Comprehensive variable-temperature Raman and spectroscopic ellipsometry investigation of bulk undoped, Fe-doped, and Ge-doped GaN: Correlated phonon, carrier, optical, and disorder dynamics

Bulk gallium nitride (GaN) substrates with controlled electrical compensation and doping are foundational materials for high-power electronics, optoelectronics, and emerging spintronic devices. Despite extensive studies on epitaxial GaN thin films, a complete and correlated understanding of crystal, phonon, free-carrier, birefringence, optical, and disorder-driven processes in high-quality bulk GaN crystals remains limited. In this work, a comprehensive temperature-dependent (25–300 °C) investigation of free-standing bulk undoped, Fe-doped, and Ge-doped GaN substrates using Raman spectroscopy, spectroscopic ellipsometry (SE), and atomic force microscopy (AFM) has been presented. A detailed analysis of phonon evolution, apparent Raman-shift-derived biaxial stress and in-plane strain, longitudinal optical phonon–plasmon coupling (LOPC), Raman-derived carrier and damping parameters, birefringence, dielectric function, optical bandgap, and Urbach energy was conducted as well. These measurements reveal systematic and dopant-dependent variations in lattice dynamics, carrier scattering, and electronic structure. Undoped GaN provides the baseline host-lattice response, with its temperature dependence dominated by intrinsic phonon anharmonicity. Fe-doped GaN exhibits semi-insulating behavior consistent with strong carrier compensation, suppressed free-carrier interaction, while AFM measurements show comparably low root-mean-square roughness for the Fe-doped relative to undoped GaN and Ge-doped samples. Ge-doped GaN exhibits pronounced LOPC effects, a high Raman-derived free-carrier concentration, and enhanced plasmon damping, while the Urbach analysis reveals temperature-dependent near-band-edge tail broadening across all three samples. The combined Raman and SE results, considered together with the AFM morphology, provide complementary information on sample-dependent lattice response, carrier dynamics, optical behavior, and surface morphology across the three bulk GaN substrates. These results establish critical material benchmarks for GaN-based high-power, radio frequency, optoelectronic, and spintronic devices, offering fundamental insights into doping-engineered bulk GaN substrates for these applications.