Large in-plane optical anisotropy in epitaxial CrN(113) films

Abstract

CrN, a transition-metal nitride crystallizing in the cubic rocksalt structure, exhibits a unique combination of mechanical robustness, semiconducting behavior, and thermoelectric functionality. To date, however, CrN has been synthesized primarily as polycrystalline material or as epitaxial films grown along high-symmetry orientations such as (001) and (111). Here, we prepare epitaxial CrN(113) films on AlN(11\bar{2}2) templates by plasma-assisted molecular beam epitaxy. The low-symmetry interface and highly direction dependent lattice mismatch are expected to modify the physical properties of CrN relative to the bulk material. Indeed, variable-angle spectroscopic ellipsometry in the photon-energy range from 1.25 to 5 eV reveals a surprisingly strong polarization dependence of the optical response despite the nearly strain-free cubic crystal structure of the films. The pseudo-optical constants n and k differ by more than 10% for orthogonal in-plane polarizations throughout the investigated spectral range. We evaluate possible origins of this effect and conclude that it most likely arises from anisotropic surface morphology of CrN(113), which creates a metasurface acting as a subwavelength grating in the strongly absorbing regime.