Periodic Reporting for period 2 - SuperGate (Gate Tuneable Superconducting Quantum Electronics.)
Periodo di rendicontazione: 2022-03-01 al 2023-02-28
The research work which we have carried out within SuperGate fits into the worldwide effort to develop technologies that can help reduce the energy costs of large-scale computers (supercomputers). The largest supercomputers fabricated nowadays need the amount of power produced by a small plant (~ 1 GW) to operate and therefore also generate thermal management issues. In SuperGate, we are setting the basis for the development and future commercialization of a new greener technology for supercomputers, which can help reduce their energy costs.
The results of the activity of our research network in SuperGate stem from the design, testing and optimization of a new class of superconducting devices, namely superconducting electric field transistors (EF-Trons), and from the study of the fundamental physical effects underlying their functioning. Our EF-Trons represent the first class of superconducting devices that operate like conventional complementary metal-oxide semiconductor (CMOS) transistors (i.e. switching between two different states via an applied gate voltage VG), but with advantage of having low dissipation, since they are made from superconductor (S) materials.
The current objectives of the SuperGate programme include the definition of the best materials and device geometries for EF-Trons, which would ensure their highest operational temperatures in combination with lowest VG values needed for switching – the latter condition is fundamental to ensure good compatibility and interfacing with CMOS technologies (typically operating with voltages less than 1 Volt). To achieve these goals, it is also essential for us to understand the physical mechanism responsible for the switching of EF-Trons under an applied VG, about which the debate within the research community is still ongoing. Other important objectives of the SuperGate programme include the characterization of the highest possible switching frequencies of EF-Trons, which in principle should be much higher than those of conventional CMOS devices, and the testing of prototypes of basic circuits and logic ports based on EF-Trons for future integration into more complex networks.
The results of the activity of our research network in SuperGate stem from the design, testing and optimization of a new class of superconducting devices, namely superconducting electric field transistors (EF-Trons), and from the study of the fundamental physical effects underlying their functioning. Our EF-Trons represent the first class of superconducting devices that operate like conventional complementary metal-oxide semiconductor (CMOS) transistors (i.e. switching between two different states via an applied gate voltage VG), but with advantage of having low dissipation, since they are made from superconductor (S) materials.
The current objectives of the SuperGate programme include the definition of the best materials and device geometries for EF-Trons, which would ensure their highest operational temperatures in combination with lowest VG values needed for switching – the latter condition is fundamental to ensure good compatibility and interfacing with CMOS technologies (typically operating with voltages less than 1 Volt). To achieve these goals, it is also essential for us to understand the physical mechanism responsible for the switching of EF-Trons under an applied VG, about which the debate within the research community is still ongoing. Other important objectives of the SuperGate programme include the characterization of the highest possible switching frequencies of EF-Trons, which in principle should be much higher than those of conventional CMOS devices, and the testing of prototypes of basic circuits and logic ports based on EF-Trons for future integration into more complex networks.
In the first 12 months since the beginning of the programme (1st March 2021), our consortium has hired all the team members needed to carry out the SuperGate activities and set up dedicated labs and equipment to the project – which also include facilities for the simulation and fabrication of logic circuits and for the testing of the switching frequencies of EF-Trons up to tens of GHz. As result of our first months of research, we have already found devices geometries and materials that reduce the voltage required for EF-Trons switching down to a few volts – which would already enable an easier integration with CMOS circuits. Our results have been presented at different international conferences and workshops by several members of our consortium and led to the writing of six scientific manuscripts to date (15th April 2022), four of which have already been published in international peer-reviewed journals.
During the second phase of the project (from March 2022 to March 2023), our consortium has continued the activities that we started during the first phase of the project. Our investigation has made us understand that superconducting materials which are made of "heavier elements", meaning elements with a higher number in the period table, are more promising towards the reduction of the switching voltages of EF-Trons - which is one of the main objectives of SuperGate, also towards future applications of the superconducting technology that we aim to develop.
We have also found out that the superconductor niobium (Nb) works quite reliably as material for the realisation of EF-Trons. Since reproducibility in the functioning of the devices is another main objective of SuperGate, we have decided to use Nb as the main superconductor for our future studies and in particular for the realisation of logic circuits based on EF-Trons.
Another important finding that we have made during the second phase of the project is that EF-Trons fabricated with a top-down approach, meaning with subtractive patterning, starting from a superconducting thin film, normally do not work, unlike EF-Trons that are made with a bottom-up approach. This is another key result from an application-related point of view because subtractive patterning is usually used by the semiconductor industry for the fabrication of CMOS devices on a large scale. The only superconductor for which we have obtained EF-Trons with reliable functioning even when fabricated with a top-down approach is Nb0.18Re0.82 (NbRe), which seems therefore the material to use for the future scale-up of circuits based on EF-Trons, once we realised them.
Over the second phase of the SuperGate project, we have also carried out the first measurements to determine the highest switching speed of EF-Trons, and we have simulated and fabricated the first circuits based on EF-Trons, including COPY ports and switches. From a theoretical point of view, we have also come up with new models that can possibly provide further insights into the mechanisms responsible for the functioning of EF-Trons, and in particular on how to control such mechanism. One of our new proposals, which we intend to confirm in the near future, is that the deposition of magnetic impurities on the surface of EF-Trons can help reduce the voltages typically needed for their operation.
In terms of publications, in addition to the above-mentioned six manuscript that have all been now published in peer-reviewed journals, we have written and published another two articles, and four more have been put on the arxiv and are currently under review or close to submission. Amongst these articles, we have written in particular a review paper which summarises the work carried out to date on EF-Trons by all the research groups working on these devices worldwide, and which critically analyses all the physical mechanisms proposed to date for their functioning.
During the second phase of the project (from March 2022 to March 2023), our consortium has continued the activities that we started during the first phase of the project. Our investigation has made us understand that superconducting materials which are made of "heavier elements", meaning elements with a higher number in the period table, are more promising towards the reduction of the switching voltages of EF-Trons - which is one of the main objectives of SuperGate, also towards future applications of the superconducting technology that we aim to develop.
We have also found out that the superconductor niobium (Nb) works quite reliably as material for the realisation of EF-Trons. Since reproducibility in the functioning of the devices is another main objective of SuperGate, we have decided to use Nb as the main superconductor for our future studies and in particular for the realisation of logic circuits based on EF-Trons.
Another important finding that we have made during the second phase of the project is that EF-Trons fabricated with a top-down approach, meaning with subtractive patterning, starting from a superconducting thin film, normally do not work, unlike EF-Trons that are made with a bottom-up approach. This is another key result from an application-related point of view because subtractive patterning is usually used by the semiconductor industry for the fabrication of CMOS devices on a large scale. The only superconductor for which we have obtained EF-Trons with reliable functioning even when fabricated with a top-down approach is Nb0.18Re0.82 (NbRe), which seems therefore the material to use for the future scale-up of circuits based on EF-Trons, once we realised them.
Over the second phase of the SuperGate project, we have also carried out the first measurements to determine the highest switching speed of EF-Trons, and we have simulated and fabricated the first circuits based on EF-Trons, including COPY ports and switches. From a theoretical point of view, we have also come up with new models that can possibly provide further insights into the mechanisms responsible for the functioning of EF-Trons, and in particular on how to control such mechanism. One of our new proposals, which we intend to confirm in the near future, is that the deposition of magnetic impurities on the surface of EF-Trons can help reduce the voltages typically needed for their operation.
In terms of publications, in addition to the above-mentioned six manuscript that have all been now published in peer-reviewed journals, we have written and published another two articles, and four more have been put on the arxiv and are currently under review or close to submission. Amongst these articles, we have written in particular a review paper which summarises the work carried out to date on EF-Trons by all the research groups working on these devices worldwide, and which critically analyses all the physical mechanisms proposed to date for their functioning.
Our ongoing research can set the basis for the realization of new supercomputers with hybrid architectures, where energy-efficient logics based on our EF-Trons is interfaced with CMOS technology – CMOS remains still the best option nowadays for memory devices with good performance. In addition to classical digital electronics, our EF-Trons can also find important applications in novel quantum computing technologies.