Energetic condensation growth of Nb thin films

Abstract

This paper describes energetic condensation growth of Nb films using a cathodic arc plasma, whose 60--120 eV ions penetrate a few monolayers into the substrate and enable sufficient surface mobility to ensure that the lowest energy state (crystalline structure with minimal defects) is accessible to the film. Heteroepitaxial films of Nb were grown on $a$-plane sapphire and MgO crystals with good superconducting properties and crystal size ($10\text{ }\text{ }\mathrm{mm}\ifmmode\times\else\texttimes\fi{}20\text{ }\text{ }\mathrm{mm}$) limited only by substrate size. The substrates were heated to temperatures of up to $700\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and coated at $125\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, $300\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, $500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, and $700\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Film thickness was varied from $\ensuremath{\sim}0.25\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ to $>3\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Residual resistivity ratio ($⟨\mathrm{RRR}⟩$) values (up to a record $⟨\mathrm{RRR}⟩=587$ on MgO and $⟨\mathrm{RRR}⟩=328$ on $a$-sapphire) depend strongly on substrate annealing and deposition temperatures. X-ray diffraction spectra and pole figures reveal that RRR increases as the crystal structure of the Nb film becomes more ordered, consistent with fewer defects and, hence, longer electron mean-free path. A transition from Nb(110) to Nb(100) orientation on the MgO(100) lattice occurs at higher temperatures. This transition is discussed in light of substrate heating and energetic condensation physics. Electron backscattered diffraction and scanning electron microscope images complement the XRD data.