🔬 applied physics

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

This study reports the successful growth of epitaxial CrN(113) films on AlN templates and reveals a surprisingly strong in-plane optical anisotropy, which is attributed to an anisotropic surface morphology acting as a subwavelength grating rather than intrinsic crystal strain.

Original authors: Duc V. Dinh, Frank Peiris, Oliver Brandt

Published 2026-10-16

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CC BY 4.0

Original authors: Duc V. Dinh, Frank Peiris, Oliver Brandt

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Light behaves differently depending on the material it travels through and the direction it takes. In a perfect, uniform crystal, light usually moves the same way regardless of which way it is polarized, much like how sound travels equally in all directions through a calm, still lake. However, when scientists grow thin films of materials on a surface, the rules can change. The way atoms line up at the interface, combined with the roughness of the surface itself, can create a hidden directionality. This phenomenon, known as optical anisotropy, means that light traveling in one direction along the surface might be absorbed or slowed down differently than light traveling in a perpendicular direction. Understanding these subtle shifts is crucial for developing new electronic and optical devices, as it reveals how the microscopic structure of a material dictates its interaction with the world of light.

Researchers at the Paul-Drude-Institut for Solid-State Electronics in Berlin and Kenyon College in Ohio have uncovered a striking example of this effect in a material called chromium nitride. This compound is known for being incredibly hard and resistant to wear, making it useful for protective coatings, while also possessing semiconducting properties that allow it to conduct electricity under specific conditions. Typically, chromium nitride forms a cubic crystal structure, which is usually optically uniform. The team, led by Duc V. Dinh, decided to grow this material in a specific, less common orientation called (113) on a template made of aluminum nitride. They used a technique called plasma-assisted molecular beam epitaxy, a process where atoms are sprayed onto a heated surface in a vacuum to build the film layer by layer. The goal was to see if this specific growth angle, combined with the unique atomic mismatch between the two materials, would alter how the film interacts with light.

The results were surprising. When the team measured the optical properties of the chromium nitride film across a wide range of energy levels, they found that the material was not uniform at all. Instead, it acted like a material with a strong internal preference for direction. Light polarized in one direction along the surface behaved significantly differently than light polarized in the perpendicular direction. The difference was not a tiny, negligible blip; the researchers measured a variation of more than 10 percent in how the material refracted light and up to 15 percent in how much it absorbed it. This level of difference is substantial, far exceeding what is usually seen in perfect crystals due to internal atomic quirks. The effect was so strong that the film essentially acted as if it were two different materials depending on the angle of the incoming light.

To understand why this was happening, the scientists had to look closely at the physical structure of the film. They examined the crystal lattice and found that while the film was largely relaxed and free of the heavy internal stress that often causes such effects, the surface itself was far from smooth. Using atomic force microscopy, they discovered that the surface was covered in tiny, elongated ridges and trenches running in a single direction, creating a striped pattern. This pattern was a direct copy of the underlying aluminum nitride template, which itself had an anisotropic, or direction-dependent, structure. The ridges were spaced so closely together that they were smaller than the wavelength of the light used to measure them.

The researchers concluded that this microscopic topography was the true cause of the optical mystery. The surface ridges and valleys formed what is known as a subwavelength grating. Because the light waves were larger than the spacing of these ridges, they could not resolve the individual bumps and valleys. Instead, the light "saw" an averaged surface that behaved differently depending on whether the light waves were vibrating parallel to the ridges or perpendicular to them. This created an artificial anisotropy, a property that emerged from the shape of the surface rather than the chemical composition of the atoms themselves. To confirm this, they compared their chromium nitride film to a similar film of scandium nitride grown on the same type of template. While the scandium nitride also showed some anisotropy, it was much weaker. This difference was explained by the fact that light penetrates deeper into scandium nitride than into chromium nitride, meaning the light in the scandium sample was less influenced by the surface texture and more influenced by the uniform bulk material underneath.

This discovery highlights how the physical shape of a material's surface can fundamentally change its optical character, even when the material itself is chemically uniform. The chromium nitride film did not need to be a new chemical compound to exhibit this behavior; it simply needed to be grown in a way that created a specific, directional texture. The findings suggest that by carefully controlling the growth conditions to engineer surface patterns, scientists can tune how materials interact with light without changing their chemical makeup. This opens a path for designing materials with custom optical properties, where the surface structure acts as a lens or a filter for specific directions of light, all while maintaining the robust mechanical and electrical properties of the underlying material.

Technical Summary: Large In-Plane Optical Anisotropy in Epitaxial CrN(113) Films

Problem Statement
Chromium nitride (CrN) is a transition-metal nitride with a cubic rocksalt structure, known for its mechanical robustness, semiconducting behavior, and thermoelectric functionality. While the optical properties of CrN have been studied in polycrystalline forms and epitaxial films grown along high-symmetry orientations like (001) and (111), these orientations typically exhibit isotropic in-plane optical responses consistent with cubic symmetry. However, optical anisotropy can arise in cubic crystals due to second-order effects related to intrinsic bulk mechanisms, surface reconstructions, or strain, particularly on low-symmetry surfaces. Despite extensive studies on anisotropic optical responses in low-symmetry semiconductor and oxide surfaces (e.g., Si(110), GaAs(110)), analogous investigations in transition-metal nitrides remain scarce. This study addresses the gap in understanding the optical properties of CrN grown along the low-symmetry (113) orientation, where reduced surface symmetry and highly anisotropic lattice mismatch may significantly alter physical properties.

Methodology
The researchers synthesized 200-nm-thick, intentionally undoped CrN(113) films on semipolar AlN(11 $\bar{2}$ 2) templates (grown on Al $_2$ O $_3$ (10 $\bar{1}$ 0) substrates) using plasma-assisted molecular beam epitaxy (PAMBE).

To characterize the films, the study employed a multi-faceted approach:

Structural Analysis: High-resolution X-ray diffraction (HR-XRD) was used to determine crystallographic orientation, epitaxial relationships, and strain states via symmetric $2\theta-\omega$ scans and reciprocal space maps (RSM). Atomic force microscopy (AFM) in contact mode was utilized to image surface morphology.
Optical Characterization: Variable-angle spectroscopic ellipsometry (SE) was performed at room temperature over a photon-energy range of 1.25–5.0 eV. Crucially, measurements were taken at four angles of incidence (55°, 60°, 65°, 70°) and nine different azimuthal orientations ( $\phi$ ) spanning 0° to 180°.
Data Analysis: A multi-azimuth approach was used to reconstruct the in-plane dielectric tensor components. The data were fitted using a Kramers–Kronig-consistent B-spline parameterization within a parametric multilayer model. This overdetermined fitting strategy allowed for the direct extraction of principal-axis optical constants ( $n$ and $k$ ) and the assessment of depolarization, avoiding the limitations of standard two-angle measurements.

Key Results

Structural Properties: The CrN(113) layers exhibit a well-defined epitaxial relationship with the AlN(11 $\bar{2}$ 2) template. XRD analysis reveals a two-component lattice tilt within the (113) plane, resulting from rotations about in-plane axes parallel to the [33 $\bar{2}$ ] $_{CrN}$ and [1 $\bar{1}$ 0] $_{CrN}$ directions. This tilt arises from the strongly anisotropic lattice mismatch between CrN and AlN (approximately -0.14% along [1 $\bar{1}$ 0] and -9.82% along [33 $\bar{2}$ ]). Despite this initial strain, the films are essentially fully relaxed in the final state.
Surface Morphology: AFM imaging shows that the CrN(113) film replicates the anisotropic, stripe-like morphology of the underlying AlN template. The surface features elongated ridges and trenches running along the [1 $\bar{1}$ 0] $_{CrN}$ direction, with a root-mean-square roughness of 5.2 nm. The ridges lack a well-defined periodicity but form a subwavelength structure relative to the SE measurement range (250–990 nm).
Optical Anisotropy: Spectroscopic ellipsometry reveals a surprisingly strong polarization dependence in the optical response, despite the nearly strain-free cubic crystal structure. The pseudo-optical constants $n$ and $k$ differ by more than 10% for orthogonal in-plane polarizations throughout the 1.25–5.0 eV range. Specifically, the relative change in reflectance ( $\Delta R/R$ ) reaches 10–12% for $n$ and 3–15% for $k$ .
Control Experiments: Reference measurements on CrN(111) films (which possess isotropic surface morphology) showed negligible anisotropy, confirming that the effect is specific to the (113) orientation and its associated morphology. Similarly, ScN(113) films with similar morphology exhibited anisotropy, but of a smaller magnitude (3–5%), suggesting that material-specific optical constants (specifically penetration depth) modulate the effect.

Significance and Conclusions
The paper reports the first demonstration of large in-plane optical anisotropy in epitaxial transition-metal nitrides. The authors conclude that the observed anisotropy (exceeding 10%) is too large to be attributed to intrinsic mechanisms or strain-induced symmetry breaking, given the relaxed state of the films. Instead, the effect is attributed to the anisotropic surface morphology of the CrN(113) films.

The authors propose that the elongated ridges and trenches on the surface act as an effective subwavelength grating (metasurface). In the strongly absorbing regime of CrN (where the optical penetration depth is limited to 20–40 nm), the light interacts significantly with this structured near-surface region, inducing form birefringence. The magnitude of the anisotropy is determined by the interplay between the geometric dimensions of the surface modulation and the complex refractive index of the material; this explains why morphologically similar ScN(113) films show weaker anisotropy due to their larger optical penetration depth.

This work establishes that surface morphology can dominate the optical response of nominally isotropic cubic materials, creating a platform for engineering optical properties through growth orientation and surface structuring without altering the bulk crystal symmetry.

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