Growth windows of epitaxial $\textrm{Nb}_x\textrm{N}$ films on c-plane sapphire and their structural and superconducting properties

TL;DR

This study investigates how growth conditions influence the structural phases and superconducting properties of NbN films on sapphire, revealing the ability to produce atomically flat $eta$-$ extrm{Nb}_2 extrm{N}$ films with specific superconducting temperatures.

Contribution

It demonstrates the growth of atomically flat $eta$-$ extrm{Nb}_2 extrm{N}$ films at high temperatures and links growth parameters to phase and superconducting properties.

Findings

Four NbN phases identified under different growth conditions.

Atomically flat $eta$-$ extrm{Nb}_2 extrm{N}$ films grown at T ≥ 1100°C.

Superconducting T_c of pure $eta$-$ extrm{Nb}_2 extrm{N}$ is between 0.35 K and 0.6 K.

Abstract

NbN films are grown on c-plane sapphire substrates by molecular beam epitaxy. The structural and superconducting properties of the film are characterized to demonstrate that growth parameters such as substrate temperature and active nitrogen flux effect the structural phase of films, and thereby the superconducting critical temperature. Four phases of NbN are identified for films grown in different conditions. In a novel finding, we demonstrate that atomically flat and highly crystalline $\beta$-$\textrm{Nb}_2\textrm{N}$ films can be grown at substrate temperatures of 1100 \degree C or higher, and that the superconducting critical temperature of phase pure $\beta$-$\textrm{Nb}_2\textrm{N}$ films is $0.35~K<T_c<0.6~K$, based on measurements of films grown at different substrate temperatures.