Growth of Ceria Films on Silicon with Rare Earth Doping

For quantum memories, a critical parameter is the coherence time, which quantifies the duration for which a quantum state can be preserved. Rare Earth Ions (REIs) are known for their extended coherence times, especially when embedded within bulk solid-state crystalline hosts [1]. However, the coherence time of qubits is often compromised by magnetic noise arising from the nuclear and electron spin moments of the host atoms. This noise can be mitigated using isotopically purified samples, wherein nuclei with non-zero magnetic moments are eliminated [2]. On the other hand, naturally occurring Cerium (Ce) stands out among the lanthanoids as its stable isotopes all possess zero nuclear spin moments. Since oxygen exhibits a minimal presence of magnetic nuclei (0.3 per thousand), the intrinsic coherence time of a two-level system is expected to be preserved in a CeO₂ host crystal [2, 3]. CeO₂ is particularly advantageous due to its large bandgap, accommodating color centers from the visible spectrum—ideal for sensing applications—to the mid-infrared range, which is crucial for quantum communication. Additionally, CeO₂'s compatibility with silicon enables the utilization of advanced nanofabrication techniques developed for silicon-based systems [4]. As a lanthanide-based host, CeO₂ effectively supports lanthanide color centers, which emit within the telecom spectrum, a key requirement for quantum communication and networking. Moreover, CeO₂'s large optical refractive index (~2.45) makes it suitable for integration into silicon-based photonic devices.<br/><br/>High-quality CeO₂ films were grown on yttria-stabilized zirconia (YSZ)-buffered Si (100) substrates using RHEED and low angle x-ray spectroscopy (LAXS)-assisted PLD systems, resulting in single-crystalline films as confirmed by RHEED and HRXRD. Rocking curve measurements yielded FWHM values of 0.8, and 0.9 for the CeO2 (200), and YSZ (200) peaks, respectively, indicating excellent film quality.<br/><br/>Future work includes doping the CeO₂ matrix with less than 1% rare earth ions (Tm, Tb, Er, Yb, Ho) and conducting optical characterization, such as photoluminescence (PL) and photoluminescence excitation (PLE), to study the optical emission and lifetimes of these films. In addition, the coherence control of spin and nuclear moments will be explored by Hahn echo experiments and Ramsey coherence measurements.<br/><br/><br/>References<br/><br/>1. Zhong, M., et al., <i>Optically addressable nuclear spins in a solid with a six-hour coherence time.</i>Nature, 2015. <b>517</b>(7533): p. 177-180.<br/>2. Wolfowicz, G., et al., <i>Quantum guidelines for solid-state spin defects.</i> Nature Reviews. Materials, 2021. <b>6</b>: p. Medium: ED; Size: p. 906-925.<br/>3. Anderson, C.P., et al., <i>Five-second coherence of a single spin with single-shot readout in silicon carbide.</i> Science Advances, 2022. <b>8</b>(5): p. eabm5912.<br/>4. Nishikawa, Y., et al., <i>Interfacial properties of single-crystalline CeO2 high-k gate dielectrics directly grown on Si (111).</i> Applied Physics Letters, 2002. <b>81</b>(23): p. 4386-4388.