Band gaps and phonons of quasi-bulk rocksalt ScN

Abstract

ScN is an emerging transition metal nitride with unique physical properties arising from the d-electrons of Sc. In this work we present the results of optical characterization techniques spectroscopic ellipsometry, Raman spectroscopy, and photoluminescence measurements of a 40 $\mu$m thick, fully relaxed, and only weakly n-type doped ($n=1.2\times 10^{18}$ cm${}^{-3}$) ScN film deposited by halide vapor phase epitaxy (HVPE) on r-sapphire substrate. Spectroscopic ellipsometry yields an indirect bandgap of 1.1 eV while the lowest direct interband transition is observed at $E_{\mathrm{g},\mathrm{opt}}=$ 2.16 eV in the dielectric function. A broad luminescence feature at 2.15 eV is observed, matching this transition. We derive an estimate for the exciton binding energy ($E_{bX}\approx$14 meV) as well as the Born effective charges $Z_{\mathrm{Sc}}^{\ast }=-Z_{\mathrm{N}}^{\ast }=3.78$. In the infrared spectral region we observe a strong phonon and a weak plasmon absorption. We precisely determine the transverse optical phonon eigenfrequency (${\omega }_{\mathrm{TO}}=$340.7 cm${}^{-1}$), the high frequency dielectric constant (${\epsilon }_{\infty }=8.3$) and the static dielectric constant (${\epsilon }_{\mathrm{stat}}=29.5$). Raman measurements using various excitation energies show resonant multi-phonon scattering up to 6LO (6th order overtone for longitudinal optical (LO) phonons) for excitation above the optical band gap ($E_{\mathrm{Laser}}>E_{\mathrm{g},\mathrm{opt}}$), where the allowed 2LO scattering is the dominant scattering mechanism for all excitation energies. Their characteristic parameters determined from Lorentzian line shape fitting yield ${\omega }_{\mathrm{LO}}=$681 cm${}^{-1}$ and an increased broadening and reduced asymmetry for higher LO scattering order $n$.