Integration of Er-doped crystalline thin film oxides with silicon nanocavities for quantum network applications

Rare earth ion defects in solid state hosts are promising candidates for use in quantum repeaters and memories needed for large-scale quantum networking, due to the ions’ inherent telecom spin-photon interface and long coherence times. Erbium-doped thin film cerium oxide (Er:CeO2)[1,2,3] and titanium oxide (Er:TiO2)[4] are both strong Er host material platforms that enable integration with silicon photonics. Cerium dioxide is appealing because its low natural nuclear spin abundance enables a long Er spin coherence time [3], and the small lattice mismatch with Si(111) allows for epitaxial growth of thin film CeO2, whereas TiO2 offers ease of synthesis, general CMOS compatibility, and facile etching chemistry [4]. However, the long optical lifetime of erbium’s telecom-band transition requires enhancement via coupling to resonant optical cavities to increase photon emission rates. Purcell enhancement of erbium ions in thin-film TiO2 has been demonstrated [4-6] using silicon one-dimensional nanophotonic crystal cavities, but the fabrication of such devices in CeO2 on Si is limited by the challenge of etching CeO2, which is impervious to most dry etching processes. In this work, we will discuss two approaches taken to integrate Er:Ox with silicon photonic cavities: (1) “Er:Ox first,” based upon MBE and ALD growth of Er:Ox on silicon followed by device top-down nanofabrication, as has been used for Er:TiO2 previously, and (2) “Er:Ox last,” by growing Er:CeO2 or Er:TiO2 directly on pre-fabricated silicon devices (with demonstrated Q factors > 60,000), a novel, material-agnostic approach that significantly simplifies integration. We will describe the device simulations, process flow, and nanofabrication processes we are developing for silicon 1D nanophotonic crystal cavities and these strategies for etching and/or integrating Er:Ox layers onto these devices. Following this we will describe the results of optical characterization studies of low-doped (a few ppm and below) oxides on Si enabled by device integration and Purcell enhancement of Er in thin film oxides integrated with silicon photonic devices.