved with low fabrication costs using silicon CMOS fabrication process. Silicon (Si) is transparent at the near infrared (NIR) wavelength regime where the photon energy is lower than its bandgap energy of ~1.1 eV. The optical transparency of Si at the NIR range facilitates light confinement and wave-guiding with little absorption losses, but this property also makes direct electron-hole pair generation through the NIR light difficult. For this reason, heterogeneous integration with other semiconductor materials, such as germanium, has been envisioned to detect the NIR light in the silicon photonics platforms. Although this approach provides a way towards efficient NIR photo-detection, epitaxial growth of germanium on silicon might require complex processing steps, and there have been growing interests in demonstrating purely Si-based sub-bandgap photo-detectors. One way to develop such Si-based PDs at the NIR wavelengths is to form deep-level defects in the Si lattice by dopant implantation with special post-annealing processes. Alternative ways without relying on the extrinsic defects are to use internal absorption processes, such as two-photon absorption, impurity-assisted Franz-Keldysh absorption effects, and internal photo-emission (IPE). In this paper, I propose a design strategy to realize a compact and broadband, Schottky PD consisting of a chain of sub-micrometer-scale metal nanobricks integrated on a typical silicon ridge waveguide. A spatial gradient of polarizabilities from the tapered array of metal nanobricks provides a highly efficient photonic-plasmonic coupling mechanism as well as slow light effects for nearly perfect absorption with little reflection. Strong light confinement and absorption near the MS interface can efficiently generate photocurrents over the wide range of input frequencies. According to our simulation, the maximum absorptivity and the responsivity are 95% and 0.125 A/W, respectively, with a total device length of only 830 nm at an input wavelength of ~1550 nm under the applied reverse bias voltage of -3 V. Furthermore, the extremely small device footprint allows a high operation bandwidth of >40 GHz with a small dark current level of <10 nA. Our design strategy suggests that it is possible to achieve high responsivities and low dark current levels by taking advantages of the perfect absorption principle.