Band gaps and phonons of quasi-bulk rocksalt ScN

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-m-thick fully relaxed, and only weakly n-type doped (n=1.2×10¹⁸0.16em0excm^-30.16em0ex) ScN film deposited by halide vapor phase epitaxy (HVPE) on r-sapphire substrate. Spectroscopic ellipsometry yields an indirect band gap of 1.10.16em0exeV, while the lowest direct interband transition is observed at Eg,opt=2.160.16em0exeV in the dielectric function. A broad luminescence feature at 2.150.16em0exeV is observed, matching this transition. We derive an estimate for the exciton binding energy (EbX≈140.16em0exmeV) as well as the Born effective charges ZSc*=-ZN*=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 (ωTO=340.70.16em0excm^-1), the high frequency dielectric constant (ε_∞=8.3), and the static dielectric constant (εₛₜₐₜ=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 (ELaser>Eg,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 ωLO=684.50.16em0excm^-1 and an increased broadening for higher LO scattering order n.