The carrier concentration dependence of the interaction between free carriers and longitudinal optical (LO) phonons of InN is studied by Raman scattering and Fourier transform infrared measurements. InN is grown on a sapphire (0001) surface by plasma-assisted molecular beam epitaxy. The carrier concentration is varied from 1.8\ifmmode×\else\texttimes\fi10181.8\ifmmode\times\else\texttimes\fi{}{10}^{18} to 1.5\ifmmode×\else\texttimes\fi1019cm\ensuremath−31.5\ifmmode\times\else\texttimes\fi{}{10}^{19}{\mathrm{cm}}^{\ensuremath{-}3} by Si doping. The infrared reflection spect