Very high residual resistivity ratios of heteroepitaxial superconducting niobium films on MgO substrates

Abstract

We report residual resistivity ratio (RRR) values (up to RRR-541) measured in thin film Nb grown on MgO crystal substrates, using a vacuum arc discharge, whose 60–160 eV Nb ions drive heteroepitaxial crystal growth. The RRR depends strongly upon substrate annealing and deposition temperatures. X-ray diffraction spectra and pole figures reveal that, as the crystal structure of the Nb film becomes more ordered, RRR increases, consistent with fewer defects or impurities in the lattice and hence longer electron mean free path. A transition from Nb(110) to purely Nb(100) crystal orientation on the MgO(100) lattice occurs at higher temperature.

Energetic condensation growth of Nb thin films

“…700=500 gave the highest hRRRi value (150) but 700=700 dropped hRRRi down to 80. The data for Nb on MgO revealed a similar trend of very strong increase of hRRRi with preheat and coating temperatures as published elsewhere [19]. Both T c and hRRRi show improvement with substrate heating.…”

“…The RRR is denoted as hRRRi in what follows because our 4-point probe measurement is an average of the RRR across the $few m film thickness. In an earlier paper [19] we showed this average might be related to the local value nearer the surface of the film. It is evident that hRRRi increases rapidly (from 7 up to 333) as the substrate-annealing temperature and the deposition temperature are both increased.…”

“…Several tools were used to characterize the films: SEM, XRD, EBSD, cross-section EBSD, TEM, and AFM. The details of the hRRRi measurement and results obtained in Nb films on MgO are published elsewhere [19]. Here we show companion results obtained on a-sapphire and repeat selected MgO data for completeness.…”

“…Figure 2 shows a photograph of the coaxial energetic condensation (CED TM ) apparatus [19] with a 2.2 GHz copper rf cavity adjacent to it, shown for scale. Figure 3 is a schematic drawing of the operation of the CED TM apparatus.…”

“…In an earlier article [19] we presented a simple model for the variation of RRR with film thickness. The physical basis for this model is that as the film grows, the texture changes from fibrous, small-grained crystals to larger grains with higher crystalline order, due to the nonequilibrium subplantation effects of energetic condensation.…”