The control of rare-earth ion doping profiles is a key challenge for several photonic applications and quantum technologies that require spatially localized emitters. In this work, we propose to use atomic layer deposition (ALD) followed by an annealing post-treatment to localize europium emitters close to the surface of a Y2O3 film or a Y2SiO5 single crystal by exploiting in-diffusion. Indeed, ALD is a conformal method that can provide in-depth nanometer-scale positioning accuracy on a large scale. However, the post-thermal annealing required to achieve higher crystalline quality and activate diffusion needs to be precisely controlled to maximize our ability to localize ions. In this paper, we evaluate europium ion diffusion in an ALD-grown Eu2O3/Y2O3/Si stack using Rutherford backscattering spectroscopy (RBS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). We then extend this approach to investigate diffusion from a Eu2O3 ALD film into a single-crystalline substrate of Y2SiO5 (YSO), a technologically relevant material system for quantum applications. We determine the Eu3+ diffusion coefficients in both cases and show that diffusion starts at 950 °C in the polycrystalline Y2O3 ALD submicron film, whereas it becomes significant only above 1200 °C in single-crystal YSO. Finally, spectral hole burning of such in-diffused emitters revealed homogeneous lines as narrow as 2 MHz. This study indicates that appropriate annealing of ALD-grown rare-earth oxide films can be harnessed to create localized dopants that preserve their outstanding optical properties, a prerequisite for their integration into photonic and quantum devices.