Strain Effects on the Crystal Growth and Superconducting Properties of Epitaxial Niobium Ultrathin Films

Abstract

Superconducting ultrathin films grown epitaxially onto crystalline substrates exhibit strained epitaxial growth due to lattice mismatch, which can have a significant effect on their superconducting properties. We present a complete correlation of the surface morphology, crystal growth, strain, microstructure, and superconducting properties in single-crystal Nb(110) thin films sputter deposited on a-plane sapphire substrates. Notably, we observe that the lattice mismatch between Nb and sapphire induces the formation of a hexagonal surface structure during the first three atomic layers. This is followed by a strained bcc Nb(110) phase whose in-plane lattice parameter progressively relaxes to bulk value. Similar lattice relaxation was also observed in the direction perpendicular to the interface using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Significant perpendicular strain in films up to 30 nm thick was found to ultimately affect the superconducting properties of the Nb thin films as demonstrated with AC susceptibility measurements, where dissipative effects in the lattice associated with the presence of strain and associated defects were identified.

MgB2 ultrathin films fabricated by hybrid physical chemical vapor deposition and ion milling

“…The strained interfacial layer has a larger c lattice parameter compared to that of the bulk. 24 Figure 2 displays the ex situ atomic force microscopy (AFM) images of the 40 nm and 2.9 nm-thick unprotected MgB 2 films. The surface roughness in an area of 100 µm 2 is 2 nm and 0.85 nm, respectively, for the two films.…”

Roughness analysis applied to niobium thin films grown on MgO(001) surfaces for superconducting radio frequency cavity applications

“…We note that since Nb growth over MgO substrates proceeds in 3D fashion, i.e., Volmer-Weber mode; RHEED cannot be used as an effective means to extract growth rates and thickness via recording intensity oscillations of the electron beam [14]. Thus, RHEED was employed mainly to obtain in situ information regarding the evolution of the film microstructure, including the epitaxial registry of the Nb films on MgO, as well as the strain evolution and surface faceting of Nb films deposited on MgO substrates as well as sapphire which are described elsewhere [15]. Characteristic RHEED patterns were collected for each film along two distinct crystallographic directions corresponding to MgO[100] and MgO[110] azimuths.…”

Superconducting Radio-Frequency Technology R&D for Future Accelerator Applications

“…A collaboration led by JLab has been characterizing Nb film growth on a variety of model substrates as a function of substrate preparation and energy of arriving Nb ions [96][97][98][99][100][101][102][103][104][105]. The methods of creating the energetic ions vary from ECR plasma biased extraction, to cathodic arc discharge plasma, to high power impulse magnetron sputtering (HiPIMS) [106] championed by Andre Anders of LBNL.…”