Scanning Tunneling Spectroscopy of Superconducting Nitridized Aluminum Thin Films

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 $$\hbar \omega =250~\mathrm {\mu eV}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>ħ</mml:mi> <mml:mi>ω</mml:mi> <mml:mo>=</mml:mo> <mml:mn>250</mml:mn> <mml:mspace/> <mml:mrow> <mml:mi>μ</mml:mi> <mml:mi>eV</mml:mi> </mml:mrow> </mml:mrow> </mml:math> and that there is a distribution of values of the superconducting gap around $$\Delta _0=360~\mathrm {\mu eV}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Δ</mml:mi> <mml:mn>0</mml:mn> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>360</mml:mn> <mml:mspace/> <mml:mrow> <mml:mi>μ</mml:mi> <mml:mi>eV</mml:mi> </mml:mrow> </mml:mrow> </mml:math> , close to the BCS expectation $$\Delta =1.76 k_{\textrm{B}}T_{\textrm{c}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>1.76</mml:mn> <mml:msub> <mml:mi>k</mml:mi> <mml:mtext>B</mml:mtext> </mml:msub> <mml:msub> <mml:mi>T</mml:mi> <mml:mtext>c</mml:mtext> </mml:msub> </mml:mrow> </mml:math> . 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.