This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record . Please cite this article as doi: 10.1002/pssr.201900196 This article is protected by copyright. All rights reserved Wave-vector Dependent Raman Scattering from Coupled Plasmon- LO Phonon Modes and Fano Resonance in n-type Scandium Nitride (ScN) Krishna Chand Maurya 1,2,3 , Bidesh Biswas 1,2,3 , Magnus Garbrecht 4 and Bivas Saha 1,2,3 1 International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, 560064, INDIA. 2 Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, 560064, INDIA. 3 School of Advanced Materials, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, 560064, INDIA. 4 Australian Centre for Microscopy and Microanalysis, The University of Sydney, NSW 2006, Australia. Raman scattering from coupled plasmon-LO phonon modes in polar semiconductors is an effective tool to determine electronic properties such as carrier concentration, mobility, carrier freeze-out, relaxation times etc., as well as to understand different types of the electron (hole)- phonon interactions. The physics of such coupling mechanism traditionally utilize the Drude dielectric permittivity that predicts an increase in coupled plasmon-LO phonon mode (LPP+) frequencies with an increase in carrier concentrations. Here we demonstrate that for n-type epitaxial ScN thin-films, the frequencies of the coupled plasmon-LO phonon Raman modes exhibit red-shift with increasing carrier concentrations, which is contrary to the predictions from the Drude theory. Utilizing the generalized Lindhard dielectric function that takes into account both the frequency and wave-vector components of free electron plasma, we demonstrate that such decrease in the frequencies of the coupled plasmon-LO phonon mode in Raman spectra is related to the non-conserved wave-vectors due to inelastic scattering from magnesium (Mg) impurities. Modeling of the experimental Raman line-shape, intensity and frequencies illustrate that the wave-vector dependence of the coupled modes decreases with increasing electron concentrations. An asymmetric broadening of LO Raman modes was observed in films having large electron concentrations (>10 20 cm -3 ) that are explained by Fano resonance. Accepted Article