Selective area growth and stencil lithography for in situ fabricated quantum devices

Abstract

The interplay of Dirac physics and induced superconductivity at the interface of a 3D topological insulator (TI) with an s-wave superconductor (S) provides a new platform for topologically protected quantum computation based on elusive Majorana modes. To employ such S–TI hybrid devices in future topological quantum computation architectures, a process is required that allows for device fabrication under ultrahigh vacuum conditions. Here, we report on the selective area growth of (Bi,Sb)2Te3 TI thin films and stencil lithography of superconductive Nb for a full in situ fabrication of S–TI hybrid devices via molecular-beam epitaxy. A dielectric capping layer was deposited as a final step to protect the delicate surfaces of the S–TI hybrids at ambient conditions. Transport experiments in as-prepared Josephson junctions show highly transparent S–TI interfaces and a missing first Shapiro step, which indicates the presence of Majorana bound states. To move from single junctions towards complex circuitry for future topological quantum computation architectures, we monolithically integrated two aligned hardmasks to the substrate prior to growth. The presented process provides new possibilities to deliberately combine delicate quantum materials in situ at the nanoscale.

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Fig. 1: Schematic of an S–TI–S junction.
Fig. 2: In situ fabricated JJs.
Fig. 3: Frequency dependency of Shapiro response of device 1 at T = 1.5 K.
Fig. 4: To employ the stencil technology to networks of nanostructures, the growth of TI has to be restricted to selected areas only.
Fig. 5: Combined in situ process.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

A. Braginski and F. Hassler are acknowledged for enlightening discussions. M. Geitner and K.-H. Deussen are acknowledged for the deposition of Si3N4 and SiO2 layers. The authors thank G. Nagda and C. Beale for proofreading the manuscript. This work is supported by the German Science Foundation (DFG) under the priority program SPP1666 “Topological Insulators”, as well as by the Helmholtz Association via the “Virtual Institute for Topological Insulators” and the IVF project “Scalable Solid State Quantum Computing”.

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P.S., D.R., T.W.S., M.S. and E.B. fabricated the substrates in the clean room. S.T. performed electron-beam lithography. P.S., M.S., A.R.J., S.S., M.W. and G.M. grew the TI thin films via MBE. B.B. grew the superconducting Nb. U.P. capped the sample with stoichiometric Al2O3. P.S., D.R., C.L. and T.W.S. performed the electrical transport measurements on Josephson devices. D.R. and J.K. investigated the magnetotransport on Hall bars. T.G. removed the stencil mask via mechanical polishing. L.K., D.M. and M.L. prepared the focused ion-beam lamellae and performed high-resolution scanning transmission electron microscopy measurements. A.B. and A.A.G. carried out the Eilenberger and Usadel fitting. P.S., D.R. and A.B. wrote the paper with contributions from all the co-authors. P.S. initiated the project, which was supervised by N.T., A.A.G., A.B., T.S. and D.G.

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Correspondence to Peter Schüffelgen.

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Peer review information: Nature Nanotechnology thanks Torsten Karzig and other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Figs. 1–8 and Supplementary Table 1.

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Schüffelgen, P., Rosenbach, D., Li, C. et al. Selective area growth and stencil lithography for in situ fabricated quantum devices. Nat. Nanotechnol. 14, 825–831 (2019). https://doi.org/10.1038/s41565-019-0506-y

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