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Conductors with infrared plasma frequencies are potentially useful hosts of surface plasmon polaritons (SPP) with sub-wavelength mode confinement for sensing applications. A challenge is to identify such a conductor that also has sharp SPP excitation resonances and the capability to be functionalized for biosensor applications. In this paper we present experimental and theoretical investigations of IR SPPs on doped silicon and their excitation resonances on doped-silicon gratings. The measured complex permittivity spectra for p-type silicon with carrier concentration 6×1019 and 1×1020 cm-3 show that these materials should support SPPs beyond 11 and 6 μm wavelengths, respectively. The permittivity spectra were used to calculate SPP mode heights above the silicon surface and SPP propagation lengths. Reasonable merit criteria applied to these quantities suggest that only the heaviest doped material has sensor potential, and then mainly within the wavelength range 6 to 10 μm. Photon-to-plasmon coupling resonances, a necessary condition for sensing, were demonstrated near 10 μm wavelength for this material. The shape and position of these resonances agree well with simple analytic calculations based on the theory of Hessel and Oliner (1965).