J. Appl. Phys. 136, 124402 (2024)
Rare-earth ion doped oxide thin films integrated on silicon substrates provide a route toward scalable, chip-scale platforms for quantum coherent devices. Erbium-doped is an attractive candidate: the optical transition is compatible with C-band optical fiber communications, while is an insulating dielectric compatible with silicon process technology. Through structural and optical studies of Er-doped thin films grown via molecular beam deposition on silicon, , and sapphire substrates, we have explored the impact of polycrystallinity and microstructure on the optical properties of the Er emission. Comparing polycrystalline (rutile)/Si with single-crystalline (rutile)/r-sapphire and polycrystalline (anatase)/Si with single-crystalline (anatase)/ , we observe that the inhomogeneous linewidth ( ) of the most prominent peak in the Er spectrum (the – transition, 1520 and 1533 nm in rutile and anatase ) is significantly narrower in the polycrystalline case. This implies a relative insensitivity to extended structural defects and grain boundaries in such films (as opposed to, e.g., point defects). We show that the growth of an undoped, underlying buffer on Si can reduce by a factor of 4–5. Expectedly, also reduces with decreasing Er concentrations: we observe a 2 order of magnitude reduction from 1000 ppm Er to 10 ppm Er. then gets limited to a residual value of 5 GHz that is insensitive to further reduction in the Er concentration. Based upon the above results, we argue that the optical properties in these thin films are limited by the presence of high “grown-in” point defect concentrations.