Transmission electron microscopy analysis of atomic layer deposited NbTiN/NbN superconducting thin films

Abstract

Atomic-scale processing and precise control of superconducting thin films are essential for the advancement and large-scale implementation of superconducting quantum technologies. Consequently, detailed analysis of the structural features and elemental composition of such superconducting films is a key element in developing highly sensitive and efficient superconducting nanowire single photon detectors. In this work, we use advanced techniques in scanning transmission electron microscopy (STEM), specifically 4-dimensional STEM (4DSTEM) and electron energy loss spectroscopy (EELS), to analyze the structure and chemistry of two few-nanometer-thick films of NbN and NbTiN deposited by plasma-enhanced atomic layer deposition. Digital dark field imaging is used to image the crystalline core of the films, separate from the silicon substrate and protective platinum overlayer, and the data are used for quantitative measurement of lattice parameters. EELS mapping correlates the structural data with local chemistry and indicates the coexistence of superconducting NbC within the films. Crystalline rock-salt structured carbonitrides are found in both cases, and their lattice parameters can be accurately and reliably measured from hundreds of datapoints from different pixels in the scan area. These correlate well with the expected chemical composition. Both films feature a Si–N rich reaction layer, with Ti also present in NbTiN films. Interestingly, significant diffusion seems to occur in both films, differing from the atomic-layer sharpness sometimes presumed. Nevertheless, the presence of a continuous film with an appropriate structure and composition confirms that the process is suitable for superconducting applications, although further optimization could improve interface control and composition.