Synthesis of V₂AlC thin films by thermal annealing of nanoscale elemental multilayered precursors: Incorporation of layered Ar bubbles and impact on microstructure formation

Highlights

• Single-phase and basal-plane oriented V₂AlC films via thermal annealing of nanoscale V/C/Al multilayered thin film precursors.

• Comprehensive microstructural and compositional analyses at the nanoscale by STEM and APT.

• Preferential incorporation of Ar atoms into amorphous carbon nanolayers within the as-deposited multilayered precursors.

• Precipitation of layered, nanoscale Ar bubbles primarily at the grain boundaries during annealing.

Abstract

Periodically stacked nanoscale V/C/Al multilayered thin film precursors were deposited by magnetron sputtering using elemental targets. Their temperature-dependent phase transformation during subsequent thermal annealing in argon towards V₂AlC MAX phase formation was investigated. Compositional and microstructural analyses at the nanoscale revealed undulation growth of the nanolayers and preferential incorporation of Ar atoms into amorphous carbon nanolayers within the as-deposited multilayered precursors. Single-phase and basal-plane-oriented V₂AlC films were formed at annealing temperatures above 560 °C. The incorporated Ar atoms migrated and aggregated into high-density nanoscale Ar clusters/bubbles located essentially at grain boundaries after annealing, displaying pronounced layered distribution characteristics especially adjacent to the substrate/film interface. The heterogeneous incorporation of argon atoms or clusters in heterostructured films composed of multicomponent sublayers likely represents a common phenomenon during thin film nucleation and growth, while its impact on the macroscopic properties of films remains to be explored.

Introduction

M_n+1AX_n phases (MAX phases, M: transition metal, A: mainly A-group element, X: carbon or nitrogen, n: typically 1–3) are a class of intrinsically nanolaminated ternary compounds [1]. These nanolaminated compounds possess unusual and remarkable properties combining attributes of both metals and ceramics that stem from their unique layered structure and bonding characteristics. More specifically, their crystal structures comprise twinned M_n+1X_n slabs of edge-sharing M₆X octahedrons interleaved with atomic layers of the A element, with strong covalent-ionic M-X bonds and relatively weak metallic M-A bonds. Considerable pioneering work has been done on the processing and properties characterization of sintered bulk MAX phase materials [1], [2]. In parallel, there has been tremendous and continuously increasing interest in synthesizing MAX phase thin films and coatings for applications in surface engineering [3], [4].

Thin-film synthesis of MAX phase materials has been predominantly carried out by physical vapor deposition (PVD) techniques, such as magnetron sputtering, cathodic arc evaporation, and pulsed laser deposition. The synthesis procedures comprise two main approaches, i.e. (1) direct growth with deliberate heating of substrates, and (2) solid-state reaction synthesis by thermal annealing of as-deposited multilayered or amorphous precursors [3]. Early studies on MAX phase films have focused on the epitaxial growth of MAX phase structures on single crystal substrates and determination of their fundamental properties [5], [6]. Intensive efforts are currently underway to optimize procedures for synthesizing high-quality (i.e. single-phase and textured) and relatively thick MAX phase films (at least a few microns) on different types of substrate materials [7], [8], [9]. However, their complex chemistry and lattice structures raise difficulties in achieving single-phase MAX phase films on technologically relevant substrates with favorable crystallographic orientation and reduced processing temperature. Our previous studies revealed that phase-pure and basal-plane textured Ti₂AlC and Ti₃AlC₂ films can be readily synthesized by thermal annealing of magnetron-sputtered multilayered thin film precursors with pre-defined nanostructured architectures (i.e. periodical stacking of nanometer-thin layers of the constituent elements) [10], [11]. Several further studies demonstrated similarly that using element targets in PVD, following a layer-by-layer approach, offers advantages for precise control of individual element fluxes towards accurate precursor design and for enabling synthesis of high-purity MAX phase thin films [12], [13], [14]. For instance, Stevens et al. have grown epitaxial Cr₂AlC MAX phase thin films by sequential layer-by-layer deposition at 600 °C using pulsed laser deposition [14].

Among the previously reported MAX phase films synthesized by PVD, V₂AlC was found to require a relatively low processing temperature (∼600 °C) and possess a fairly low resistivity (30 μΩ·cm), which makes it attractive for various electrical applications [3]. In addition, synthesis of V₂AlC films using multilayered precursors has not been reported yet. This study focuses on synthesis of V₂AlC MAX phase films via thermal annealing of periodically stacked V/C/Al multilayered thin film precursors deposited by magnetron sputtering using a layer-by-layer approach. The temperature-dependent phase formation of the annealed multilayered precursors was investigated by in-situ high-temperature X-ray diffraction (HT-XTD) and ex-situ XRD. In particular, comprehensive microstructural and compositional analyses at the nanoscale, combining high-resolution scanning transmission electron microscopy (HR-STEM) and atom probe tomography (APT), were performed to gain a deeper understanding of the thermally induced phase formation and microstructural evolution. We observed a preferential incorporation of Ar atoms into amorphous carbon nanolayers within the multilayered thin film precursors. During the annealing process towards V₂AlC MAX phase formation, these Ar atoms migrate and aggregate into nanoscale Ar clusters/bubbles that are distributed in a well-defined, periodically layered arrangement.