Structural analysis of rf and dc sputtered Nb3Ge thin films containing oxygen

Abstract

Using electron microscopy, diffraction, microprobe, scanning microscopy, and nuclear back-scattering techniques to analyze rf and dc sputtered N3Ge thin films, it was found that the film structure was strongly dependent on film preparation methods. Amorphous Nb3Ge films were prepared by both sputtering techniques, on substrates held at liquid nitrogen temperature. Identical crystallization heat treatments were then carried out in a He furnace at 750°C for 65 h. Analyses by nuclear back-scattering indicated that crystallized rf-prepared films contained about three times as much oxygen as crystallized dc films. Structure studies showed that although both types of crystallized films exhibited the A-15 structure, the data on the rf films indicate the presence of the crystalline oxide, Nb2O5. In addition, the lattice parameter for the rf film was slightly smaller * dc-prepared films while the grain size of the former is about 10 times larger. These data appear to be correlated to differences in the superconducting transition temperature of both types of films. A high Tc (∠16 K) was observed for rf-prepared samples. The annealed dc-sputtered films did not become superconducting for temperatures above 1.5 K. In summary, the results of this work indicate that large grain size is conducive to high Tc superconductivity in Nb–Ge system and provide some evidence for oxygen stabilization of the high TcA-15 phase.