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.
Highlights
This paper is organized as follows: We begin with a brief description of ‘‘energetic condensation’’, followed by a description of our cathodic arc deposition apparatus
We conclude this paper with a discussion of the differences between our energetic condensation process and other processes, such as electron cyclotron resonance (ECR) plasma deposition [6] or much lower energy film growth processes such as magnetron sputtering and e-beam evaporative deposition
This paper has presented examples of Nb thin films grown on various crystal substrates using energetic condensation
Summary
This paper is organized as follows: We begin with a brief description of ‘‘energetic condensation’’, followed by a description of our cathodic arc deposition apparatus. Films and describe how the crystallographic orientation and quality are affected by deposition conditions This is followed by presentation of electron backscattered diffraction (EBSD) data for the films. These data include both surface and cross-section EBSD images that are discussed in light of deposition conditions. Cathodic arc plasmas are known [2] to produce both energetic ions (60–120 eV with 3þ charge in the case of Nb) as well as $0:1–10 m macroparticles. The ability of our energetic condensation process to drive crystal growth on an amorphous substrate has implications for future development of cast Al SRF cavities that have high quality, superconducting Nb thin films on their inside surface. We conclude this paper with a discussion of the differences between our energetic condensation process and other processes, such as electron cyclotron resonance (ECR) plasma deposition [6] or much lower energy film growth processes such as magnetron sputtering and e-beam evaporative deposition
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