Scanning tunneling spectroscopy of superconducting nitridized aluminum thin films

TL;DR

This study uses scanning tunneling spectroscopy to investigate the microscopic superconducting properties of nitridized aluminum thin films, revealing a larger, more homogeneous gap than pure aluminum, relevant for quantum device applications.

Contribution

First detailed STM analysis of nitridized aluminum's superconducting density of states, showing a larger, spatially uniform gap compared to pure aluminum.

Findings

Superconducting gap around 360 μeV close to BCS prediction.

Zero in-gap density of states up to 250 μeV.

Spatial variation of the gap by about 10% across the sample.

Abstract

Nitride-based superconductors represent a family of superconducting thin film materials displaying higher quality than their corresponding bare superconductor when used in devices for applications such as cosmic radiation sensing. In recent times, Niobium-based and Titanium-based nitrides were used to improve the quality of superconducting devices in quantum technology applications. Recently, nitridized Aluminum (NitrAl) has been found to display higher critical temperatures and enhanced resilience to magnetic fields compared to those of Al, making it a new interesting candidate for superconducting quantum circuit applications. However, the microscopic properties of NitrAl remain highly unexplored. Here we use Scanning Tunneling Microscope (STM) to measure the superconducting density of states of a thin film sample of nitridized-Aluminum (NitrAl), with a room temperature resistivity between pure Al and fully insulating aluminum nitride. We show that the in-gap density of states is zero up to about $\hbar\omega=250~\mathrm{\mu eV}$ and that there is a distribution of values of the superconducting gap around $\Delta_0=360~\mathrm{\mu eV}$, close to the BCS expectation $\Delta=1.76 k_{\mathrm{B}}T_{\mathrm{c}}$. We also find varying superconducting gap values at the nanometer scale, by approximately 10\%, when probing different regions of the sample. These results suggest a gap which is larger than the one of pure Al, and is spatially more homogeneous than the superconducting gap values often found in thin films. Our work demonstrates that STM is as a powerful tool to screen materials for quantum devices through the measurement of the spatial dependence of the superconducting density of states.