Atomic-scale microscopy insights into order and disorder phenomena in AlxGa1-xN.

Order and disorder in III-V compound semiconductors critically determine their electronic, optical, magnetic, thermal, and mechanical properties, directly impacting device performance and reliability. In these materials, disorder manifests through elemental distribution in ternary and quaternary alloys, substitutional arrangements on sublattices, solute impurity distribution, point and extended defects, and compositional variations across heterostructure interfaces. These phenomena affect band-gap engineering, carrier mobility and recombination rates, emission wavelengths in lasers and LEDs, absorption spectra in photodetectors and solar cells, thermal management, and emerging spintronic functionalities. While reciprocal-space techniques like X-ray diffraction have provided valuable ensemble-averaged information for decades, understanding local atomic arrangements and nanoscale compositional fluctuations in III-V semiconductors requires direct-space imaging with atomic resolution. This presentation examines three decades of research applying atomic-scale microscopy techniques, namely atom-probe tomography (APT) and scanning transmission electron microscopy (STEM), to III-V materials, and in particular AlxGa1-xN [1]. APT enables three-dimensional compositional mapping of alloys with near-atomic resolution, quantifying chemical species distribution and interfacial abruptness in heterostructures. Aberration-corrected STEM offers versatile imaging and spectroscopy for characterizing crystal structure, interface quality, and defect configurations [2,3]. We comprehensively review the application of these techniques to key phenomena in AlxGa1-xN: spontaneous atomic ordering, compositional fluctuations (fig. 1), phase separation, short-range and long-range order quantification, interface roughness, and the interplay between cellular disorder (chemical distribution) and structural disorder (dislocations, stacking faults, inversion domains) (fig. 2). Analytical and statistical methods specific to each technique are discussed, along with correlative approaches connecting microscopy with functional properties. This work establishes a methodological reference framework for III-V semiconductor researchers, synthesizing achievements and identifying future opportunities. [1] E. Di Russo, et al., Order and disorder at the atomic scale: Microscopy applied to semiconductors. Reviews of Modern Physics 97.2, 025006 (2025). [2] L. Amichi et al., Three-dimensional measurement of Mg dopant distribution and electrical activity in GaN by correlative atom probe tomography and off-axis electron holography. Journal of Applied Physics 127.6 (2020). [3] C. Bougerol et al., Solubility limit of Ge dopants in AlGaN: A chemical and microstructural investigation down to the nanoscale. ACS applied materials & interfaces 13.3, 4165-4173 (2021).