Study of Niobium Mononitride Thin Films Grown Using High Power Impulse Magnetron Sputtering

Abstract

Niobium mononitride (NbN) has gained considerable attention because of its superconducting and refectory properties making it a promising material in devices like spin filters, Josephson junctions, superconducting qubits, and transistors especially when combined with other semiconductors and/or magnetic materials. [1][2][3][4][5][6][7] The ease of formation along with a relatively higher superconducting transition temperature (T C > 10 K) and a lower superconducting energy gap (Δ(0) % 3 meV) has made NbN to be one of the most studied superconducting materials for a wide range of applications. [8] Additionally, it has been widely used as an oxidation, wear-resistant layer, and diffusion barrier in protective and hard coatings. [9,10] NbN thin films can be synthesized using different growth techniques such as chemical vapor deposition, [11] ion beam-assisted deposition, [12] reactive direct current magnetron sputtering (dcMS), molecular beam epitaxy, [1] atomic layer deposition, [13] pulsed laser deposition, [14] etc.Among these, the dcMS has been widely used to grow thin films due to its cost effectiveness and the ability to grow uniform thin films. In a typical dcMS process, the plasma is mostly dominated by neutrals and the fraction of ionized species is very small (typically less than 5%). [15] The structural properties of the sputtered films strongly depend on the kinetic energies and ionization state of the impinging atomic and molecular species on the surface of growing films. It is well-known that the low ionization in dcMS process results into a porous and loosely packed microstructure and forms a large density of defects in the grown films. [16,17] To overcome these issues high power impulse magnetron sputtering (HiPIMS) has been developed. [18,19] In the HiPIMS technique, high power pulses are applied to a sputtering target contrary to a constant DC voltage in the dcMS. The use of high power pulses onto the sputtering target results in a large fraction of ionized-sputtered species. In addition, gases that are present in the vicinity of target also get ionized. [20] Earlier experimental and theoretical reports suggest that high ion to neutral ratios during HiPIMS process results into better thin-films' qualities in terms of denser microstructure, smoother surface, and interfaces. [21][22][23] Recent experimental and theoretical results [24,25] suggest that Nb vacancies are the most dominant

Structure and Superconductivity of Epitaxial and Polycrystalline VN Thin Films

“…30 Similar effects are also seen in the recent experiments in NbN thin films. 46,47 As such a direct correlation in ρ n and T C cannot be established but a higher ρ n can be attributed to an increased disorder. Additionally, many studies have been conducted recently on the impact of CS on the T C .…”