Epitaxial Er-doped Y₂O₃ on silicon for quantum coherent devices

Rare-earth ions (REIs) have incomplete 4f shells and possess narrow optical intra-4f transitions due to shielding from electrons in the 5s and 5p orbitals, making them good candidates for solid-state optical quantum memory. The emission of Er³⁺ in the telecom C-band (1530 nm-1565 nm) makes it especially attractive for this application. In order to build practical, scalable devices, the REI needs to be embedded in a non-interacting host material, preferably one that can be integrated with silicon. In this paper, we show that Er³⁺ can be isovalently incorporated into epitaxial Y₂O₃ thin films on Si (111). We report on the synthesis of epitaxial, single-crystalline Er:Y₂O₃ on Si with a narrow inhomogeneous linewidth in the photoluminescence (PL) spectra, 5.1 GHz (<100 mK), and an optical excited state lifetime of 8.1 ms. The choice of Y₂O₃ was driven by its low nuclear spin and small lattice mismatch with Si. Using PL and electron paramagnetic resonance, we show that Er³⁺ substitutes for Y in the crystal lattice. The role of interfacial SiO_x, diffusion of silicon into the film, and the effect of buffer layers on the inhomogeneous PL linewidth are examined. We also find that the linewidth decreased monotonically with film thickness but surprisingly exhibits no correlation with the film crystalline quality, as measured by the x-ray rocking curve scans, suggesting other factors at play that limit the inhomogeneous broadening in Y₂O₃ films.