Piecing Together the Electronic Identity of Chromium Nitride: Electrical and Optical Bandgaps Resolved

Researchers at the Paul-Drude-Institut für Festkörperelektronik (PDI, Berlin), the Leibniz-Institut für Kristallzüchtung (IKZ, Berlin), Kenyon College (Ohio, USA), and the University of Toledo (Ohio, USA) have published two complementary studies that together resolve the long-debated electronic structure of chromium nitride (CrN). A hard, corrosion-resistant compound with potential in electronics, thermoelectrics, and spintronics, CrN had resisted reliable characterization for years, with reported bandgap values ranging from 0.02 to 0.7 eV depending on measurement technique and sample quality. By combining precision electrical transport measurements with broadband optical spectroscopy on high-quality epitaxial films, the team has produced a coherent and mutually consistent picture for the first time.

The electrical study (Advanced Electronic Materials, 2026) examined ~75-nm-thick CrN films grown by plasma-assisted molecular beam epitaxy and characterized by Hall-effect measurements across an exceptional temperature range of 4 to 920 K. Analysis of the temperature-dependent carrier density between 300 and 920 K yielded two activation energies associated with intrinsic conduction: 0.15 eV, attributed to the fundamental bandgap, and 0.50 eV, representing a higher energy transition. Critically, this wide temperature range was essential, as prior estimates based on resistivity measurements limited to below 400 K were unable to cleanly separate extrinsic from intrinsic conduction regimes, leading to systematic underestimates. Carrier mobility was found to be governed by ionized-impurity scattering below 400 K and phonon scattering above 400 K, consistent with a nondegenerate semiconductor with an unusually heavy electron effective mass. 

The optical study (Physical Review Materials, 2026) connects directly to these findings. Epitaxial CrN thin films were grown simultaneously on two substrates and characterized by spectroscopic ellipsometry over a broad energy range from 0.04 to 5.5 eV, revealing two direct interband transitions with onsets at approximately 0.35 and 0.60 eV. The lower optical transition corresponds directly to the 0.50 eV electrical bandgap identified in the transport study, with the optical value being slightly smaller due to the presence of bandtail states. The absence of any optical signature near 0.15 eV supports the conclusion that the fundamental bandgap is indirect and therefore optically silent in 200-nm-thick films. The two studies thus reinforce each other: the electrical measurements establish the gap energies, and the optical measurements confirm their direct or indirect character. 

In the infrared, the optical data reveal a pronounced Reststrahlen band, a spectral window of near-total infrared reflectance, bounded by transversal and longitudinal optical phonon modes at ~403 and ~629 cm⁻¹. This is the first direct observation of this feature in epitaxial CrN via spectroscopic ellipsometry. A Born effective charge of ~2.7 indicates that CrN is partially ionic, with dynamic charge transfer between chromium and nitrogen atoms during lattice vibrations. Together, the dielectric constants, phonon frequencies, and Born effective charges provide a detailed picture of the interplay between lattice dynamics and carrier transport.

The combined dataset, spanning electrical transport from cryogenic temperatures to 920 K and optical constants from the far-infrared through the ultraviolet, establishes CrN as a narrow-gap semiconductor with an indirect fundamental gap of ~0.15 eV and two direct transitions at ~0.35 and ~0.60 eV. For industry, the Reststrahlen band in the 15–25 µm range positions CrN as a candidate for mid-infrared photonic and thermal management applications, while its coexistence of semiconducting behavior and antiferromagnetic ordering makes it relevant to emerging spintronic device concepts.

Title: Reststrahlen band and optical bandgaps in semiconducting CrN films
Authors: D. V. Dinh, X. Lü, O. Brandt, D. Sen, O. Fairlamb, F. Peiris, F. Lima, A. Bordovalos, C. Suresh, A. Shan, N. J. Podraza 
Source: Phys. Rev. Mater., 10, 044602
DOI: 10.1103/gt8b-1psw

Title: Carrier transport and electrical bandgaps in epitaxial CrN layers
Authors: D. V. Dinh, J. Herfort, A. Fiedler, O. Brandt 
Source: Adv. Electron. Mater., 12, e70364
DOI: 10.1002/aelm.70364

CReA: Nitride Semiconductors