Charge carrier spatial distribution effects in photomodulated reflectivity of 4H-SiC and GaN

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Photoinjecting free carriers into polar semiconductors can drastically modify the infrared (IR) optical properties of the material via coupling of the longitudinal optical phonon to the free-carrier plasma. We report a detailed description of how the IR response depends on the distribution of free carriers, which, in turn, depends on the penetration depth of the excitation. The three-dimensional spatial overlap of this distribution with a probe pulse dictates the IR response. We measure transient infrared reflectivity in the reststrahlen band region of 4H-SiC and GaN using multiple excitation wavelengths in order to show the effect of carrier spatial distributions. The sensitivity of the IR response to the carrier distribution can potentially offer an avenue for tuning that response in devices.