Orbital-mediated local symmetry in ScAlN probed by scandium K-edge XAFS

Dear Editor,

X-ray absorption fine structure (XAFS), especially X-ray absorption near-edge structure (XANES), delivers an element-specific fingerprint of local bonding. Photoelectric absorption at characteristic edges responds to near-neighbor coordination through edge shifts (effective charge) and near-edge modulations (transition pathways), making it exquisitely sensitive to short-range order, defects, and mixed coordination that elude diffraction. In III-nitride semiconductors, this sensitivity is pivotal. Sc doping in AlN platforms markedly tunes electrical, piezo/ferroelectric, and linear/nonlinear optical responses while remaining process compatible, positioning ScAlN for use in power electronics, acoustic devices, memories, and neuromorphic hardware. Critically, Sc lowers the energy barrier between wurtzite and layered hexagonal motifs and can drive local transitions among wurtzite (w-ScN), metastable layered hexagonal (h-ScN), and rock-salt (r-ScN) phases, each with distinct symmetry/coordination. Diffraction signatures—e.g., systematic weakening/broadening of the ScAlN (002) peak with rising Sc content—reflect disorder yet cannot resolve the short-range distortions that govern polarity and property changes.

The Sc K-edge (4,492 eV) lies in the tender X-ray regime, where air absorption and detector limitations degrade the signal-to-noise ratio. Standard Lytle-based fluorescence often lacks suitable filters for Sc, yielding suboptimal spectra; therefore, low-pressure beam paths and high-count-rate solid-state detectors are essential for reliable measurements (Figure 1A). Thanks to advances in synchrotron and laboratory tender XAFS instrumentation,7 the Sc K-edge XAFS collection is expected to become a routine procedure similar to X-ray diffraction (XRD) analysis. This development is crucial for the accurate identification of the Sc doping state in materials. Here, we leverage Sc K-edge XAFS to map the interplay between local symmetry, p-d hybridization, and local strain in ScAlN, revealing how Sc coordination plays roles in the Sc-III-nitride semiconductors.