Enhancement of the superconducting critical temperature of NbN ultra-thin films in a CMOS-compatible silicon nitride photonic platform on 200-mm-diameter wafers

Overview & Synopsis

In this paper, we investigate the superconducting critical temperature, morphology and structural properties of NbN ultra-thin films (5–7 nm) deposited by reactive sputtering on top of silicon nitride (SiN) over 200-mm silicon wafers. We demonstrate the effectiveness of a 10-nm thick spacer layer of AlN in raising the superconducting critical temperature from 5.0 K to 7.2 K and from 7.3 K to 9.3 K for 5-nm and 7-nm NbN thickness respectively. By combining investigations with X-ray diffraction and transmission electron microscopy, we highlight significant modifications of the crystalline properties of the NbN film on top of the AlN spacer layer. When directly deposited on SiN, the grains of the NbN polycrystalline layer are randomly oriented. In contrast, when deposited on top of the AlN (0001) spacer layer, the NbN layer is textured along the (111) direction, leading to increased critical temperatures. As AlN is a CMOS-compatible material, these findings are particularly relevant in view of the future integration of high-performance Superconducting Nanowire Single Photon Detectors (SNSPDs) on large-scale quantum photonic chips.