  Silicon photonics has recently been proposed for a diverse set of
applications at mid-infrared wavelengths, the implementation of which require
on-chip photodetectors. In planar geometries, dopant-based extrinsic
photoconductors have long been used for mid-infrared detection with Si and Ge
acting as host materials. Leveraging the dopant-induced sub-bandgap trap-states
used in bulk photoconductors for waveguide integrated mid-infrared detectors
offers simple processing, integration, and operation throughout the
mid-infrared by appropriate choice of dopant. In particular, Si doped with Zn
forms two trap levels ~ 0.3 eV and ~ 0.58 eV above the valence band, and has
been utilized extensively for cryogenically cooled bulk extrinsic
photoconductors. In this letter, we present room temperature operation of Zn+
implanted Si waveguide photodiodes from 2200 nm to 2400 nm, with measured
responsivities of up to 87 mA/W and low dark currents of < 10 microamps.
