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Epitaxial Rhenium, a clean limit superconductor for superconducting Qbits

Abstract : The epitaxial growth condition and the superconducting properties of nanostructured devices made of rhenium (superconducting below T=1.7 K) on sapphire were explored. Epitaxial growth of rhenium thin films onto a single crystal α-Al2O3(001) substrate was realised using molecular beam epitaxy. The cleanness of the substrate was verified using XPS, and the growth of rhenium was monitored using RHEED. The orientations of the two crystals are (0001)Al2O3//(0001)Re and <2110>Al2O3//<0110>Re, which was confirmed using X-ray diffraction. The in-plane misfit between the lattices is -0.43% at room temperature, which allows us to estimate the critical thickness of rhenium to be between 10 nm and 15 nm.For deposition, rhenium was heated using an electron beam. Substrates were heated during growth using either a Joule-heated W filament located behind the sample or electron bombardment. Generally deposition temperatures of 800◦C and 900◦C gave reproducible results.The effect of deposition temperature was studied on samples that had the same thickness but were deposited at different temperatures. Three thickness groups were selected: 25 nm, 50 nm and 100 nm. Every sample was dominated by the (001) epitaxial orientation. Orientations (110), (100), (101) were present, but their intensities were small and decreased with increasing deposition temperature. AFM imaging was used to observe the morphology of the films. The 25 nm thick films were decorated with grains. The diameter of the grains (∼ 50 nm) did not vary significantly on the 25 nm thick sample, however, they became more uniform with increasing deposition temperature, and the surface became smoother. On the 50 nm and 100 nm thick films spirals and holes can be observed. Diameter of spirals on the 50 nm thick film increased from 100 nm to 500 nm when the temperature of the deposition was increased from 800◦C to 900◦C. XRD rocking curves measured on all samples got narrower with increasing deposition temperatures, indicating lower mosaicity of the (001) crystals. High-resolution θ-2θ scans evidenced a disorder in the 50 nm thick film, corresponding to strain values in the range of 0.01. Deposition temperature of 1000◦C lead to the dewetting of a 50 nm thick sample, islands with atomically flat surfaces were formed.The frequently observed spirals are most likely the result of screw dislocations. The origin of the holes that accompany the spirals is a dewetting process that starts when the thickness of the film reaches ~10 nm. We quantified the temperature evolution of the film during growth taking into account emission, reflection and transmission between all surfaces. This thermal model confirmed that the temperature of the film increases as the thickness of the rhenium film grows. The dewetting was studied using Mullins’ theory of thermal grooving. A surface diffusion coefficient of 4E−12 cm2/s was obtained, which is consistent with the observed dimensions of the surface topography.Wires with widths ranging from 100 nm to 3 μm and SQUIDs were fabricated from the rhenium films. Transport measurements confirmed that the lithography process does not affect the superconducting properties of rhenium. Critical temperatures between 1.43 K and 1.96 K were measured. We could correlate the superconducting transition temperature with the topography and the crystallinity of the films. Mean free path of electrons, and the superconducting coherence length were obtained, for two of the films both mean free path and effective coherence length were over 100 nm. These two films were in the clean limit, but the fabricated wires were in the dirty limit.On one film SQUIDs of 1 um diameter with 50 nm and 20 nm wide nanobridges acting as Josephson junctions were fabricated. The SQUIDs were cooled down using a dilution refrigerator. Critical current oscillations were measured. The flux noise values obtained were as low as 2.6E−5 Φ0/Hz1/2.
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Submitted on : Thursday, April 5, 2018 - 11:31:09 AM
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Kitti Ratter. Epitaxial Rhenium, a clean limit superconductor for superconducting Qbits. Superconductivity [cond-mat.supr-con]. Université Grenoble Alpes, 2017. English. ⟨NNT : 2017GREAY074⟩. ⟨tel-01759239⟩

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