ScN is an emerging transition metal nitride with unique physical properties arising from the $d$ electrons of Sc. In this letter, we present the results of optical characterization techniques: spectroscopic ellipsometry, Raman spectroscopy, and photoluminescence measurements of a $40\text{−}\mu \mathrm{m}$-thick fully relaxed, and only weakly $n$-type doped ($n=1.2\times {10}^{18}{\mathrm{cm}}^{-3}$) ScN film deposited by halide vapor phase epitaxy (HVPE) on r-sapphire substrate. Spectroscopic ellipsometry yields an indirect band gap of $1.1\mathrm{eV}$, while the lowest direct interband transition is observed at $E_{\mathrm{g},\mathrm{opt}}=2.16\mathrm{eV}$ in the dielectric function. A broad luminescence feature at $2.15\mathrm{eV}$ is observed, matching this transition. We derive an estimate for the exciton binding energy ($E_{\mathrm{bX}}\approx 14\mathrm{meV}$) as well as the Born effective charges $Z_{\mathrm{Sc}}^{*}=-Z_{\mathrm{N}}^{*}=3.81$. In the infrared spectral region, we observe a strong phonon and a weak plasmon absorption. We precisely determine the transverse optical (TO) phonon eigenfrequency (${\omega }_{\text{TO}}=340.7{\mathrm{cm}}^{-1}$), the high frequency dielectric constant (${\varepsilon }_{\infty }=8.3$), and the static dielectric constant (${\varepsilon }_{\text{stat}}=29.5$). Raman measurements using various excitation energies show resonant multiphonon scattering up to 6LO [sixth-order overtone for longitudinal optical (LO) phonons] for excitation above the optical band gap ($E_{\text{Laser}}>E_{\mathrm{g},\text{opt}}$), where the allowed $2\mathrm{LO}$ scattering is the dominant scattering mechanism for all excitation energies. Their characteristic parameters determined from Lorentzian line shape fitting yield ${\omega }_{\text{LO}}=684.5{\mathrm{cm}}^{-1}$ and an increased broadening for higher LO scattering order $n$.

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• Dielectric properties
• Phonons
• Ellipsometry
• Optical techniques
• Raman spectroscopy