Phonon-Dressed Quasiparticles in Epitaxial Scandium Nitride Resolved by Soft X-ray ARPES
Electron-phonon interactions govern quasiparticle energies, lifetimes, and temperature-dependent band structures in semiconductors. However, direct momentum-resolved experimental evidence of their impact on quasiparticle electronic structure remains scarce, particularly in nitride semiconductors. Here, we present conclusive experimental evidence of electron-phonon interaction-induced quasiparticle renormalization and pronounced spectral broadening in p-type scandium nitride (ScN), using soft x-ray angle-resolved photoemission spectroscopy. First-principles spectral-function calculations due to electron-phonon coupling reproduce the measured spectra, capturing a pronounced quasiparticle kink ~2.3 eV below the valence-band maximum, phonon satellite features arising from long-range electron-phonon coupling, and a direct bandgap zero-point renormalization of -124.3 meV within the non-adiabatic Allen-Heine-Cardona approach. We further show that the anomalous temperature-induced increase in the direct bandgap in ScN originates from the combined effects of thermal expansion and electron-phonon coupling, contributing comparably near room temperature. Our results establish ScN as a model platform for visualizing electron-phonon coupling and provide a quantitative benchmark for predictive first-principles descriptions of quasiparticle renormalization in semiconductors.