Josephson junctions with high critical current density grown by molecular beam epitaxy
Lead Research Organisation:
University College London
Department Name: London Centre for Nanotechnology
Abstract
Future error-corrected quantum computers will need ~106 qubits per chip. The current world-leading solid-state qubit technology is based on resonator-coupled Josephson junctions. To fit 106 qubits inside a dilution fridge their linear dimensions will need to be an order of magnitude smaller than current technology. This in turn requires the qubit to operate at higher frequencies, which in turn requires the Josephson junctions to have an order of magnitude higher critical current density than at present.
Current Josephson device technology is limited to current densities in the 0.1 to 10 kAcm-2 range since higher values necessitate thinner barriers (less than 3 nm) which are susceptible to pinholes. By using molecular-beam epitaxy (MBE) and plasma-assisted in situ oxidation we will be able to make ultra-thin oxide barriers, enabling critical current densities up to 100 kAcm-2. This will open up applications in the short term for Josephson junctions in (classical) high-speed digital logic and in the longer term for future-generations of high-packing-density qubit circuits.
We are looking for a motivated experimentalist to fabricate Zn-ZnO-Zn trilayers using the oxide-plasma-assisted MBE. In-situ electron diffraction and optical interferometry will be used to develop ultra-thin oxide barriers with atomically-sharp interfaces. These will be characterised using transmission electron microscopy (TEM). The trilayers will be fabricated into Josephson devices at the LCN cleanroom. The critical current density will be determined using low-temperature transport measurements at 300 mK, with device uniformity characterised by measurements of the dependence of the critical current on an in-plane magnetic field. The absolute magnitude and the process variability of the critical current density will be correlated with the barrier properties as determined by TEM.
Current Josephson device technology is limited to current densities in the 0.1 to 10 kAcm-2 range since higher values necessitate thinner barriers (less than 3 nm) which are susceptible to pinholes. By using molecular-beam epitaxy (MBE) and plasma-assisted in situ oxidation we will be able to make ultra-thin oxide barriers, enabling critical current densities up to 100 kAcm-2. This will open up applications in the short term for Josephson junctions in (classical) high-speed digital logic and in the longer term for future-generations of high-packing-density qubit circuits.
We are looking for a motivated experimentalist to fabricate Zn-ZnO-Zn trilayers using the oxide-plasma-assisted MBE. In-situ electron diffraction and optical interferometry will be used to develop ultra-thin oxide barriers with atomically-sharp interfaces. These will be characterised using transmission electron microscopy (TEM). The trilayers will be fabricated into Josephson devices at the LCN cleanroom. The critical current density will be determined using low-temperature transport measurements at 300 mK, with device uniformity characterised by measurements of the dependence of the critical current on an in-plane magnetic field. The absolute magnitude and the process variability of the critical current density will be correlated with the barrier properties as determined by TEM.
Organisations
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/N509577/1 | 01/10/2016 | 24/03/2022 | |||
| 1781327 | Studentship | EP/N509577/1 | 01/10/2016 | 31/03/2021 | Matthew Sparks |
| Description | We have been able to produce a two-dimensional electron gas (2DEG) with mobilities on the order or 10,000 cm^2/Vs, an order of magnitude improvement compared to earlier attempts by the group. These 2DEGs were achieved by thermally treating low-quality ZnO substrates to remove residual Li defects which arise as part of the hydro-thermal growth. The Coherence length of electrons within such a 2DEG is estimated to be on the order of 500nm at 5k. Such values make it possible to fabricate gated Josephson Junctions which have potential quantum computing applications. Attempts to make such junctions are still ongoing but we expect a prototype can be demonstrated by the end of this award. We have demonstrated a link between rapid thermal annealing of the substrate and reduced defect density. This improvement is conducive to 2DEG formation. The results of this can be seen in a peer-reviewed paper: 'Rapid Thermal Annealing for Surface Optimisation of ZnO Substrates for MBE-Grown Oxide Two-Dimensional Electron Gases', https://doi.org/10.3390/cryst10090776 |
| Exploitation Route | The primary goal of this award, to produce a 2DEG-based controllable Josephson Junction still has not been realized. The high mobility 2DEGs produced thus far is one step closer. Experiments must be conducted to see if these 2DEGs can be reproduced. Once that is done Josephson Junctions must be fabricated using electron beam lithography (EBL). Experiments are already underway to perfect the fabrication such devices. Attempts are already being made to retro-fit existing 2DEG devices from earlier group members with Josephson devices. Once such a device has been demonstrated the next step is to apply an electric gate and tune the 2DEG. |
| Sectors | Digital/Communication/Information Technologies (including Software),Electronics |
| URL | https://doi.org/10.3390/cryst10090776 |
| Description | Training in MBE |
| Geographic Reach | Europe |
| Policy Influence Type | Influenced training of practitioners or researchers |