InN layers have electron accumulation structure around the surfaces and interfaces with the substrates. Infrared (IR) spectroscopy enables the analysis of the inside bulk region in spite of the high sheet electron density of the order of 1013 cm−2 in the accumulation layers using the dispersion of the penetration depth. The polarization spectroscopy shows the anisotropic electron and hole scattering rates. The higher scattering rates of electron and hole plasmons vibrating along the c axis are attributed to edge-type dislocations, while the scattering by Mg or related complex dominates the damping processes of hole plasmons vibrating vertical to the c axis in highly Mg-doped p-InN. The smaller photoluminescence (PL) intensity of p-InN is attributed to the smaller activation energy of carriers to be captured by deep levels than that from the captured carriers to the ground state.