A crystal structure that combines a semiconductor and superconductor is a tantalizing prospect to create energy-efficient computers, or quantum computers, which leverage the unique quantum mechanical properties of superconductors. Superconductors carry current with little to no energy loss, while semiconductors offer the control and versatility that has made them an essential feature of transistor technology.

The challenge is how to combine the two states and make sure you get the best of both electrical worlds – and can still isolate them.

A collaboration between researchers from Cornell and the Paul Scherrer Institute in Switzerland grew a thin film, only a few atomic layers in thickness, of the one of the oldest known superconductors, niobium nitride, on top of gallium nitride, a semiconductor that in recent decades has become a vital component in optical and power electronics. For the first time, the researchers have successfully measured the electronic properties of the junction between the two materials, a crucial step toward creating hybrid superconductor-semiconductor quantum devices.

The groups’ paper, “Momentum-Resolved Electronic Structure and Band Offsets in an Epitaxial NbN/GaN Superconductor/Semiconductor Heterojunction,” published Dec. 22 in Science Advances. The paper’s co-lead authors are Cornell doctoral student John Wright and Tianlun Yu with the Paul Scherrer Institute.

“We have vastly different electronic systems that are now coexisting in the same materials platform. Knowing more about the interface between these two materials, and the electronic properties at the interface, could allow us to design applications that take advantage of the unique electronic properties of both materials,” Wright said. “Sometimes the separation between the states of the two materials is the property we would want. And sometimes we may actually want interaction between the electronic states.”

Cornell’s team was led by Debdeep Jena, the David E. Burr Professor of Engineering in the School of Electrical and Computer Engineering and Department of Materials Science and Engineering, and Huili Grace Xing, the William L. Quackenbush Professor of Electrical and Computer Engineering and of Materials Science and Engineering, both in the College of Engineering.

The Jena-Xing Lab previously demonstrated they could integrate niobium nitride with gallium nitride in epitaxial structures and, in the process, leverage thrilling quantum properties. For their new project, they wanted to understand the electronic properties of both materials independently, as well as the properties at the junction, or interface, where the materials meet.

The group used molecular beam epitaxy to grow a precisely controlled, high-quality crystalline superconducting sample only several nanometers thick – itself a significant accomplishment. Then they developed an intricate capping procedure to preserve the quality and purity of the sample and shipped it to their partners at the Paul Scherrer Institute, who used soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) to measure the electronic properties both at the surface of the material, as well as deep inside it.

“Historically, niobium nitride was one of the oldest known superconductors. Still, people had failed to measure its complete quantum mechanical electronic structure,” Jena said. “The experiment that was initiated with this collaboration actually succeeded in getting there.”

While the Paul Scherrer Institute team performed its ARPES measurements, the Cornell group fabricated test devices at the Cornell NanoScale Science and Technology Facility (CNF) and conducted its own electronic transport measurements to gain a macroscopic look at the electrons moving through the material interface and the energetic barriers they encounter there.

“The beauty of this work is that when you put gallium nitride with niobium nitride, the fundamental properties of each of these materials do not change,” said research associate Betul Pamuk, who conducted the computational modeling and was one of the paper’s co-authors.

The researchers were surprised to find niobium nitride’s electronic states differed slightly from what the modeling and theory had suggested – a discrepancy that would have been impossible to discover without the combination of the ARPES measurements and the high crystalline quality and chemical purity of the sample they grew.

One of the superconductor-semiconductor’s most promising applications is for high-quality crystalline Josephson junctions, which can enable nonlinear electrical behavior and, as a result, boost high-speed electronics and create qubits – or quantum bits – for quantum computing.

The project’s impact extends beyond gallium nitride and niobium nitride.

“What’s really nice here is that, because gallium nitride is so well studied as a Group-III nitride semiconductor, by understanding this information for this set of materials we can understand how to design interfaces with other materials, like aluminum nitride, indium nitride and their combinations,” said research associate and paper co-author Guru Khalsa.

The paper’s co-senior author was Vladimir N. Strocov with the Paul Scherrer Institute. Other co-authors include David Muller, the Samuel B. Eckert Professor of Engineering in the School of Applied and Engineering Physics; Celesta Chang, Ph.D. ’20; and researchers from the Paul Scherrer Institute, the German Electron Synchrotron and Shanghai Research Center for Quantum Sciences.

The research was supported by the National Science Foundation through Cornell’s Platform for the Accelerated Realization, Analysis and Discovery of Interface Materials (PARADIM), and the Office of Naval Research. CNF is a member of the NSF’s National Nanotechnology Coordinated Infrastructure.