On the hunt for new substrates for superconducting circuits

—by Eric He, Peter Jacobson and Zach Degnan

EQUS researchers have developed new methods to analyse the surface chemistry of substrate materials for superconducting devices.

Two widely used substrates for superconducting circuitry are silicon (Si) and sapphire (Al2O3), but imperfections in these materials reduce device performance.  Although all materials contain defects, finding substrates whose imperfections have less of a detrimental effect could lead to higher-quality superconducting quantum computing chips.  However, it's not as simple as choosing the material with the fewest defects; for example, gallium arsenide (GaAs), one of the highest-quality materials available (fewest imperfections), is a terrible platform for superconducting devices.

To properly assess new substrate materials, we need to carefully prepare and characterise them before putting the superconductor on the surface.  Doing so while ensuring the materials are compatible with conventional nanofabrication techniques for superconducting circuitry is challenging.  As the mechanical and chemical properties of materials vary, the fabrication and characterisation methods must be adjusted accordingly.  Therefore, we need to first understand the materials but then also have the expertise to measure the superconducting devices.  This kind of collaboration is uncommon for proof-of-concept devices, but important to optimise device performance.

The EQUS team—Zach Degnan, Xin (Eric) He, Alejandro Gomez Frieiro, Yauhen (Eugene) Sachkou, Arkady Fedorov and Peter Jacobson—has developed a new method for analysing surface chemistry.  Whereas other exploratory work has focused on materials widely used in the semiconductor industry (such as GaAs), we tested two materials widely used in the ‘oxide electronics’ community.

Although we didn't beat silicon or sapphire on our first try, one of the materials (spinel, MgAl2O4) turned out to be surprisingly good, even in the presence of substantial disorder (defects or imperfections).  Silicon and sapphire have decades of experience behind them, but our study, combined with the low-temperature device performance, clearly points to smoother surfaces as a way of improving our devices.

Our result is important because it verifies an approach to exploring new substrate materials to reduce losses in superconducting quantum devices. In principle, it could allow us to find a good material candidate for building a practical quantum computer.  The next steps are to optimise the approach and analyse more materials.

Read the full paper here: https://arxiv.org/abs/2201.06228.  This work fits within EQUS’ Designer Quantum Materials research program.  Zach and Alejandro are PhD students, Eric and Eugene are Research Fellows, Arkady is a Chief Investigator and Peter is an Associate Investigator, all based at UQ.

Major funding support

Australian Research Council

The Australian Research Council Centre of Excellence for Engineered Quantum Systems (EQUS) acknowledges the Traditional Owners of Country throughout Australia and their continuing connection to lands, waters and communities. We pay our respects to Aboriginal and Torres Strait Islander cultures and to Elders past and present.