Periodic Reporting for period 1 - SuperCONtacts (Solid state diffusion for atomically sharp interfaces in semiconductor-superconductor hybrid structures)
Berichtszeitraum: 2021-07-15 bis 2023-07-14
The emerging field of semiconductor-superconductor hybrid-structure physics is increasingly gaining momentum. Building on the vast and application-oriented knowledge acquired in semiconductor physics in the last century, this new field combines it with the extraordinary properties described by superconductor physics, such as dissipation less current and electron coupling through Cooper pair formation. Theoretical proposals of novel devices capable of revolutionizing our life are plentiful. Most notably these are hybrid light-emitting diodes, novel superconductor-based lasers, entangled photon detectors and quantum light sources which have the potential to impact future quantum processing, communication, and encryption. The p-n junction and quantum dot (QD) superconducting light sources can be based on the integration of Josephson junctions (JJs). These and superconductor coupled wave guides and photonic Bell-state analyzers are all based on contacting the superconducting material with one or two contact leads which form the connection between the superconductor and the semiconductor. They are the injection point of Cooper pairs into semiconductor and therewith the critical liaison between the two where the properties of one type of material can interact with those of the other material. The quality of the interface is therefore of utmost importance for the hybridization of the nanodevice. Up to now, major experimental problems have occurred in the fabrication of such high-quality interfaces making the promises of superconducting light emitting sources far from reality.
In this project, I will fill this gap by proposing a technology transfer of the solid-state diffusion technique from metal- to superconductor-interfaces with a semiconductor. Thanks to this technique, atomically sharp interfaces with an epitaxial relationship with the adjacent materials have been produced. Such clean and very controlled interfaces would be a big advancement for superconductor-semiconductor hybrid structures. I want to employ nanowires (NWs) as the semiconducting component which are already a well-established building block for this technique and very efficient for optoelectronic devices. Through this project it will be possible to make an important step towards the realization and control of JJs in n- and p-doped NWs. This will be essential to extend the knowledge of superconductor-semiconductor interfaces at the nanoscale and will provide a useful technique for future device miniaturization. There are still gaps in the experimental know-how in the field and with this project I propose a new fabrication protocol of superconductor-semiconductor material systems, a material selection which in the near future will allow the development of JJ-based devices (such as the superconducting quantum light sources or detectors) and novel outstanding devices which will significantly improve the state-of-the-art. It will therewith represent one important aspect towards the realization and commercialization of other semiconductor-superconductor structures.
In this project, I will fill this gap by proposing a technology transfer of the solid-state diffusion technique from metal- to superconductor-interfaces with a semiconductor. Thanks to this technique, atomically sharp interfaces with an epitaxial relationship with the adjacent materials have been produced. Such clean and very controlled interfaces would be a big advancement for superconductor-semiconductor hybrid structures. I want to employ nanowires (NWs) as the semiconducting component which are already a well-established building block for this technique and very efficient for optoelectronic devices. Through this project it will be possible to make an important step towards the realization and control of JJs in n- and p-doped NWs. This will be essential to extend the knowledge of superconductor-semiconductor interfaces at the nanoscale and will provide a useful technique for future device miniaturization. There are still gaps in the experimental know-how in the field and with this project I propose a new fabrication protocol of superconductor-semiconductor material systems, a material selection which in the near future will allow the development of JJ-based devices (such as the superconducting quantum light sources or detectors) and novel outstanding devices which will significantly improve the state-of-the-art. It will therewith represent one important aspect towards the realization and commercialization of other semiconductor-superconductor structures.
The work performed in the project has achieved most of its objectives and milestones both into the scientific and training objectives. In particular:
1-The material search was pursued to find the best candidates for solid-state diffusion tests including: InAs, Ge, NiBr2 as semiconductors and Al, Al/EuS as superconductors.
-It was found that InAs nanowires (NW), as opposed to Ge NWs, suffer from As degassing during the annealing process. That degrades the crystal quality of the nanowire itself because of the formation of voids which were observed in TEM imaging. A solutions was to anneal the samples in As over pressure. The results were presented at a specialistic conference.
-Investigations on Al/EuS thin demonstrated that the ferromagnetic insulator (EuS) able to induce magnetic correlations in the adjacent superconducting Al. Tests on thin films demonstrated superconductivity and strong magnetism in the Al. The non-reciprocal spintronic transport is at the base of two publications and the Patent.
-In the last part of the project, the van-der-Waals 2D semiconductor NiBr2 has been investigated. We observed non-reciprocal superconductivity it in the NiBr2/Al system and this scientific result is now in preparation for publication.
2. Ultra-short JJ: Fabrication and characterization of the first JJ with the solid-state diffusion technique.
The fabrication and electrical characterization of the devices was developed thanks to the interplay between the CNRS in Grenoble and the CNR-NEST in Pisa. Ther successful propagation of Al in to InAs is achieved from both ends, using rapid thermal annealing at 410°C for 15-35 min with subsequent observations in SEM and high angle annular dark field (HAADF) scanning TEM (STEM). Results of this technique have been presented at a Conference.
3. Demonstration of supercurrent-diodes and tunnel-diodes.
The characterization of Al/EuS films allowed the discovery of the non-reciprocal properties of spin-selective tunnel barriers. This result was disseminated through a publication and three conferences. Moreover, this result is at the base of a promising superconducting diode prototype that has been protected via a Patent. Finally, non-reciprocal superconductivity has been also observed in the supercurrent of Al/NiBr2 thin films measured at low temperature in the presence of strong in-plane magnetic fields stemming for magneto-chiral superconductivity, publication in progress.
4. Training in Superconductor physics. The researcher has been trained in the field of superconductor physics which is the main expertise of the hosting lab CNR Pisa. The focus was on the theoretical understanding of superconducting spintronics for applied electronics including non-reciprocal superconductivity and detectors. Intellectual property exploitation has also taken place and training of the researcher in the procedures for patenting.
5. Training in superconducting electronic transport characterization. The researcher has been introduced and fully trained in the experimental technique for quantum transport measurements at ultra-cryogenic temperature (<10mK) and at high magnetic fields (~Tesla). This included the mastering of cryogen-free dilution fridges equipped with single or vectorial magnets, for which she wrote user manuals and followed maintenance operations. She was also trained in the use of the low noise electronics for low and medium frequency (<100 kHz) characterizations.
1-The material search was pursued to find the best candidates for solid-state diffusion tests including: InAs, Ge, NiBr2 as semiconductors and Al, Al/EuS as superconductors.
-It was found that InAs nanowires (NW), as opposed to Ge NWs, suffer from As degassing during the annealing process. That degrades the crystal quality of the nanowire itself because of the formation of voids which were observed in TEM imaging. A solutions was to anneal the samples in As over pressure. The results were presented at a specialistic conference.
-Investigations on Al/EuS thin demonstrated that the ferromagnetic insulator (EuS) able to induce magnetic correlations in the adjacent superconducting Al. Tests on thin films demonstrated superconductivity and strong magnetism in the Al. The non-reciprocal spintronic transport is at the base of two publications and the Patent.
-In the last part of the project, the van-der-Waals 2D semiconductor NiBr2 has been investigated. We observed non-reciprocal superconductivity it in the NiBr2/Al system and this scientific result is now in preparation for publication.
2. Ultra-short JJ: Fabrication and characterization of the first JJ with the solid-state diffusion technique.
The fabrication and electrical characterization of the devices was developed thanks to the interplay between the CNRS in Grenoble and the CNR-NEST in Pisa. Ther successful propagation of Al in to InAs is achieved from both ends, using rapid thermal annealing at 410°C for 15-35 min with subsequent observations in SEM and high angle annular dark field (HAADF) scanning TEM (STEM). Results of this technique have been presented at a Conference.
3. Demonstration of supercurrent-diodes and tunnel-diodes.
The characterization of Al/EuS films allowed the discovery of the non-reciprocal properties of spin-selective tunnel barriers. This result was disseminated through a publication and three conferences. Moreover, this result is at the base of a promising superconducting diode prototype that has been protected via a Patent. Finally, non-reciprocal superconductivity has been also observed in the supercurrent of Al/NiBr2 thin films measured at low temperature in the presence of strong in-plane magnetic fields stemming for magneto-chiral superconductivity, publication in progress.
4. Training in Superconductor physics. The researcher has been trained in the field of superconductor physics which is the main expertise of the hosting lab CNR Pisa. The focus was on the theoretical understanding of superconducting spintronics for applied electronics including non-reciprocal superconductivity and detectors. Intellectual property exploitation has also taken place and training of the researcher in the procedures for patenting.
5. Training in superconducting electronic transport characterization. The researcher has been introduced and fully trained in the experimental technique for quantum transport measurements at ultra-cryogenic temperature (<10mK) and at high magnetic fields (~Tesla). This included the mastering of cryogen-free dilution fridges equipped with single or vectorial magnets, for which she wrote user manuals and followed maintenance operations. She was also trained in the use of the low noise electronics for low and medium frequency (<100 kHz) characterizations.
Successful propagation of the superconductor Al into the semiconductor material InAs, here in the shape of a NW, has been demonstrated. An epitaxial relationship has been found. This study paves the way for improved Cooper pair transfer in strong spin-orbit Josephson junctions, in this case an Al/InAs/Al NW with epitaxial interfaces. It likewise allows for detailed electrical and low temperature measurements of the Josephson coupling enhanced by the hybridization and low dimensionality of the semiconducting channel.
Moreover, the realization of a spintronic superconducting tunnel diode exploited for non-reciprocal electronics represents an additional progress beyond the state of the art. The prototype of a superconducting diode is a promising innovation protected by a patent with the potential to create important benefits for society and promises to positively influence fields such as encryption and supercomputers.
Moreover, the realization of a spintronic superconducting tunnel diode exploited for non-reciprocal electronics represents an additional progress beyond the state of the art. The prototype of a superconducting diode is a promising innovation protected by a patent with the potential to create important benefits for society and promises to positively influence fields such as encryption and supercomputers.