Research Article
Deposition rate controls nucleation and growth during amorphous/nanocrystalline competition in sputtered Zr-Cr thin films

https://doi.org/10.1016/j.jallcom.2022.168258Get rights and content

Highlights

  • Amorphous/Crystalline competitive growth has been found in Zr-Cr and Zr-V.

  • Deposition rate is a new lever to control microstructure and surface morphology.

  • Nucleation and growth dynamics are controlled by the deposition rate.

  • Zr-Cr, Zr-V, Zr-W and Zr-Mo show different nucleation and growth dynamics.

Abstract

Dual-phase Zr-based thin films synthesized by magnetron co-sputtering and showing competitive growth between amorphous and crystalline phases have been reported recently. In such films, the amorphous phase grows as columns, while the crystalline phase grows as separated cone-shaped crystalline regions made of smaller crystallites. In this paper, we investigate this phenomenon and propose a model for the development of the crystalline regions during thin film growth. We evidence using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), that this competitive self-separation also exists in co-sputtered Zr-Cr thin films with Cr contents of ∼84–86 at%, corresponding to the transition between the amorphous and crystalline compositions, and in the Zr-V system. Then, to assess the sturdiness of this phenomenon, its existence and geometrical characteristics are evaluated when varying the film composition and the deposition rate. The variation of geometrical features, such as the crystalline cone angle, the size and density of crystallites, is discussed. Is it shown that a variation in the deposition rate changes the nucleation and growth kinetics of the crystallites. The surface coverage by the crystalline phase at a given thickness is also calculated for each deposition rate. Moreover, comparison is made between Zr-Cr, Zr-V, Zr-Mo and Zr-W dual-phase thin films to compare their nucleation and growth kinetics.

Introduction

During the last decades, interest for new functional surfaces has continuously been growing [1], [2], [3]. This is because the functionalities of surface-modified materials are manifold: antibacterial surfaces [4], [5], [6], solar cells [7], wear or corrosion protection [8], [9], [10], to name a few. Many advances in the field of thin films have been made thanks to elaboration techniques such as magnetron sputtering, allowing to inexpensively synthesize thin films at low temperatures. Among these thin films, Zr-based thin film metallic glasses (TFMGs) have been intensively investigated due to their good mechanical properties such as hardness beyond 10 GPa, improvement of fatigue resistance of 316 L stainless steel, and their corrosion resistance, for instance [11], [12], [13].

The ease to synthesize such TFMGs is characterized by the glass-forming ability (GFA). The higher the GFA, the easier to sputter-deposit a TFMG. For the GFA to be the highest, the sputtered film must contain different elements, with a difference greater than 10 % in atomic radii, have a negative mixing enthalpy, and a high quenching rate [14]. Moreover, in the case of Zr-X systems (X = Cr, V, Mo, W), intermetallic phases can be found in the equilibrium phase diagram, which increase the GFA, despite the atomic radii of the different elements is rather close and the mixing enthalpy is only slightly negative (−4 to −12 kJ/mol) [15]. This means that although it is possible to find amorphous films in a wide range of compositions, it is still possible to sputter-deposit crystalline films outside of this range. Thus, sputtering films at the transition between the amorphous and crystalline compositions could lead to new microstructures, such as the dual-phase crystalline and amorphous thin films that have been reported in the literature. This dual-phase morphology has already been observed in Zr-W [16], [17], [18] and Zr-Mo [19]. Moreover, similar morphologies have been reported in the literature for different systems (Ti-Al [20], Ti-O [21], Al-N [22]), where it has only been mentioned without much attention given to its development. These dual-phase thin films are the result of a competitive growth between the amorphous and crystalline phases during film growth, leading to unique surface morphologies [16], [19]. A possible origin for crystallization in a given range of composition is that GFA is favored, meaning that the liquid is relatively stable compared to the crystalline phase. In other words, when the composition gets closer to the crystalline composition nucleation becomes easier. Moreover, Borroto et al. have evidenced that, in the Zr-W system, films undergo evolution from amorphous, to nucleation at the column boundaries, to random nucleation as the tungsten content increases and nucleation rate increases [16].

Even if this dual-phase morphology has been observed in different binary systems, the underlying mechanisms governing the development of competitive growth of the two phases are currently not well understood. Also, the sturdiness of the phenomenon regarding the deposition conditions is still unknown, and it is unknown if this phenomenon is present regardless of the deposition conditions, such as deposition rate, or not. This is mainly due to the facts that this phenomenon has only been first observed recently, and the composition range in which it occurs is narrow, making it difficult to observe experimentally.

In this study, we show that this phenomenon can be extended to Zr-Cr co-sputtered thin films. The competitive growth is tested by changing the deposition rate between 3.8 and 74 nm/min to test if it can exist over a wide range of deposition rates, if the composition range in which it occurs changes, and to compare the geometrical features of the obtained crystalline regions. Finally, nucleation and growth evolutions are explored in order to explain our results and generalize our understanding of the competitive growth process in Zr-based sputtered thin films. It is shown that the angle of crystalline cones decreases when increasing the deposition rate, and that it is due to a difference in nucleation and growth kinetics of the crystallites inside the cones. Then, these results are compared with dual-phase Zr-V, Zr-Mo and Zr-W thin films in regard to the geometrical features of the crystalline phase.

Section snippets

Thin films synthesis

Nanostructured Zr-Cr thin films were deposited onto (100) monocrystalline Si substrates (15 ×15 mm²) using DC magnetron co-sputtering of Zr and Cr metallic targets (targets dimensions: 2 in. diameter, 3 mm thickness and purity higher than 99.9 %) in an argon atmosphere. The two targets were in confocal configuration and the substrate-holder rotation was set at 15 rpm to ensure homogeneity of the deposited films. The sputtering chamber was pumped down via a mechanical and a turbo-molecular pumps

Results and discussion

When synthesizing Zr-Cr thin films by magnetron sputtering, a composition-driven transition from amorphous to crystalline Cr(Zr) solid solutions has been reported at ∼89 at%Cr [23]. This work aims to study what happens at the transition range, and aims to search for a competitive growth between an amorphous and crystalline phases, as has been seen for other Zr-based binary systems in magnetron sputtering [16], [17], [18], [19]. For this purpose, different thin films at compositions around the

Conclusion

To summarize, the competitive growth phenomenon that occurs between amorphous and crystalline phases that has been observed in the literature for sputtered Zr-Mo, Zr-W and Zr-V films has been also found in the Zr-Cr system. This study shows that it exists in a wide range of deposition rate, indicating its resilience against changes of deposition conditions and giving promises for its experimental observation in other systems. However, changing the deposition rate also changes the geometrical

CRediT authorship contribution statement

Q. Liebgott: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data curation, Writing – original draft, Writing – review & editing, Visualization. A. Borroto: Conceptualization, Methodology, Writing – review & editing, Visualization. Z. Fernández-Gutiérrez: Investigation. S. Bruyère: Formal analysis, Investigation. F. Mücklich: Supervision, Funding acquisition. D. Horwat: Conceptualization, Methodology, Resources, Writing – review & editing, Supervision, Project

Declaration of Competing Interest

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 “Université Franco-Allemande” (UFA) and the “Ministère de l′Enseignement Supérieur, de la Recherche et de l'Innovation” are deeply acknowledged for the Ph.D. scholarship of Quentin Liebgott within the PhD-track in Materials Science and Engineering at UFA.

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