Silicon-doped InP is grown on top of semi-insulating iron-doped and sulfur-doped InP substrates by metal-organic vapor phase epitaxy (MOVPE), and the growth parameters are adjusted to obtain various free carrier concentrations from 1.05×1019 cm−3 up to 3.28×1019 cm−3. Mid-infrared (IR) reflection spectra of the samples with different carrier concentrations are used to retrieve pertaining dielectric functions as the key factor for understanding plasmonic behavior of InP:Si in the mid-IR wavelength range.