  The use of trivalent erbium (Er$^{3+}$), typically embedded as an atomic
defect in the solid-state, has widespread adoption as a dopant in
telecommunications devices and shows promise as a spin-based quantum memory for
quantum communication. In particular, its natural telecom C-band optical
transition and spin-photon interface makes it an ideal candidate for
integration into existing optical fiber networks without the need for quantum
frequency conversion. However, successful scaling requires a host material with
few intrinsic nuclear spins, compatibility with semiconductor foundry
processes, and straightforward integration with silicon photonics. Here, we
present Er-doped titanium dioxide (TiO$_2$) thin film growth on silicon
substrates using a foundry-scalable atomic layer deposition process with a wide
range of doping control over the Er concentration. Even though the as-grown
films are amorphous, after oxygen annealing they exhibit relatively large
crystalline grains, and the embedded Er ions exhibit the characteristic optical
emission spectrum from anatase TiO$_2$. Critically, this growth and annealing
process maintains the low surface roughness required for nanophotonic
integration. Finally, we interface Er ensembles with high quality factor Si
nanophotonic cavities via evanescent coupling and demonstrate a large Purcell
enhancement (300) of their optical lifetime. Our findings demonstrate a
low-temperature, non-destructive, and substrate-independent process for
integrating Er-doped materials with silicon photonics. At high doping densities
this platform can enable integrated photonic components such as on-chip
amplifiers and lasers, while dilute concentrations can realize single ion
quantum memories.
