Heterogeneous Integration of Solid State Defect and Dopant Qubit Systems on Silicon

Solid state defect or dopant qubits embedded in wide gap semiconductor and insulator hosts are important for devices in quantum information processing such as sensors [1], quantum memories [2] and single photon emitters [3]. Many of these materials belong to the “exotic” to “expensive” class (e.g. diamond, SiC, YIG, various rare earth and other oxides): they can be difficult to process, ,or are limited in availability and purity. In addition to meeting performance specifications, turning these compounds into members of the “electronics materials proletariat”--making them chip-scale, on Si platforms, processable using scalable cleanroom techniques, and integrable with electronics--is the challenge. With this in mind, we will describe our work in two ongoing directions.

The Er atom offers a unique spin-photon interface compatible with long-distance optical fiber communications due to its electron spin qubit combined with its 1.5 mm (telecom C-band) optical transition. This has led to broad effort on establishing Er-doped oxide host systems [4] as quantum memory devices, a limiting component for the quantum repeaters necessary for long distance quantum communication. Our work has targeted developing such Er-doped metal oxide thin films (TiO₂, CeO₂, CaMoO₄, Y₂O₃) on Si substrates [5-8]. Correlating optical (inhomogeneous & homogeneous linewidths, spectral diffusion via photoluminescence excitation) and microstructural (electron microscopy and X-ray diffraction) analysis with growth studies that explore crystallinity, Er density, and film thickness across a large number of samples and different materials, we have explored the factors that control optical properties – for example, showing evidence of the role of charged defects and Er density variation. We have further explored processing and nanofabrication of 1-D photonic cavities demonstrating Purcell enhancement of ~300X in Er:TiO₂/Si heterostructures [9], the isolation of emission from single Er⁺³ ions [10], and the adaptation of atomic layer deposition (ALD) techniques for ppm doped Er:TiO₂ and Er: CeO₂ [11].

We are also studying the heterogeneous integration of materials by “spalling” 1-50 mm thick large area layers from single crystal wafers of materials relevant to quantum technologies such as SiC, diamond and YIG/GGG and integrating them onto arbitrary platforms without the need for high thermal budgets or ion implantation. While spalling of low fracture toughness Si and GaAs is well established, the spalling of significantly harder materials relevant for us is new and we will describe our design of such spalling methods. We have spalled commercially available high purity 4H-SiC and achieve coherent spin control of neutral divacancy (VV⁰) qubit ensembles with quasi-bulk spin coherence in the exfoliated films [12].

[1] C. Degen, et al., Rev. Mod. Phys., 89, 035002 (2017).
[2] D. Awschalom, et al., PRX Quantum, 2, 017002 (2021).
[3] I. Aharonovich, et al., Nature Photonics, 10, 631-641 (2016).
[4] P. Stevenson, et al., PRB, 105, 224106 (2022).
[5] M. Singh, et al., APL Materials, 8, 3 (2020).
[6] M. Singh, et al., JAP, 136, 12 (2024).
[7] G. Grant, et al., APL Materials, 12, 2 (2024).
[8] J. Zhang, et al., npj Quantum Information, in press (2024); arXiv:2309.16785
[9] A. Dibos, et al., Nano Letters, 22.16, 6530-6536 (2022).
[10] C. Ji, et al., Appl. Phys. Lett. 125, 084001 (2024).
[11] C. Ji, et al., ACS Nano, 18.14, 9929-9941 (2024).
[ 12] C. Horn et al., ACS Nano, in press (2024) ; arXiv : 2404.19716.