Elsevier

Applied Surface Science

Volume 320, 30 November 2014, Pages 863-870
Applied Surface Science

On the synthesis of a compound with positive enthalpy of formation: Zinc-blende-like RuN thin films obtained by rf-magnetron sputtering

https://doi.org/10.1016/j.apsusc.2014.09.158Get rights and content

Highlights

  • RuN thin films in the zinc-blende structure have been synthesized by rf-magnetron sputtering.

  • Contribute is given to the understanding of phase-formation mechanisms in systems that under ambient conditions present positive enthalpies of formation.

  • Contribute is given to the understanding of phenomena occurring during reactive sputtering processes.

  • Nanopillar structure: suitable for application requiring a high effective area, like sensing, catalysis, and electrode material for energy-storage devices.

Abstract

4d- and 5d-transition metal nitrides are of interest both because of their importance for the understanding of mechanisms of phase formation in systems that under ambient conditions present positive enthalpies of formation and because of their appealing structural and electronic properties. In this study, we report the synthesis of thin films of ruthenium mononitride (RuN) in the zinc-blende structure by radio-frequency-magnetron sputtering. Films present a characteristic structure of packed columns ending with tetrahedral tips. The effect of changing the synthesis parameters was investigated in detail. It was found that RuN can be formed if the nitrogen partial pressure exceeds a minimum value and that the addition of argon has the major effect of increasing the deposition rate because of its higher sputter ability. Temperature plays an important role: if it is too high, decomposition/desorption effects overcome those leading to the formation of the compound. Phenomena resulting in the formation of RuN occur at the surface of the growing films and are related to the interactions of ruthenium with energetic nitrogen ions, or atoms, which can penetrate the first atomic layers by low energy implantation. Because of its properties and structure, this material is a promising candidate for applications like sensing, catalysis, and electrode material for energy-storage devices.

Introduction

4d- and 5d-transition metal nitrides and carbides are of great interest both because of their importance for the understanding of mechanisms of phase formation in systems that under ambient conditions present positive enthalpies of formation and because of their appealing structural and electronic properties. In particular, very high bulk moduli are expected and have been measured in some cases. A review of the early (2004–2008) results on platinum group metal nitrides and carbides was given by Ivanovskii [1]. Since then, a number of papers reporting on first-principles studies [2], [3], [4], [5], [6], [7], [8] and on experimental synthesis [9], [10], [11], [12], [13], [14] appeared. The first and most of the subsequent experimental studies [1–9 and references therein] were performed on samples synthesized in diamond anvil cells under extreme conditions of pressure (several tens of GPa) and temperature (thousands of Kelvin). Thin films of nitrides of a few elements were also synthesized using different techniques: gold nitride was obtained by ion implantation [15] and reactive ion sputtering and plasma etching [16]; platinum nitride was synthesized by pulsed laser ablation [17]. The most studied was ruthenium nitride, with a focus on its possible application as electrochemical sensor [11], [18], diffusion barrier [12], electrode material for energy-storage devices (supercapacitors) [13], [14] and, in alloy with hafnium, as a gate electrode [19]. Pulsed laser ablation [20] and reactive ion sputtering (both radio-frequency (rf) and continuous current (dc)) [11], [12], [13], [14], [18], [19] techniques were used. In some cases, the nitrogen to ruthenium atomic ratios were a fraction of unity, and, in general, papers are lacking a detailed discussion of the influence of the synthesis parameters, reporting only on the used recipes. Although results of first-principles calculations [3], [5], [7] converged towards the conclusion that the stability of the zinc-blende-like structure is higher than that of the rock-salt-like one, and both are higher than those of the other possible RuN polymorphs, on the basis of X-ray diffraction data and calculated lattice constants, the only two papers reporting on the formation of stoichiometric, cubic, RuN, reached different conclusion about the crystallographic structure of the nitride: rock-salt (NaCl)-like [20] and zinc-blende (ZnS)-like [14].

It is also worth noting that the stoichiometric nitride was synthesized by pulsed laser ablation [20] and dc-magnetron sputtering [14], while researchers that used rf-magnetron sputtering obtained compounds with much lower amounts of nitrogen. On the basis of our experience on the rf-magnetron sputtering of ruthenium and iridium oxide film cathodes [21], [22], we undertook a detailed investigation on the influence of many parameters affecting the synthesis of ruthenium nitride thin films by rf-magnetron sputtering, in order to find, and optimize, the conditions for the synthesis of the stoichiometric compound, and definitely identify the crystalline structure of this nitride. Several analytical techniques have been used, with results that can be set in a well-defined framework.

Section snippets

Experimental details

Sputter depositions were performed in a custom-built rf-magnetron sputtering system (13.56 MHz), equipped with three independent circular (2 in. in diameter), water-cooled, planar magnetron sources and an rf-biased sample holder. The cylindrical process chamber (equipped with a load-lock system) has both the diameter and the height equal to 60 cm and is evacuated by the concurrent action of a turbomolecular pump and a cryogenic one. Throttle valves are placed between the pumps and the chamber to

RBS analysis

Since Bouhtiyya et al. [14] reported that a successful synthesis of RuN occurred only at mild sputtering conditions, in an atmosphere of pure N2, we started our depositions at the mild source power of 50 W, total pressure of 0.60 Pa (standard for our apparatus) either in pure N2 (sample N) or in the N2(0.40 Pa)–Ar(0.20 Pa) gas mixture (sample NA) atmosphere.

In Fig. 1, we show the RBS spectra for samples N and NA. Arrows indicate the signals for the different elements: the leading edges of the

Conclusions

RuN was successfully obtained by rf-magnetron sputtering. XPS and XRD measurements confirmed the formation of the nitride, the composition of which was determined by RBS. Its zinc-blende-like structure was determined by combining XRD and magnetic measurements.

The compound forms both in pure nitrogen and in an Ar–N2 gas mixture, providing that a minimum quantity of nitrogen is present. The presence of argon enhances the growth rate of the nitride layer because of its higher target sputtering

Acknowledgements

We gratefully thank T. Finotto (Ca’ Foscari University of Venice, Italy), A. Glisenti (University of Padua, Italy) and A. Patelli (Veneto Nanotech, Venice, Italy) for their valuable contributions in the characterization of the samples.

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