IEEE Study Reveals Breakthroughs in High-Performance Photon Detectors

Researchers develop a fabrication technique to overcome design and performance challenges for scalable single-photon detectors

PISCATAWAY, N.J., Feb. 14, 2025 -- Superconducting nanowire single-photon detectors (SNSPDs) utilize ultra-thin superconducting wires that quickly switch between physical states when a photon strikes, enabling ultra-fast detection. The unique arrangement of nanowires in Peano arced-fractal pattern enables this detection regardless of the photon's direction or orientation, highlighting the multidisciplinary applications of arced-fractal SNSPDs (AF SNSPDs).

In a recent study published on 25 December 2024, in the IEEE Journal of Selected Topics in Quantum Electronics, Professor Xiaolong Hu and Dr. Kai Zou from Tianjin University, China, outline the necessary materials, provide a comprehensive guide to fabricating high-quality AF SNSPDs and address various challenges associated with it.

AF SNSPDs consist of nanowires for photon detection, optical microcavities to capture photons, and keyhole-shaped chips that house and align the detector with the optical fiber. The fabrication process begins with creating the optical microcavity by coating a silicon wafer with six or eight alternating layers of silicon dioxide (SiO₂) and tantalum oxide (Ta₂O₅) using ion-beam-assisted deposition (IBD) to form a bottom-distributed Bragg reflector, followed by the addition of a SiO₂ defect layer. A 9-nm niobium-titanium nitride (NbTiN) superconducting film is deposited on the defect layer using reactive magnetron sputtering, creating the photon-sensitive surface. Titanium-gold electrodes are then fabricated on this surface using optical lithography and lift-off processes.

The nanowires are patterned into a fractal design using scanning-electron-beam lithography and then transferred to the NbTiN layer through reactive-ion etching. The microcavity is completed by depositing a top SiO₂ defect layer and additional alternating layers of Ta₂O₅/SiO₂ using aligned optical lithography and IBD. The chip is shaped into its keyhole form using optical lithography, inductively coupled plasma etching, and the Bosch etching process, and packaged for optical fiber connections.

The authors also provided suggestions for optimizing the fabrication processes of nanowires, optical microcavities, and keyhole-shaped chips. Some of their recommendations include: Applying a 5-nm silicon or 3-nm SiO₂ layer as an adhesion promoter to improve bonding between the resist patterned into nanowires and the NbTiN material, using auxiliary AF nanowire patterns to ensure consistent nanowire widths, a careful design of the layout and spacing for optical microcavities to minimize photoresist deformation, and using accurate alignment markers for keyhole-shaped chips.

In conclusion, the researchers were able to develop SNSPDs with impressive sensitivity and system detection efficiency. "These advancements will help simplify the fabrication of fractal SNSPDs enabling the development of more advanced devices with additional functionalities," concludes Prof. Hu.

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