Dispersion models describing coupled systems in doped crystals. II. Infrared phonon excitations in GaAs single crystals and phonon–plasmon coupling in Zn-doped GaAs

We present a temperature-dependent infrared study of intrinsic and Zn-doped GaAs single crystals analyzed within a recently developed dispersion framework for coupled phonon–carrier systems. Reflectance, transmittance, and ellipsometry data (300–440 K) were fitted simultaneously using response functions that incorporate Gaussian-broadened dipole distributions for single-phonon excitations and triangle–delta–triangle representations for multi-phonon absorption. The approach enables a clear separation of lattice and free-carrier contributions and reveals a pronounced Fano-type asymmetry of the TO-phonon resonance in heavily doped samples, originating from the interference between lattice vibrations and the Drude background of light and heavy holes. The model provides physically consistent optical constants across the far- and mid-infrared, showing improved internal agreement compared with existing datasets. These results demonstrate that the extended dispersion formalism yields an accurate description of single-phonon, multi-phonon, and free-carrier processes in GaAs and establishes a validated basis for quantitative modeling of GaAs-based photonic and optoelectronic structures.