Reststrahlen band and optical bandgaps in semiconducting CrN films

We present a comprehensive optical characterization of 200-nm-thick CrN(111) films grown simultaneously on Al 2 O 3 ( 0001 ) and AlN / Al 2 O 3 ( 0001 ) using plasma-assisted molecular beam epitaxy. Spectroscopic ellipsometry, spanning the far-infrared to ultraviolet range ( 0.04 – 5.5 eV ), is conducted at room temperature to determine the optical constants n and k of the films. Spectral fits reveal two interband transitions at approximately 0.35 and 0.60 eV. In the infrared range, the ellipsometry data also reveal a pronounced Reststrahlen band stemming from transversal and longitudinal optical phonons at approximately 403 and 629 cm − 1 , respectively. The relative static and high-frequency permittivities are estimated to be about 39 and 15, respectively. A Born effective charge of approximately 2.7, extracted from the far-infrared region, indicates that CrN is partially ionic.