Growth temperature-induced interfacial degradation in superconducting NbN/insulator HfO2 bilayers

Abstract

This study reports on the electrical transport properties of NbN/HfO₂ bilayers grown sequentially by reactive sputtering on c-Al₂O₃ substrates. An epitaxial (111) NbN layer, 10 nm thick, was deposited at 450 °C using an N₂/argon mixture. Subsequently, an HfO₂ layer with a nominal thickness of 10 nm was grown using a reactive O₂/argon mixture at deposition temperatures ranging from room temperature to 450 °C. The crystalline structure was analyzed using X-ray diffraction. The electrical transport properties of the insulator layer were characterized by conductive atomic force microscopy at room temperature, while the superconducting properties of NbN were evaluated using a standard four-point configuration. Results indicate that increasing the deposition temperature of HfO₂ causes chemical degradation and reduces the nominal thicknesses of both NbN and HfO₂ layers due to interfacial reactions and Nb oxidation. This degradation adversely affects the electrical properties of the superconducting layers, specifically leading to a decrease in the superconducting critical temperature of NbN and an increase in the insulating properties of the HfO₂ layer. The modifications in the properties of HfO₂ are attributed not only to interfacial degradation but also to a probable reduction in structural disorder with increasing deposition temperature. Overall, these findings contribute to understanding the impact of interface disorder on electronic devices incorporating nitride and oxide layers.