AlN Thin Films Grown on MgO(111) Substrates - American Chemical

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Charge Distribution in the Single Crystalline Ti2AlN Thin Films Grown on MgO(111) Substrates Zheng Zhang,† Yanguang Nie,‡ Lu Shen,† Jianwei Chai,† Jisheng Pan,† Lai Mun Wong,† Michael B. Sullivan,‡ Hongmei Jin,*,‡ and Shi Jie Wang*,† †

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, 117602, Singapore ‡ Institute of High Performance Computing, A*STAR (Agency for Science, Technology and Research), 1 Fusionopolis Way, Connexis, 138632, Singapore. ABSTRACT: Single crystalline Ti2AlN thin films have been grown on MgO(111) substrates at 750 °C using DC magnetron sputtering from a Ti2Al alloy target in a mixed N2/Ar plasma. X-ray diffraction, atomic force microscopy, and transmission electron microscopy demonstrate layered growth of a Ti2AlN{0002} thin film on the MgO(111) substrate. X-ray photoelectron spectroscopy detects a TiN-like conducting nature of the Ti2AlN thin film. However, the binding energies (BEs) of Ti 2p3/2 and Al 2p have shifted to 454.7 ± 0.2 and 72.3 ± 0.2 eV, respectively, lower than their corresponding values in nitrides (TiN and AlN). The BE of Al 2p is even lower than that in metallic Al. The unusual shifts are attributed to charge transfer from Ti to Al as shown by the density functional theory calculations.

I. INTRODUCTION Ti2AlN is a member of the MAX phase, which is a family of about 60 nanolaminate ternary compounds with shared formula of Mn+1AXn (M, an early transition metal; A, an element in groups IIA and IVA; X, N and/or C, n = 1−3).1 After their initial discovery in the 1960s, this group of materials has regained significant attention recently owing to their unique combination of ceramic properties (high melting point and high temperature oxidation resistance) and metallic properties (good electrical and thermal conductivity, high ductility, easy machinability, superior thermal shock resistance, and damage tolerance).2 These properties lead to a variety of applications including high-temperature protective coatings on turbine blades,3 structural components and fuel pellet coatings for next generation nuclear power plants,4 and a candidate as Ohmic contact to n-GaN.5 The distinctive combination of these properties is attributed to the layered hexagonal structures of MAX consisting of sheets of distorted edge sharing [M6X] octahedrons (with C and/or N as X filling the octahedral sites) interleaved by layers of A-group elements.6 The strong covalent-ionic nature of the M−X bonds is believed to account for the ceramic properties, while the weak metallic M−A bonds are the basis for the metallic properties.1,7 Bulk polycrystalline Ti2AlN has been synthesized by hot isostatic pressing of Ti and AlN powders at 1600 °C for 4 h under a high pressure (40 MPa).8 It has not been until recently that single crystalline Ti2AlN thin films have been grown epitaxially on single crystal MgO and Al2O3 substrates through ultrahigh-vacuum (UHV) DC magnetron sputtering.7,9−12 The epitaxial growth of Ti2AlN thin films enables investigation of © XXXX American Chemical Society

the intrinsic material properties of Ti2AlN without the influence of grain boundary and grain size. However, characterization of these epitaxial Ti2AlN thin films has been predominantly carried out ex situ outside the growth chamber and focused on their bulk properties, i.e., crystal structure, resistance, and mechanical properties.6−12 Little work has been done to characterize the Ti2AlN thin films in situ (without oxidizing the films in air),7,10,11 especially in the aspects of surface chemical states, which are essential to understand their electronic properties and the subsequent oxidation behavior. Therefore, in this work, the chemical states of Ti, Al, and N in the single crystalline Ti2AlN thin films grown on the MgO(111) substrate are probed in situ by X-ray photoelectron spectroscopy (XPS) and correlated with their electronic structures calculated by the density functional theory (DFT).

II. EXPERIMENTAL METHODS Ti2AlN thin films were deposited at 750 C on MgO(111) substrates, which were cleaned ultrasonically in acetone, isopropanol, and deionized water before being introduced in the growth chamber with a base pressure of 5.0 × 10−9 mbar. The MgO substrates were thermally degassed at 800 °C for 1 h and then cleaned by a N2 atomic source for 30 min to remove surface carbonaceous contamination. A 3 in. Ti2Al alloy target (99.99% purity) was used for deposition using DC magnetron sputtering at a power of 90 W. The partial pressures of Ar and Received: March 8, 2013 Revised: May 2, 2013

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N2 during deposition were 3.2 × 10−3 and 6.0 × 10−5 mbar, respectively. After deposition, the samples were transferred to the analysis chamber of VG ESCALAB 220i-XL XPS in situ without exposing to air. A monochromatic Al Kα (1486.6 eV) X-ray with a diameter of 700 μm is employed, while the photoelectrons are collected at a normal takeoff angle (with respect to surface plane). The electron analyzer was calibrated with polycrystalline gold, silver, and copper standard samples by setting the Au 4f7/2, Ag 3d5/2, and Cu 2p3/2 peaks at binding energies of 83.96 ± 0.02, 368.21 ± 0.02, and 932.62 ± 0.02 eV. After one hour deposition, the thicknesses of Ti2AlN thin films were measured to be around 300 nm by a surface profiler. The crystalline structures of thin films were investigated using a PANalytical X’pert PRO X-ray diffractometer (XRD) operated at a voltage of 40 kV and a current of 40 mA (Cu Kα X-ray, λ = 1.54 Å). Microstructure observation and selected area electron diffraction (SAED) pattern analysis were carried out using a JEOL 2100 transmission electron microscope (TEM) operated at 200 kV. The surface morphology of the films was examined by Digital Instrument Nanoscope IV atomic force microscopy (AFM). Mechanical properties of the films were determined using a MTS Nano Indenter XP using a continuous stiffness measurement (CSM) technique with a Berkovich indenter (three-faced pyramid diamond with a 20 nm radius of curvature at the apex). During the indentation test, the indenter was pressed into the composite film with a constant strain rate (0.05 s−1), from the sample surface to 300 nm deep into the sample. A constant strain rate was chosen to load on the samples to avoid a strainhardening effect on the measurements. The method allows stiffness of the film to be recorded continuously along indentation depth. As such, continuous modulus and hardness profiles could be derived and plotted against displacement into the surface.13 The electronic structure of Ti2AlN was calculated using CASTEP which was implemented in Materials Studio from Accelrys simulation package.14 The exchange and correlation energy is treated via the generalized gradient approximation (GGA) with Perdew, Burke, and Ernzerhof parametrization. A kinetic energy cutoff of 550 eV is used with irreducible k points generated according to the Monkhorst− Pack scheme in a 15 × 15 × 6 k-point grid and ultrasoft Vanderbilt-type pseudopotentials for Ti, N, and Al. The lattice constant and ions are relaxed using the Broyden−Fletcher− Goldfarb−Shanno (BFGS) minimization scheme. The convergence criteria for energy, displacement, stress, and force are set as 5 × 10−6 eV/atom, 5.0 × 10−4 Å, 0.02 GPa, and 0.01 eV/ Å, respectively.

Figure 1. (a) Out-of-plane XRD Gonio spectrum of single crystalline Ti2AlN thin film grown on a MgO(111) substrate. (b) and (c) are the pole figures of Ti2AlN {1013̅ } and {1011̅ } planes, while (d) shows the pole figure of the MgO{111} planes. (e) In-plane XRD Phi (φ) scan shows single crystalline Ti2AlN thin film on the MgO(111) substrate.

MgO(111) substrate. The orientation relationship of Ti2AlN(0002) [12̅ 1̅0] ∥ MgO(111) [011] can be determined from the overlapping peak position in φ-angle between the film and substrate, which is consistent with previous reports.9,12 The c lattice parameter can be calculated to be 1.35 nm from the 2θ value, which is very close with c = 1.3614 nm from bulk Ti2AlN.1 Figure 2(a) shows an overview of Ti2AlN film after two hours of deposition on the MgO(111) substrate. It can be seen that Ti2AlN film is dense with layered texture, while the interface between Ti2AlN and MgO is sharp and flat. Selected area electron diffraction (SAED) pattern taken from the area marked by the green square clearly demonstrates the single crystalline nature of the Ti2AlN film (Figure 2(b)). The highresolution TEM image from the same area presented in Figure 2(c) clearly shows fine horizontal lines, which are parallel to the MgO(111) surface and are attributed to the laminar nature of the Ti2AlN phase. This layered feature confirms exclusive Ti2AlN(000l) plane growth derived from XRD results. Morphology shown by AFM demonstrates layer-by-layer growth of Ti2AlN thin film on the surface with a hint of hexagonal shape (Figure 3(a)). Flat terraces have a width of 33−160 nm. The step height is dominated by a value of 1.4 ±

III. RESULTS AND DISCUSSION A typical out-of-plane Gonio scan of the Ti2AlN film grown on the MgO(111) substrate is shown in Figure 1(a), which reveals exclusive Ti2AlN(000l) planes (l = 2, 4, 6, 10, and 12) besides MgO(111) and (222) planes. No other ternary or binary nitrides or intermetallic phases were detected. Figures 1(b), (c), and (d) display the pole figure scans of Ti2AlN{101̅3} and {1011̅ } planes and MgO{111} planes, respectively. Six equally distributed poles can be seen for Ti2AlN thin film projected along {101̅3} and {101̅1} planes, which suggest the single crystalline nature of Ti2AlN thin film with 6-fold symmetry. Inplane φ-scans along Ti2AlN{1013̅ } and {1011̅ } planes and MgO{111} planes were performed as shown in Figure 1(e). This shows well-aligned in-plane epitaxy between 6-fold symmetry of Ti2AlN(0002) film and 3-fold symmetry of the B

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Figure 2. (a) Cross-section TEM image of 1 μm Ti2AlN(0002) thin film grown on a MgO(111) substrate; (b) selected area electron diffraction (SAED) pattern along the [11̅1̅0] zone axis, and (c) high-resolution TEM image from the area marked by the green square in (a).

Figure 3. (a) 2 μm × 2 μm AFM image of the Ti2AlN(0002) thin film and line profiles across (b) one single unit cell and (c) two half-unit cells.

Figure 4. Profiles of (a) hardness (H) and (b) Young’s modulus (E) of the Ti2AlN(0002) thin film with respect to indentation depth into surface measured by nanoindentation. A bare MgO(111) wafer is also measured for reference. C

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Figure 5. High-resolution XPS spectra of (a) Ti 2p, (b) Al 2p, and (c) N 1s as well as (d) the distribution of binding energies of Ti 2p3/2, Al 2p, and N 1s across ten samples.

0.2 nm (Figure 3(b)), although a value of 0.7 ± 0.2 nm can be observed occasionally (Figure 3(c)). These two values interestingly correspond to the heights of one unit cell and half-unit cell, respectively, suggesting lateral layer growth mode by the prevailing propagation of one unit cell on the surface. Two half-unit cells can grow on top of each other to form a complete unit cell and continue the lateral growth eventually. Such a layered hexagonal surface morphology has been similarly observed from another single crystalline MAX phase, Ti3SiC2.15 The AFM results thus tally with the XRD and TEM observation of basal plane growth of single crystalline Ti2AlN(0002) thin films, which possesses a hexagonal unit cell with a lattice constant of 1.3614 nm in the c axis.1 The hardness (H) and Young’s modulus (E) of the Ti2AlN film are further evaluated by a nanoindentation test with a reference measured from a bare MgO(111) wafer. Ten indentations 100 μm apart from each other were performed on both sample surfaces. The typical H and E profiles with respect to indentation depth are shown in Figure 4. Both H and E profiles of the Ti2AlN thin film (black square curves) are found to deviate toward the profiles of MgO wafer (red circle curves) with increasing indentation depth, indicating an increasing influence from the substrate. The stabilized hardness and modulus values at deeper bulk region (marked by red

arrows) were reported as properties of the MgO(111) wafer. On the other hand, the highest hardness and modulus values slightly below the surface (marked by blue arrows) were reported as properties of Ti2AlN film. The resulting hardness and Young’s modulus of the MgO(111) wafer are determined to be 9.9 ± 0.1 and 297.0 ± 4.7 GPa, close to the reported values of 9.0 ± 0.3 and 307 ± 15 GPa.16 The hardness and Young’s modulus of the Ti2AlN film are measured to be 26.5 ± 1.8 and 364.8 ± 20.8 GPa, which appear at the same order but greater than the reported values of 16.1 ± 1 and 270 ± 20 GPa in the previous work done on single crystalline thin film.9 It is worth noting that only three maximum loads were chosen in the previous study,9 which might miss the peak values of hardness and modulus as observed in the current study by the continuous stiffness measurement (CSM) method. The hardness and modulus reported in this study are taken as the peak values averaged in the depth range of 18−20 nm and 15− 17 nm, respectively. In this range, minimal substrate contribution is found, and hence the values represent the intrinsic film properties. The results from XRD, TEM, AFM, and nanoindentation therefore confirm the growth of single crystalline Ti2AlN(0002) thin films, which are further subjected to in situ XPS surface analysis. D

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Figure 6. DFT calculations of (a) partial density of state (DOS) of Ti, Al, and N and total DOS together with the valence band spectrum from XPS and (b) charge density difference on the Ti2AlN(112̅0) plane after subtraction of corresponding atomic electron density with red and blue colors representing electron gain and loss, respectively.

while Ti transfers most of the electron charge to N (similar to what it does in TiN), it also transfers some of the electron charge to Al. To validate this postulation, DFT calculations were thus carried out for a Ti2AlN cell consisting of 4 Ti, 2 Al, and 2 N atoms. It can be seen from the total density of states (DOS) in Figure 6(a) that there is no band gap but prominent DOS at EF, which can be treated as a local minimum in a pseudogap between two adjacent peaks. The significant states in EF suggest the conductive nature of Ti2AlN, and its conductivity is mainly contributed by the Ti 3d states. The adjacent peak located between −0.4 and −1.6 eV besides EF is a result of Ti 3d−Al 3p hybridization, while the peak centered at −6.0 eV is mainly contributed from Ti 3d and N 2p states. The deeper lying position of the Ti 3d−N 2p peak (−6.0 eV) than that of the Ti 3d−Al 3p peak (−0.4−1.6 eV) suggests a much stronger Ti−N bond compared with the Ti−Al bond. The coexistence of the strong Ti−N bond and weak Ti−Al bond thus leads to anisotropic nature of Ti2AlN. The peak at −16.7 eV is largely attributed to the N 2s state with a minor contribution from Ti 4s and Ti 3d states. Clearly the valence band (VB) spectrum from XPS resembles the total DOS from DFT calculation closely (Figure 6(a)) and agrees with the previous reports.23,24 From the electron density difference distribution on the Ti2AlN(112̅0) plane in Figure 6(b), an electron transferring from the Ti atom to the N atom and the anisotropic electron density difference distribution around the Ti atom can be observed clearly. The strong electron gain and loss for the N and Ti atom, respectively, suggest the presence of covalent bonding between Ti and N atoms. The electron gain or loss for the Al atom is not explicit from Figure 6(b) because electrons on different orbitals of Al atoms are redistributed and a red triangle domain (representing electron gain) is located in between Ti, Al, and N atoms, implying a weak bond in Al−Ti or Al−N. As a result, we use Bader charge analysis25 and found

The XPS spectra of Ti 2p, Al 2p, and N 1s of the Ti2AlN(0002) thin films were taken immediately after growth in situ without exposing to air and are shown in Figure 5. It can be seen that only a single peak is detected in Ti 2p3/2, Ti 2p1/2, Al 2p, and N 1s spectra, indicating a sole chemical state for Ti, Al, and N. No carbon contamination except a small amount of oxygen of less than 4.0% is detected. The little oxygen in the film is likely from the Ti2Al alloy target, and it may exercise a negligible effect on the formation of the Ti2AlN MAX phase, as reported earlier.17 Furthermore, the binding energies (BEs) of Ti 2p3/2, Al 2p, and N 1s are determined to be 454.7 ± 0.2, 72.3 ± 0.2, and 397.4 ± 0.2 eV, respectively, far away from the values representing their oxides at 458.8, 74.4, and >402 eV.18 As Ti2AlN is metallic and conducting in nature, the charging shift on Ti 2 AlN film after photoelectron emission is insignificant, and no further BE correction is performed. It is worth pointing out that the current BE of Ti 2p3/2 in Ti2AlN is close to but lower than that in TiN (455.1 eV), although the BE of N 1s in Ti2AlN is the same as that in TiN (397.4 eV).19 These two BE values imply TiN-like bond formation on the surface. More interestingly, the BE of Al 2p is lower than those in AlN (74.4 eV) and even metallic Al (72.9 eV) but higher than that in TiAl alloy (71.4 eV).18,20 The distribution of Ti 2p3/2, Al 2p, and N 1s over 10 samples can be seen clearly in Figure 5(d) with reasonable repeatability. A similar shift of Al 2p has been observed previously in Ti4AlN3, Ti3AlC2, and Nb2AlC phases ex situ (with the presence of oxides) and explained by screening of Al from M6X blocks.21,22 The shift in BE is usually an indication of charge transfer with an increase in BE suggesting loss of electron density and vice versa. The smaller BE of Ti 2p in Ti2AlN than in TiN implies that Ti does not completely transfer all the valence electrons to N in Ti2AlN, as it does in TiN. The shift of BE of Al 2p in Ti2AlN to a lower value from that in metallic Al indicates that Al is gaining electron density. Therefore, one possible explanation is that, E

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Behavior of Cr2AlC Coating on Ti6242 Alloy. Surf. Coat. Technol. 2010, 204, 2343−2352. (4) Hoffman, E. N.; Vinson, D. W.; Sindelar, R. L.; Tallman, D. J.; Kohse, G.; Barsoum, M. W. MAX Phase Carbides and Nitrides: Properties for Future Nuclear Power Plant In-Core Applications and Neutron Transmutation Analysis. Nucl. Eng. Des. 2012, 244, 17−24. (5) Borysiewicz, M. A.; Kaminska, E.; Piotrowska, A.; Pasternak, I.; Jakiela, R.; Dynowska, E. Ti-Al-N MAX Phase, a Candidate for Ohmic Contacts to n-GaN. Acta Phys. Pol., A 2008, 114, 1061−1066. (6) Farber, L.; Levin, I.; Barsoum, M. W.; El-Raghy, T.; Tzenov, T. High-Resolution Transmission Electron Microscopy of Some Tin+1AXn Compounds (n=1, 2; AAl or Si; XC or N). J. Appl. Phys. 1999, 86, 2540−2543. (7) Beckers, M.; Schell, N.; Martins, R. M. S.; Mucklich, A.; Moller, W.; Hultman, L. Nucleation and Growth of Ti2AlN Thin Films Deposited by Reactive Magnetron Sputtering Onto MgO(111). J. Appl. Phys. 2007, 102, 074916. (8) Barsoum, M. W.; Brodkin, D.; El-Raghy, T. Layered Machinable Ceramics for High Temperature Applications. Scr. Mater. 1997, 36, 535−541. (9) Joelsson, T.; Horling, A.; Birch, J.; Hultman, L. Single-Crystal Ti2AlN Thin Films. Appl. Phys. Lett. 2005, 86, 111913. (10) Beckers, M.; Schell, N.; Martins, R. M. S.; Mucklich, A.; Moller, W. Phase Stability of Epitaxially Grown Ti2AlN Thin Films. Appl. Phys. Lett. 2006, 89, 074101. (11) Beckers, M.; Schell, N.; Martins, R. M. S.; Mucklich, A.; Moller, W.; Hultman, L. Microstructure and Nonbasal-Plane Growth of Epitaxial Ti2AlN Thin Films. J. Appl. Phys. 2006, 99, 034902. (12) Joelsson, T.; Flink, A.; Birch, J.; Hultman, L. Deposition of Single-Crystal Ti2AlN Thin Films by Reactive Magnetron Sputtering from a 2Ti: Al Compound Target. J. Appl. Phys. 2007, 102, 074918. (13) Lucas, B. N.; Oliver, W. C.; Swindeman, J. E. The Dynamics of Frequency-Specific, Depth-Sensing Indentation Testing. Mater. Res. Soc. Symp. Proc. 1998, 522, 3−14. (14) Materials Studio; Accelrys Software Inc.: San Diego, CA, 2001− 2011. (15) Emmerlich, J.; Hogberg, H.; Sasvari, S.; Persson, P. O. A.; Hultman, L.; Palmquist, J. P.; Jansson, U.; Molina-Aldareguia, J. M.; Czigany, Z. Growth of Ti3SiC2 Thin Films by Elemental Target Magnetron Sputtering. J. Appl. Phys. 2004, 96, 4817−4826. (16) Ljungcrantz, H.; Oden, M.; Hultman, L.; Greene, J. E.; Sundgren, J. E. Nanoindentation Studies of Single-Crystal (001)-, (011)-, and (111)-Oriented TiN Layers on MgO. J. Appl. Phys. 1996, 80, 6725−6733. (17) Zhang, T.; Myoung, H. B.; Shin, D. W.; Kim, K. H. Syntheses and Properties of Ti2AlN MAX-phase Films. J. Ceram. Process. Res. 2012, 13, S149−S153. (18) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data; Physical Electronics, Inc.: 6509 Flying Cloud Drive, Eden Prairie, Minnesota 55344, United States of America, 1995. (19) Bertoti, I. Characterization of Nitride Coatings by XPS. Surf. Coat. Technol. 2001, 151, 194−203. (20) Mencer, D. E.; Hess, T. R.; Mebrahtu, T.; Cocke, D. L.; Naugle, D. G. Surface Reactivity of Titanium Aluminum-Alloys - Ti3Al, TiAl, and TiAl3. J. Vac. Sci. Technol. A 1991, 9, 1610−1615. (21) Myhra, S.; Crossley, J. A. A.; Barsoum, M. W. Crystal-Chemistry of the Ti3AlC2 and Ti4AlN3 Layered Carbide/Nitride Phases Characterization by XPS. J. Phys. Chem. Solids 2001, 62, 811−817. (22) Barsoum, M. W.; Crossley, A.; Myhra, S. Crystal-Chemistry from XPS Analysis of Carbide-Derived Mn+1AXn (n=1) NanoLaminate Compounds. J. Phys. Chem. Solids 2002, 63, 2063−2068. (23) Zhou, Y. C.; Sun, Z. M. Electronic Structure and Bonding Properties of Layered Machinable Ti2AlC and Ti2AlN Ceramics. Phys. Rev. B 2000, 61, 12570−12573. (24) Djedid, A.; Mecabih, S.; Abbes, O.; Abbar, B. Theoretical Investigations of Structural, Electronic and Thermal Properties of Ti2AlX(XC,N). Phys. B 2009, 404, 3475−3482.

that in bulk Ti2AlN each Ti atom loses 1.23 electrons, while each N and Al atom gain −1.75 and −0.71 electrons, respectively. The larger electron gain in N compared to Al agrees with a larger negative shift in BE of N 1s than Al 2p (from their respective elemental states) as detected by XPS. As each Al atom gains 0.71 electrons from Ti, core electrons in the Al atom are screened by the extra charge, and the BE of the Al 2p peak in Ti2AlN is expected to be lower than that in metallic Al atoms without additional screening. This is indeed consistent with in situ XPS measurement and explains the origin of the unusual negative BE shift of the Al 2p peak in Ti2AlN. It is worth noting that the same three elements in the Ti2AlN MAX phase can also form Ti1−xAlxN cubic phase material when adding Al into the TiN matrix.26 It is essentially a mixture of TiN and AlN where both Ti and Al transfer charges to N. The resulting BE of Ti 2p3/2, Al 2p, and N 1s is located at 454.8, 74.5, and 397.0 eV, respectively, representing the chemical states of TiN and AlN. On the other hand, the BEs of Ti 2p3/2 and Al 2p in titanium−aluminum alloys (Ti3Al, TiAl, and TiAl3) are in the range of 453.3−453.8 and 71.4−71.5 eV, respectively.20 Hence, the characteristic BE of Ti 2p3/2 and Al 2p at 454.7 and 72.3 eV as well as the unique shape of VB from our XPS measurement can serve as fingerprints for the Ti2AlN MAX phase.

IV. CONCLUSION In summary, single crystalline Ti2AlN thin films have been fabricated using DC magnetron sputtering from the Ti2Al alloy in Ar/N2 plasma. Out-of-plane Gonio scan and in-plane φ scan from XRD and selected area electron diffraction pattern from TEM confirm the growth of single crystalline Ti2AlN(0002) on MgO(111) substrates. A cross-section micrograph from highresolution TEM reveals layer-by-layer basal plane growth, while surface morphology from AFM displays hexagonal laminar morphology with a c lattice parameter of 1.4 nm. The films have a hardness and Young’s modulus of 26.5 ± 1.8 and 364.8 ± 20.8 GPa, respectively. XPS reveals the nitride-like nature of the film with good conductivity. In addition, an unusual shift of Al 2p to 72.3 ± 0.2 eV is discovered, which is explained by charge transfer from Ti to Al and additional screening of Al by Ti shown by DFT calculations. This shift can therefore serve as a fingerprint for the Ti2AlN MAX phase. These results therefore show strong evidence and a better understanding of Ti2AlN material for promising applications such as wearresistant coatings and metallic contact layers.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +65-64191332. Fax: +65-64632536. E-mail address: [email protected]. Tel.: +65-68748184. Fax: +6567744657. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Barsoum, M. W. The Mn+1AXn Phases: A New Class of Solids; Thermodynamically Stable Nanolaminates. Prog. Solid State Chem. 2000, 28, 201−281. (2) Eklund, P.; Beckers, M.; Jansson, U.; Hogberg, H.; Hultman, L. The Mn+1AXn Phases: Materials Science and Thin-Film Processing. Thin Solid Films 2010, 518, 1851−1878. (3) Wang, Q. M.; Renteria, A. F.; Schroeter, O.; Mykhaylonka, R.; Leyens, C.; Garkas, W.; Baben, M. tFabrication and Oxidation F

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(25) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Clarendon Press: Oxford, 1990. (26) Shum, P. W.; Zhou, Z. F.; Li, K. Y.; Shen, Y. G. XPS, AFM and Nanoindentation Studies of Ti1‑xAlxN Films Synthesized by Reactive Unbalanced Magnetron Sputtering. Mater. Sci. Eng., B 2003, 100, 204− 213.

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