The potential of Ge as n-type dopant for AlGaN materials is currently under consideration. Ge is known to generate significantly less local tensile strain than silicon, whatever the Al mole fraction of the alloy. However, at high doping concentrations, the issue of the effective dopant incorporation and its spatial distribution is relevant. Formation of precipitates or diffusion of dopants along structural defects may occur [1]. Recently, we have demonstrated plasma-assisted molecular-beam epitaxy growth of AlxGa1−xN planar layers with Ge nominal concentrations up to 1021 cm-3 [2]. Here, we present a structural analysis of AlxGa1−xN:Ge samples with x covering the full compositional range (0≤x≤1), together with the measurement of Ge concentration and its spatial distribution from the µm range down to the nm scale [3]. Our study is based on a combination of X-ray diffraction (XRD) and quantitative energy dispersive X-ray spectrometry (EDX) coupled to high-resolution scanning transmission electron microscopy (HR-STEM) and atom probe tomography (APT). In AlxGa1−xN:Ge samples with x>0.4, top-view SEM observations reveal the presence of µm-size crystallites at the surface. The nature of such crystallites is pure Ge, as verified by EDX. Inside those layers, we have also identified Ge-rich inclusions with a size of tens of nanometers, generally associated with Ga-rich regions around structural defects. With these local exceptions, the AlxGa1−xN:Ge matrix presents homogenous Ge composition. The Ge concentration in the matrix, i.e. away from the precipitates, was obtained from the ratio of the EDX signal between doped and undoped samples. The Ge content in the binary AlN matrix is extremely low, and it increases linearly with the Ga mole fraction in ternary AlxGa1−xN, which suggests that the Ge incorporation takes place by substitution of Ga atoms. The maximum percentage of Ga sites occupied by Ge saturates around 1%. These solubility issues and the Ge segregation phenomena should play a role in the efficiency of Ge as n-type dopant, even at Al concentrations where Ge DX centers are not expected to manifest. In view of these results, the extracted solubility limit of Ge in AlxGa1−xN can have direct impact on the performance of AlxGa1−xN-based UV light emitting diodes. In principle, the Ge content should be kept below these limits to prevent a degradation of carrier transport due to scattering at structural defects. Co-doping or the use of surfactant species should be explored to attain Ge concentrations beyond the solubility limit. References [1] L. Amichi, et al., Nanotechnol. 31, 045702 (2020). [2] R. Blasco, et al., J. Phys. D.: Appl. Phys. 52, 125101 (2019). [3] C. Bougerol, et al., ACS Applied Materials & Interfaces 13, 4165 (2021).