Elsevier

Surface Science

Volume 700, October 2020, 121637
Surface Science

Surface topography and composition of NiPd alloys under oblique and normal gas cluster ion beam irradiation

https://doi.org/10.1016/j.susc.2020.121637Get rights and content

Highlights

  • very high enrichment with one of the alloy components if found

  • the steady state dose for cluster ions higher than for atomic ones;

  • ripples develop under oblique cluster incidence

  • alloy components distribution along surface repeats the ripple pattern

  • etching rate depends on the crystallite orientation

Abstract

Polycrystalline NiPd and Ni5Pd alloys were irradiated with argon gas cluster ion beam and atomic ion beam. In situ XPS measurements showed surface enrichment with Ni. For cluster ions, the degree of the enrichment was significantly higher, and the ion current density influenced on it. Under oblique cluster ion beam, ripples developed on the surface, and the elements of the alloy redistributed along the surface according to the ripple pattern. For normal cluster ion beam direction, sputtering rate was determined by a crystalline orientation, which limited the smoothing effect.

Introduction

Gas cluster ion beams (GCIB) have emerged in the recent few decades as a new object of investigations and are already widely used in fundamental research and technological processes [1], [2], [3]. A gas cluster ion consists of from a few of atoms up to a few tens of thousands of atoms, which are bound with weak van-der-Waals bonds. After ionization, such a cluster carries a charge of one or a few elementary charges, and after acceleration up to a keV range energy each atom of the cluster can have quite low kinetic energy. For example, an atom of a 10 keV Ar1000+ cluster has the average energy of only 10 eV. When such a cluster collides with a solid surface, it is easily disassembled and do not penetrate into depth [4,5]. Nevertheless, since all the carried energy is deposited in a small surface area, this local area heats up to high temperatures (>104 K), and a shock wave can be generated [6,7].

Cluster size distribution in GCIB usually ranges from a few atoms to a few thousands of atoms. It can be varied by the GCIB source adjustments and narrowed with mass-filters. Applying the desired acceleration voltage, the energy per a cluster atom can be precisely tuned. All these unique features lead to extensive employment of accelerated cluster ions for surface analysis. Gas clusters are widely used in organic secondary ion mass spectrometry (SIMS) as the probing beam since they are able to detach a large molecule from the sample surface without its fragmentation [8,9]. As well, applying cluster ions in SIMS for profiling sputtering is shown to give sharper borders between layers because of lower intermixing caused with the cluster ion beam [10]. Another powerful analytical technique is x-ray photoelectron spectroscopy (XPS), which utilizes GCIB for surface cleaning and surface profiling. Thus, in [11,12] it was shown that argon cluster ions with energy low enough effectively remove carbon contaminations and do not change the chemical state of the sample components. Nevertheless, it is well known that ion irradiation can change surface composition through selective sputtering, radiation-induced segregation, etс [13,14]. This fact is crucial for quantitative analytical techniques such as XPS. For gas cluster ions, there is a lack of information on the influence of the ion bombardment on the surface composition. We can mention the works [15,16], which describes the formation of nanostructures on InP under Ar300 clusters at 8 keV. Metallic In nanoparticles had rapidly developed on the surface and determined its consequent evolution. Such behavior is known for the case of atomic ion irradiation as well [17]. Another example is preferential removal of selenium and chemical reduction of copper, indium, and gallium in CIGS materials by 10 keV Ar1500+ [18]. On the other hand, in the same work, damage layer induced by 1 keV Ar+ ion was removed from PET and PTFE polymers by the clusters. In [19] HfO2 and SrTiO3 profiling with atomic and cluster ion beams were compared, and clusters were found to introduce less damage. It is worth emphasizing that InP, CIGS, HfO2 and SrTiO3 are compounds and have fixed ratio between the elements, which determines the process of the altered layer formation; it makes it difficult to understand the physical basis of the process and to predict the altered layer properties for a substance.

Another problem ion beam depth profiling faces is surface relief development under ion irradiation. Indeed, if one wants to examine nm-thickness layers, and the developed relief has the same or even larger height, no exact thickness value can be measured. Typically, GCIB gun in an XPS machine is placed under the angle of 30 - 60° from the surface normal. But surface ripples are known to develop for off-normal cluster irradiation [20]. Such ripples were found, for example, on silicon [4,21] and gold [22,23] and typically have wavelength and height of tens or hundreds nanometers. The exact mechanism of the relief emergence under cluster ion beam is not described yet, but some ideas can be found in [20,24]. Understanding the laws of the ripples formation is extremely interesting not only from the point of view of surface profiling improvement, but as well for practical applications. Such applications can be plasmonics, biosensing, biocompatibility improvement [15,20]. Alongside with the well-known effect of GCIB surface smoothing [25,26], this can open new perspectives in surface nanostructuring.

In our study, we investigate the surface composition and topography of atomic and cluster ions irradiated Ni-based alloys (NiPd, Ni5Pd). Such alloys are actually solid solutions, which means that Ni and Pd atoms can easily replace each other and prevents us from the chemical compounds effects in selective sputtering and segregation. In situ XPS measurements allowed us to observe the evolution of surface composition during the irradiation, and secondary electron microscopy (SEM) and atomic-force microscopy (AFM) studies revealed the interrelation between the surface relief and its composition for normal and off-normal ion bombardment.

Section snippets

Materials and methods

Polycrystalline NiPd and Ni5Pd with 99.99 at. % purity were cut into rectangular samples 4 × 10 mm with 2 mm thickness. The samples surfaces were mechanically polished, cleaned with organic solvents and had no texture. Volume composition was controlled using SEM with energy dispersive x-ray spectroscopy (EDS) and equaled to the nominal ratio. Relevant parameters of Ni and Pd are shown in the Table 1.

Ion irradiation and XPS measurements were performed with XPS machine PHI 5000 VersaProbe II by

Surface composition under oblique ion incidence

Fig. 2 shows the surface composition evolution of NiPd and Ni5Pd alloys within 20 kV cluster ion bombardment at 55° from the surface normal. At the very first moment, the amount of palladium is significantly higher than the nominal ratio. It can be explained by the influence of the mechanical polishing. Surface cleaning with 5 keV cluster ions could not cause this high concentration of Pd, since the XPS spectra obtained before and after the cleaning process showed almost the same ratio of Ni

Conclusions

Cluster ion bombardment of NiPd and Ni5Pd alloys both under normal and inclined incidence resulted in significant surface enrichment with Ni. This can be explained by preferential sputtering of Pd by cluster ions and by its stimulated segregation to the surface. For cluster ions, the transition to the steady state regime occurs at significantly higher doses than for atomic ions. For inclined beam incidence, ripples appeared on the surface, and the alloy components redistributed according to the

CRediT authorship contribution statement

A.E. Ieshkin: Methodology, Visualization, Writing - original draft. D.S. Kireev: Investigation. A.A. Tatarintsev: Investigation. V.S. Chernysh: Conceptualization, Supervision, Writing - review & editing. B.R. Senatulin: Investigation. E.A. Skryleva: Resources, Supervision.

Competing Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work is performed using equipment of the Joint Research Center «Material Science and Metallurgy» NUST MISIS.

The work was partly carried out with the financial support of the Ministry of science and higher education of the Russian Federation (the unique identifier of PNIER RFMEFI57918 × 0157, Agreement on the provision of the Federal budget in 2018-2020 grant in the form of subsidies from 30.11.2018 № 075-11-2018-173).

References (38)

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