Low-temperature growth of epitaxial Ti2AlC MAX phase thin films by low-rate layer-by-layer PVD

ABSTRACT Here we report on the structural and tribo-mechanical characterization of epitaxial single-crystalline Ti2AlC MAX phase thin films, grown by means of electron beam physical vapor deposition at relatively low temperature (700°C). The growth of phase pure Ti2AlC at a relatively lower temperature when compared to other PVD methods was achieved utilizing a relatively low deposition rate and layer-by-layer deposition technique. The epitaxial growth is evidenced through the combination of XRD, HR-TEM and Raman spectroscopy measurements. The nanomechanical and micro-scale tribological properties of the Ti2AlC thin films were studied by means of nanoindentation and nanoscratch tests. GRAPHICAL ABSTRACT IMPACT STATEMENT The growth temperature of phase pure single-crystalline Ti2AlC MAX phase thin films was reduced to 700°C utilizing the low-rate layer-by-layer PVD technique.


Introduction
M n + 1 AX n (n = 1,2,3) phases are inherently lamellar ternary ceramic compounds (i.e. nitrides and carbides) with hexagonal structure with P6 3 /mmc symmetry, where M is an early transition group metal, A is an A-group element and X is carbon or nitrogen. Their inherently nanolaminated structure assumes different bonding between the layers of the constituent elements: the M-A are metallic, while the M-X are covalent. This combination of those bonding gives a unique combination of properties, typical for both metals (thermal and electrical conductivity) and ceramics (low coefficient of friction, high resistance to thermal shocks, high-temperature chemical stability). Such combination of properties makes them very promising functional materials for high-temperature structural applications, protective coatings, sensors, low friction surfaces, electrical contacts, tunable damping films for microelectromechanical systems, etc. [1]. Moreover, MAX phases are used as precursors for the synthesis of twodimensional solids, MXenes, by etching of the A-element, which have recently shown promise as electrodes for supercapacitors, Li-S batteries and Li-, Na-and K-ion batteries [2,3]. The combination of metallic/covalent bonds and inherently laminated configuration makes the hexagonal crystal structure of MAX phases highly anisotropic that consequently promotes properties to be anisotropic. In this regard, the synthesis of MAX phases in thin films form via a controllable epitaxial growth, with desired crystallographic orientation on lattice matched substrates, makes possible the measurement of fundamental physical properties and their anisotropies. Moreover, continuous epitaxial MAX phase thin films are of special importance for the synthesis of large specific surface area MXenes [4]. Therefore, the growth of epitaxial and phase pure MAX phases are of tremendous scientific and technological importance.
Among the Ti n + 1 AlC n MAX phase family, Ti 2 AlC has gained a special interest of the research community due to a unique combination of excellent physical and functional properties [1,5]. The epitaxial Ti 2 AlC MAX phases were grown by physical vapor deposition (PVD) methods at temperatures of order 900°C (see Supplementary note 2). However, the growth of pure single-crystal Ti 2 AlC thin films by PVD at relatively lower temperatures remains challenging and limits the use of temperature sensitive substrates and thus hinder the applicability and studies on Ti 2 AlC MAX phases and related composites. Recently, a rather innovative approach based on layer-by-layer deposition of MAX phase forming atomic species was reported by Vishnyakov et al. [6], which allowed to reduce the growth temperature of the polycrystalline Ti 3 SiC 2 substantially from 900°C down to 650°C.
In this work, we utilize the layer-by-layer approach and additionally reduce the deposition rate for the growth of epitaxial Ti 2 AlC MAX phase thin films by electron beam physical vapor deposition at 700°C. The nanomechanical and micro-scale tribological characterization of the Ti 2 AlC thin films are also reported.

Materials and deposition
The growth of the films was carried out in cryopumped ultra-high vacuum (UHV) environment (with a base pressure of 6.7 × 10 −10 Torr) by means of electronbeam physical vapor deposition ( Figure S1). The evaporation materials were Al (99.96% pure), Ti (99.99% pure) pellets and C pieces (99.99% pure) placed into water-cooled intermetallic (BN-TiB 2 ), TiC and graphite crucibles, respectively. The MAX phase thin film was deposited with a deposition rate of approximately 0.3, 0.4, and 0.1 Å/s for Ti, Al and C, respectively. The Ti 2 AlC film was grown by sequential layer-by-layer deposition 120 Å of Ti, 60 Å of C and 72 Å of Al up to the total thickness of 1200 Å. The 200 Å-thick TiC seed layer was grown by sequential deposition 60 Å of Ti and 40 Å of C. Due to the exchange of the crucibles and the heating of the materials up to evaporation (sublimation of C) and subsequent cooling before the exchange, the overall deposition time was 8 h.
The substrate was a polished 10 × 10 mm singlecrystal Al 2 O 3 (0001) wafer. The substrate temperature during the deposition was kept at 700°C. See Supplementary materials for details on the deposition.

Structural characterization
Structural characterization of the thin film was performed be means of X-ray diffraction (XRD) by -2 , Éÿ, and pole-figure (ψ-Éÿψ) scans. High-resolution transmission electron microscopy (HR-TEM) with energy dispersive analysis (EDS), atomic force microscopy (AFM) and Raman spectroscopy. The cross-section for TEM studies was prepared by means of a focused ion beam system (FIB) with Ga ions.

Nanomechanical testing
The nanoindentation, nanoscratch and nanowear tests were performed at room temperature using Hysitron Tri-boIndenter TI 950 equipped with a Berkovich diamond tip (see Supplementary materials for the details).

Results and discussions
The XRD analysis allowed to reveal Ti 2 AlC and TiC phases together with peaks from Al 2 O 3 substrate (Figure 1(a)). Moreover, the peak around 36°(and the 222 peak at 76°, not shown) is an indication of TiC (111), originating from the seed layer, which is slightly shifted to higher angles compared to stoichiometric TiC. The peak shift is associated with reduced cubic unit cell due to the substitution of Ti atoms to smaller Al atoms as a result of inward diffusion of the latter from the substrate or/and Ti 2 AlC layer, as was revealed by the TEM-EDS mapping (see Figure S2). This is an important process for initial reactions at the surface during nucleation and growth of Ti 2 AlC [7]. Only the (000l) type peaks from the MAX phase Ti 2 AlC and (lll) TiC are observed that is typical for epitaxial growth of the MAX-phase film with out-of-plane orientation (0001)Ti 2 AlC (111)TiC (0001)Al 2 O 3 . In such growth configuration ( Figure 1(a), inset), (111) TiC layer acts as a template for nucleation and epitaxy of (000l) Ti 2 AlC MAX phase because the (111) TiC and (000l) surfaces are isomorphous due to relatively similar structure and orientation of (111) TiC to TiC slabs in Ti 2 AlC MAX phase structure and forms the first basal plane of the MAX phase. The epitaxial growth is additionally con-    demonstrates bright and dark stripes typical for MAX phases.
The SAED pattern in Figure 2(c) obtained from the area of Ø220 nm of the sample together with a part of the substrate recorded with the electron beam parallel to the [1010] Al 2 O 3 zone axis. The indexation of the SAED pattern reveals symmetric reflections from the (lll) TiC, (000l) MAX phase and (000l) Al 2 O 3 sequentially arranged along the growth direction (see Figure 2(c), inset) and are slightly superimposed due to a small difference in the corresponding interplanar distances. Figure 2(d) shows a high-resolution image of the TiC/Ti 2 AlC interface recorded along Al 2 O 3 [1010] zone axis. The typical stacking sequence of the dark bands separated by thin bright lines can be seen. Such a contrast is characteristic for the Ti 2 AlC phase with TiC slabs interleaved with a square-planar Al-layers. The filtered image of the reciprocal-lattice spots on the FFT pattern (Figure 2(e)) corresponding to (111) TiC planes and (0008) Ti 2 AlC planes visualize the epitaxial growth and sharp interface between the MAX phase and the seed layer. Hence, these results give much stronger support to the XRD measurements thus giving sufficient evidence to conclude that the phase pure (000l) Ti 2 AlC MAX phases grow epitaxially on (lll) TiC seed layer.
The presence of ω1, ω 2/ω3 and ω4 Raman-active modes of Ti 2 AlC (Figure 3) additionally confirms and support the XRD and HR-TEM measurements that the grown thin film is truly Ti 2 AlC MAX phase with TiC seed layer. The growth of Ti 2 AlC is additionally evidenced by hexagonally shaped crystals with low-energy faces (0001) and {1010} revealed at the surface by means of AFM (Figure 3, inset), which are commonly observed on the surface of MAX phases [1].
Consequently, the single-crystal phase pure epitaxial Ti 2 AlC MAX phase is grown at a relatively lower temperature of 700°C compared to simultaneous deposition by means of magnetron sputtering and pulsed cathodic arc deposition at 900°C (see Supplementary note 2). Generally, Ti 2 AlC starts to nucleate already at 700°C while phase pure Ti 2 AlC has been grown only at 900°C (see Supplementary note 2). Typically, in order to promote the formation of a large unit cell of MAX phases, the nucleation dynamics and growth conditions should fulfill long adatom diffusion length. Since the mean diffusion distance per adatom on the surface is proportional to the surface diffusion coefficient and inversely proportional to the deposition rate (Equation (1)), desired mobility of adatoms can be tuned by properly adjusting growth temperature and growth rate, as follows from the equation [10]: where D s is the surface diffusion coefficient, ν is the vibrational frequency of surface atoms, a is the individual jump distance, t ∼ a/R, where R is the deposition rate, E D is the activation energy for surface diffusion, k is Boltzmann's constant and T is the substrate temperature. It is clear that the mean diffusion distance can be less than one interatomic spacing below a critical temperature or above a critical deposition rate. Therefore, it is reasonable to suggest that in our approach a relatively low deposition temperature was compensated by low deposition rate during layer-by-layer growth that facilitates sufficient adatom diffusion length and growth of single-crystalline Ti 2 AlC MAX phase. Moreover, the sublimation of Al is a limiting factor for nucleation and growth dynamics of MAX phases during low-rate deposition at high temperatures [7]. Although an excess of Al with respect to 2:1:1 stoichiometry was supplied during the deposition in our experiment, a slight outward diffusion of Al was observed (see Figure S2). The driving forces for Al release from the crystal are the low diffusion barrier for Al atom self-diffusion along the (0001) plane [11], lower energy for vacancy formation [12] and higher isotropic thermal motion of Al [13] in comparison to Ti and C. However, defective Ti 2 AlC may sustain phase stability down to an Al sub-stoichiometry of Ti 2 Al 0.5 C [11]. Importantly, O inward diffusion from the substrate is less pronounced than Al (see Figure S2) that is of considerable importance for the synthesis of phase pure MAX phases, since O tends to substitute C atoms in the hexagonal lattice [7]. The more pronounced Al inward diffusion from the sapphire substrate can be assigned to lower activation energy for diffusion of Al than of O in sapphire [14].
It is well-known that the thin film growth kinetics can additionally be affected by the energy delivered to the growing film by the species impinging on the growing films (i.e. ions and neutrals). Considering that the vapor flux energy is significantly different between magnetron sputtering, cathodic arc deposition and electron beam evaporation, it is impossible to critically compare our results with those reported before. However, the outcomes of the present study can be an excellent starting point for further optimization and understanding of MAX phase thin film grown by both magnetron sputtering and cathodic arc deposition. Figure 4 shows the extracted values of hardness and Young's modulus from load-displacement curves ( Figure  S4) for 1200 Å-thick Ti 2 AlC thin film. The substrate independent hardness and reduced elastic modulus values of 4.8 ± 2 and 182.5 ± 10 GPa, respectively, were obtained for the Ti 2 AlC thin film. These hardness values are in a good agreement with those for bulk Ti 2 AlC reported by Barsoum et al. [15], while the elastic modulus of 280.4 ± 10 GPa (calculated taking Poisson's ratio of 0.19) is slightly lower than theoretical and experimental values (see Table S1). The hardness and modulus values increase with increasing indentation depth approaching much harder TiC layer and the substrate. Figure 4(a), inset shows the residual impression after  nanoindentation under maximum load, revealing a pileup around the indent, which forms due to typical deformation mechanism for MAX phases proceeding through cohesive delamination of the basal planes and kink band formation (Figure 4(b), inset). The averaged hardness (Figure 4(c)) and reduced elastic modulus (Figure 4(d)) for depth 10-20 nm are presented as histograms with kernel fitting [16].
The nanowear test was performed over the area of 5 × 5 μm of the Ti 2 AlC thin film ( Figure 5(a-c)). The nanowear volume increases with the applied load from for the Ti 2 AlC thin film. The same trend was observed for the wear depth from ∼ 2 nm at 100 μN load to about 15 nm at 500 μN. The removed surface parts (debris) due to delamination and kinking are well-visible at the edges of the wear track after tests at higher loads. This process in MAX phases, according to Emmerlich et al. [17], associated with basal plane gliding and slipping when the load exceeds the deformation energies. The nanowear volume and wear depth of the Ti 2 AlC thin film are in the range of the values reported for ultra-low nanowear nanocomposite films and much harder thin films (see Table S5).
The low nanowear of the film can be associated with a low coefficient of friction of the Ti 2 AlC (Figure 5(d-f)) due to the layered structure with relatively weak metallic bonding. An abrupt increase of the coefficient of friction to about 0.3 at 500 μN can be due to the pushing of the debris along the scratch and their subsequent penetration beneath the indenter, which are well-visible at the top end of the scratch. Importantly, as most functional properties, tribological properties of MAX phases are known to be anisotropic [18], and therefore the measured friction coefficient at micro-scale is related to the c-plane oriented Ti 2 AlC. These outcomes point on the outstanding micro-scale tribological properties of the Ti 2 AlC film at micro-Newton loads.

Conclusions
In summary, herein we demonstrated that low-rate layer-by-layer electron-beam physical vapor deposition method is suitable for the growth of epitaxial singlecrystalline Ti 2 AlC MAX phase thin films. Utilizing three independent sources, epitaxial Ti 2 AlC thin films were deposited at a relatively low temperature of 700°C in comparison to other PVD methods due to properly adjusting deposition rate and layer-by-layer deposition. The reported approach offers prospects for epitaxial growth of other MAX phases and Ti n + 1 AlC n MAX phases with higher stoichiometry and related composites by PVD methods. The micro-scale tribological characterization revealed ultra-low nanowear of the MAX phases due to low friction coefficient that can ultimately contribute to improved performance in a wide range of applications where low friction coefficient and good wear resistance is required at micro-Newton loads.