We present a comprehensive optical characterization of 200-nm-thick CrN(111) films grown simultaneously on Al2O3(0001) and AlN/Al2O3(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 reveals 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.
@article{arxiv.2511.22484,
title = {Reststrahlen band and optical bandgaps in semiconducting CrN films},
author = {Duc V. Dinh and Xiang Lü and Oliver Brandt and Dilara Sen and Olivia Fairlamb and Frank Peiris and Farihatun Lima and Alexander Bordovalos and Suresh Chaulagain and Ambalanath Shan and Nikolas J. Podraza},
journal= {arXiv preprint arXiv:2511.22484},
year = {2025}
}