Optical and microstructural studies of erbium-doped TiO2 thin films on silicon, SrTiO3, and sapphire

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 TiO2 is an attractive candidate: the Er3+ optical transition is compatible with C-band optical fiber communications, while TiO2 is an insulating dielectric compatible with silicon process technology. Through structural and optical studies of Er-doped TiO2 thin films grown via molecular beam deposition on silicon, SrTiO3, and sapphire substrates, we have explored the impact of polycrystallinity and microstructure on the optical properties of the Er emission. Comparing polycrystalline TiO2(rutile)/Si with single-crystalline TiO2(rutile)/r-sapphire and polycrystalline TiO2(anatase)/Si with single-crystalline TiO2(anatase)/ SrTiO3, we observe that the inhomogeneous linewidth ( Gamma(inh)) of the most prominent peak in the Er spectrum (the Y-1- Z(1) transition, 1520 and 1533 nm in rutile and anatase TiO2) 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 TiO2 buffer on Si can reduce Gamma(inh) by a factor of 4-5. Expectedly, Gamma(inh) also reduces with decreasing Er concentrations: we observe a similar to 2 order of magnitude reduction from similar to 1000 ppm Er to similar to 10 ppm Er. Gamma(inh) then gets limited to a residual value of similar to 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.