The metal-substrate interface determines atomic structure and could affect qubit performance
UPTON, N.Y. — Researchers from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and DOE’s Pacific Northwest National Laboratory (PNNL) have uncovered an unexpected interface layer that may be hindering the performance of superconducting qubits, the building blocks of quantum computers. While examining this layer through a combination of imaging techniques and theoretical models, they discovered the underlying cause of puzzling structural differences in qubits.
The unexpected layer is called a metal-substrate interface, or M-S interface, because it lies between a layer of tantalum metal and a sapphire substrate. Researchers from the Co-design Center for Quantum Advantage (C2QA), a DOE National Quantum Information Science Research Center led by Brookhaven Lab, have fabricated high-performing superconducting qubits made up of a tantalum thin film deposited on a sapphire substrate. But to unlock the potential power of quantum computers, qubits must exhibit a higher “coherence time,” meaning they need to retain quantum information for longer.
Quantum researchers have dedicated significant efforts to determining which constituent materials and fabrication techniques yield qubits with the highest coherence times. But there are several other elements of qubit architecture that could also affect coherence times. For example, when a qubit is exposed to air, the surface-level tantalum reacts with oxygen. This results in a tantalum oxide layer on the surface of the qubit, and C2QA researchers have found that the interface between this oxide layer and the tantalum thin film hinders the qubit performance. They’ve even explored coating tantalum to prevent the oxidation from occurring.
“We knew that the interface between tantalum oxide and tantalum had a pretty big effect on the performance of qubits made with tantalum thin films,” explained Aswin kumar Anbalagan, a researcher at the National Synchrotron Light Source II (NSLS-II) and first author on the recent Advanced Science publication. “That led us to question whether the other interface, the one between the tantalum and the sapphire, was also affecting qubit performance.”
Thinner samples, deeper insights: probing the M-S interface
The high-performing superconducting qubits fabricated by C2QA researchers are typically between 150 and 200 nanometers thick. Though they are incredibly thin — for context, a human hair is 80,000-100,000 nanometers wide — they are too thick to characterize with certain X-ray techniques.
Anbalagan and his mentors at NSLS-II wanted to explore the region where the tantalum metal meets the sapphire substrate, so they partnered with researchers from the Center for Functional Nanomaterials (CFN) to fabricate thinner samples — around 30 nanometers thick — made from the same materials as traditional qubits.
“At CFN, we have developed a technique to fabricate high-quality tantalum thin films for quantum circuitry applications,” said Mingzhao Liu, senior scientist at CFN and co-author on the paper. “In this case, we adopted the same technique to fabricate tantalum films that are much thinner, with a very smooth surface and interface against sapphire.” NSLS-II and CFN are DOE Office of Science user facilities at Brookhaven Lab.
“We started with some reasonably straightforward measurements at NSLS-II to see the interface below the tantalum thin film,” said Andrew L. Walter, a lead beamline scientist in NSLS-II’s electronic structure techniques program and one of the lead authors on the paper.
The researchers conducted X-ray reflectivity experiments at the Beamline for Materials Measurement (BMM). These studies offered insights into the thickness and density of each layer in the sample. They also leveraged the Spectroscopy Soft and Tender 2 (SST-2) beamline to take X-ray spectroscopy measurements that revealed the chemical makeup of the layers. The BMM and SST-2 beamlines at NSLS-II are funded and operated by the National Institute of Standards and Technology (NIST).
Read more on BNL website
Image: Brookhaven Lab researchers discovered an unexpected interface layer between a tantalum (Ta) thin film and the sapphire substrate it was grown on. To better understand this metal-substrate interface, the team conducted several techniques, like scanning transmission electron microscopy (top right circle), and collaborated with researchers from Pacific Northwest National Laboratory, who carried out computational simulations (bottom right circle). This research, conducted as part of the Co-design Center for Quantum Advantage, revealed that the concentration of oxygen atoms (O) at the sapphire’s surface influences the direction of tantalum’s deposition. Aluminum (Al) is a core component of sapphire, in addition to oxygen.
Credit: Nathan Johnson | Pacific Northwest National Laboratory