Optical properties of ScN layers grown on Al$_{2}$O$_{3}$(0001) by plasma-assisted molecular beam epitaxy

An accurate knowledge of the optical constants (refractive index $n$ and extinction coefficient $k$) of ScN is crucial for understanding the optical properties of this binary nitride semiconductor as well as for its use in optoelectronic applications. Using spectroscopic ellipsometry in a spectral range from far infrared to far ultraviolet (0.045-8.5 eV), we determine $n$ and $k$ of ScN layers grown on Al$_{2}$O$_{3}$(0001) substrates by plasma-assisted molecular beam epitaxy. Fits of ellipsometry data return the energies of four oscillators representing critical points in the band structure of ScN, namely, 2.03, 3.89, 5.33, and 6.95 eV. As the infrared range is dominated by free carriers, the vibrational properties of the layers are examined by Raman spectroscopy. Despite the rocksalt structure of ScN, several first-order phonon modes are observed, suggesting a high density of point defects consistent with the high electron density deduced from Hall measurements. Finally, photoluminescence measurements reveal an emission band slightly above the lowest direct bandgap. We attribute the redshift of the peak emission energy from 2.3 to 2.2 eV with increasing layer thickness to a reduction of the O concentration in the layers.