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

TL;DR

This study investigates how the substrate miscut angle affects the structural and electronic properties of NbN thin films grown on c-plane sapphire, revealing improved structural quality and slight increases in superconducting transition temperature with larger miscut angles.

Contribution

It provides new insights into how substrate miscut angles influence the structural quality and superconducting properties of NbN films grown by molecular beam epitaxy.

Findings

Increased miscut angle reduces FWHM of rocking curves, indicating better structural quality.

Critical thickness of NbN is around 10 nm regardless of miscut.

Superconducting transition temperature slightly increases with larger miscut angles.

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.