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,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 $Z_\mathrm{Sc}^* = -Z_\mathrm{N}^* = 3.78$. 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}$), the high frequency dielectric constant ($\varepsilon_\infty = 8.3$) and the static dielectric constant ($\varepsilon_\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,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 $n$.