Effect of substrate miscut angle on critical thickness, structural and electronic properties of MBE-grown NbN films on c-plane sapphire

TL;DR

NbN epitaxy on c-plane sapphire is challenged by symmetry mismatch, and this study tests large substrate miscuts along the m-axis to enhance structural quality using PAMBE growth and comprehensive characterisation (XRD, RC, STEM, and transport). Increasing the miscut sharpens NbN 111 rocking curves and narrows 2θ-ω peaks, indicating reduced defects and mosaicity, while STEM shows a ~10 nm coherently strained NbN region before columnar growth and misfit dislocations at the interface. The structural improvement accompanies a modest rise in Tc from 12.1 K to 12.5 K, with room-temperature resistivity largely unchanged. The authors discuss Nagai tilt as a potential mechanism for defect reduction and highlight future work to generalise the approach across miscut directions and related materials.

Abstract

We report the structural and electronic properties of niobium nitride (NbN) thin films grown by molecular beam epitaxy on c-plane sapphire with miscut angles of $0.5^\text{o}$, $2^\text{o}$, $4^\text{o}$, and $10^\text{o}$ towards m-axis. X-ray diffraction (XRD) scans reveal that the full width at half maximum of the rocking curves around the 1 1 1 reflection of these NbN films decreases with increasing miscut. Starting from 76 arcsecs on $0.5^\text{o}$ miscut, the FWHM reduces to almost 20 arcsecs on $10^\text{o}$ miscut sapphire indicating improved structural quality. Scanning transmission electron microscopy (STEM) images indicate that NbN on c-sapphire has around 10 nm critical thickness, irrespective of the substrate miscut, above which it turns columnar. The improved structural property is correlated with a marginal increment in superconducting transition temperature $T_\text{c}$ from 12.1 K for NbN on $0.5^\text{o}$ miscut sapphire to 12.5 K for NbN on $10^\text{o}$ miscut sapphire.

Figures (8)

• Figure 1: Schematic of the Nagai tilt. Substrates with a miscut angle $\mu$ form terraces with atomic steps on the surface to lower the surface energy. The terrace width $L$ depends on the miscut angle and the step height $d_s$ which is the inter-planar spacing of the low energy planes. The difference in the inter-planar spacings of the film $d_f$ and the substrate $d_s$ causes an epitaxial tilt $\alpha$ which is the angle between the low energy planes of the film and the substrate.
• Figure 2: Symmetric $2\theta$-$\omega$ coupled scans around $2\theta=35.4^\text{o}$ expected for NbN 1 1 1 peak after aligning the instrument to NbN 1 1 1 reflection show narrower peaks with increasing miscut angle of the substrate and the peak position shifts closer towards the bulk NbN peak position.
• Figure 3: Normalized $\omega$ rocking curves (RCs) around the NbN 1 1 1 reflection show decreasing FWHM with increasing miscut angle of the substrate, the FWHM value of each RC is indicated in arcsecs (") [3600 arcsecs (") = 1 degree ($^\text{o}$)].
• Figure 4: a), d) Wide field-of-view ADF-STEM images of NbN on 0.5$^\text{o}$ and 10$^\text{o}$ miscut sapphire, respectively. Both samples show a clean epitaxial NbN film in the initial 10 nm followed by columnar growth. b), e) Atomic resolution ADF-STEM images of the NbN to 0.5$^\text{o}$ and 10$^\text{o}$ miscut sapphire interface, respectively. The sapphire interface is aligned normal to the growth direction, and the NbN (1 1 1) planes prefer to be parallel to the sapphire (0 0 1) planes. c), f) Atomic resolution iDPC image of the NbN to 0.5$^\text{o}$ and 10$^\text{o}$ miscut sapphire interface aligning the sapphire (0 0 1) plane. The iDPC captures the tilts and disorder within the NbN that leads to blur in the image. The misfit dislocation between NbN and sapphire is noted in yellow across the interface. Both samples show a similar dislocation density $\sim$1.6$\times$10$^{12}$/cm$^2$ (assuming a 50 nm projection thickness). The high dislocation density reflects the large difference and lattice constant between the sapphire and relaxed NbN.
• Figure 5: a) Room temperature resistivity of NbN films versus substrate miscut angle showing no impact of the substrate miscut. b) Marginal increment in NbN film's superconducting critical temperature, $T_\text{c}$, with increasing miscut angle of the sapphire substrate is seen from the normalized resistance vs temperature curve.