Functionality of superconducting thin-film devices such as superconducting nanowire single photon detectors stems from the geometric effects that take place at the nanoscale. The engineering of these technologies requires high-resolution patterning, often achieved with electron beam lithography. Common lithography processes using hydrogen silsesquioxane (HSQ) as the electron beam resist rely on tetramethylammonium hydroxide (TMAH) as both a developer and a resist adhesion promoter. Despite the strong role played by TMAH in the fabrication of superconducting devices, its potential influence on the superconducting films themselves has not yet been reported. In this work, the authors demonstrate that a 25% TMAH developer damages niobium nitride (NbN) thin films by modifying the surface chemistry and creating an etch contaminant that slows reactive ion etching in CF4. They also show how the identity of the contaminant may be revealed through characterization including measurement of the superconducting film properties and Fourier transform infrared spectroscopy. Although workarounds may be available, the results reveal that processes using 25% TMAH as an adhesion promoter are not preferred for NbN films and that changes to the typical HSQ fabrication procedure will need to be made in order to prevent damage of NbN nanoscale devices.

A complete description of the work may be found here.

Summer Program: Studying superconducting thin film

This summer, Lily Hallett investigated superconducting MoN thin films for use in nanowire single-photon detectors. She optimized deposition conditions for DC Magnetron Sputtering of MoN films and studied their superconducting and electrical properties. Low-pressure annealing experiments were performed in the sputtering chamber to increase the critical temperature of 5 nm films.

Studying Superconducting Thin Films

Many superconducting technologies such as rapid single-flux quantum computing and superconducting quantum-interference devices rely on the modulation of nonlinear dynamics in Josephson junctions for functionality. More recently, however, superconducting devices have been developed based on the switching and thermal heating of nanowires for use in fields such as single-photon detection and digital logic. In this paper, we use resistive shunting to control the nonlinear heating of a superconducting nanowire and compare the resulting dynamics to those observed in Josephson junctions. We show that interaction of the hotspot-impedance with an external shunt produces high-frequency relaxation oscillations with similar behavior to that observed in Josephson junctions due to their ability to be modulated by a weak periodic signal. In particular, we use a microwave drive to pull and mix the oscillation frequency, resulting in phase-locked features that resemble the Shapiro steps observed in the ac Josephson effect. Microwave nanowire devices based on these conclusions have promising applications in fields such as parametric amplification and frequency mixing.

A complete description of the work may be found here.

Citation:

Emily Toomey, Qing-Yuan Zhao, Adam N. McCaughan, Karl K. Berggren. “Frequency pulling and mixing of relaxation oscillations in superconducting nanowires” Physical Review Applied 9, 6 (2018)

Congratulations to Navid Abedzadeh for being awarded Best Electron Beam and congratulations to Emily Toomey, Marco Colangelo and Navid Abedzadeh for the honorable mention in the Micrograph Contest at this year’s EIPBN conference in Puerto Rico. All the competing micrographs can be found here.
View Navid’s winning micrograph “Black hole in an SEM”

View Emily, Marco and Navid electron micrograph “Why did the chicken cross the wafer?”

Coincidence detection of single photons is crucial in numerous quantum technologies and usually requires multiple time-resolved single-photon detectors. However, the electronic readout becomes a major challenge when the measurement basis scales to large numbers of spatial modes. Here, we address this problem by introducing a two-terminal coincidence detector that enables scalable readout of an array of detector segments based on superconducting nanowire microstrip transmission line. Exploiting timing logic, we demonstrate a sixteen-element detector that resolves all 136 possible single-photon and two-photon coincidence events. We further explore the pulse shapes of the detector output and resolve up to four-photon events in a four-element device, giving the detector photon-number-resolving capability. This new detector architecture and operating scheme will be particularly useful for multi-photon coincidence detection in large-scale photonic integrated circuits.

A complete description of the work may be found here.

Citation:

Di Zhu, Qing-Yuan Zhao, Hyeongrak Choi, Tsung-Ju Lu, Andrew E. Dane, Dirk R. Englund, Karl K. Berggren. “A scalable multi-photon coincidence detector based on superconducting nanowires” Nature Nanotechnology (2018)