Computational Design and Experimental Development of Functional Thin Films

Wednesday, May 1, 2013 2:10 PM in Room Royal Palm 1-3

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2:10 PM B7-2-1 On the Structure and Growth of Reactive Magnetron Sputtered Ta2O5
Robert Hollerweger (Christian Doppler Laboratory for Application Oriented Coating Development at Montanuniversitat Leoben and Vienna University of Technology, Austria); Matthias Bartosik (Vienna University of Technology, Austria); Mirjam Arndt, Richard Rachbauer (OC Oerlikon Balzers AG, Liechtenstein); Peter Polcik (PLANSEE Composite Materials GmbH, Germany); Jörg Paulitsch (Vienna University of Technology, Austria); David Holec (Montanuniversität Leoben, Austria); Paul Mayrhofer (Vienna University of Technology, Austria)

Especially due to its high relative permittivity and refraction index, Ta2O5 is frequently being discussed for electronically and optically applications. These properties are very sensitive on the atomic arrangement within the crystal structure as even slight variations could cause changes in the band structure resulting in e.g. conductivity or loss of transmittance. However, the arrangement of oxygen atoms in the a-b plane of the orthorhombic structured Ta2O5, but also the structure itself, can be controlled by varying the deposition conditions during reactive magnetron sputtering. Nevertheless, the formation energy of the amorphous and crystalline Ta2O5 is similar, which makes it challenging to synthesize crystalline Ta2O5 at moderate temperatures. Therefore, Ta2O5 films were deposited by reactive DC magnetron sputtering at 500°C using O2/Ar flow rates (Γ) ranging from Γ = 50 to 100%. Our investigations indicate an amorphous interlayer, which decreases with increasing Γ before the crystalline oxide phase is formed. Simultaneously, the deposition rate decreases with increasing Γ from about 120 to 10 nm/min and the chemical composition ratio O/Ta increases from 2.33 to 2.5, respectively. This trend was also confirmed by nano-indentation as the hardness and Young’s modulus increase from ~14 to 16 GPa and ~180 to 235 GPa, respectively. Nano-beam measurements across the whole substoichiometric Ta2O5 film thickness of 15 µm exhibit a highly texturized 110 / 200 growth and strained a and b, but hardly affected c lattice parameters.

Driven by these experimental results, a new orthorhombic pentoxide crystal structure was designed and calculated by the DFT-GGA ab-initio method yielding lattice parameters of a = 6.32, b = 3.73, and c = 3.96 Å, a band gap Eg of at least 2.5 eV, and energy of formation Ef = –3.158 eV/atom. Moreover, when oxygen is removed to reach an O/Ta ratio of 2.33 as obtained by the experiments, the energy of formation remains nearly constant (Ef = –3.134 eV/atom). These calculations support the here proposed structure for tantalum pentoxide, which is formed under non-equilibrium conditions of sputtering.

2:30 PM B7-2-2 Probing Temperature-induced Ordering in Ti0.33Al0.67N Coatings
Cecilia Århammar (Sandvik Coromant R&D S-126 80 Stockholm, Sweden); Jose Endrino (Instituto Abengoa Research S. L., Spain); Muhammad Ramzan (Uppsala University, Sweden); David Horwat (Université de Lorraine, Institut Jean Lamour, CNRS, Institut Jean Lamour, UMR 7198, Nancy, F-54000, France Division of Molecular and Condensed Matter, France); Andreas Blomqvist (Sandvik Coromant R&D, Sweden); Jan-Erik Rubensson, Rajeev Ahuja (Uppsala University, Sweden)
Cubic TiAlN is one of the most common coatings used as a protective layer on cutting tools. The TiAlN structure and its evolution with temperature have been under careful study by techniques such as X-Ray Diffraction (XRD), Near Edge X-ray Absorption Fine-structure (NEXAFS) [1], Transmission Electron Microscopy (TEM), first principles and phase field modeling. Previous modeling has provided estimations of the thermodynamical, kinetic and mechanical driving force for spinodal decomposition of TiAlN. In this paper we instead interpret the measured spectral features directly by spectra calculated from first principles. The ordering of supersaturated cubic titanium aluminum nitride (c-Ti 0.33Al0.67N) coatings is probed by its electronic structure from room temperature up to and above the point of spinodal decomposition, using X-ray Emission Spectroscopy (XES), NEXAFS, along with Density Functional Theory (DFT) and the G0W0-approach. A simple ordered c-TiAlN model structure along with a Special Quasi random Structure (SQS) were used to correlate measured spectral features to local changes in partial and orbital projected density of states. The N K edge spectra along with the calculated N p density of states suggest that non-bonding Ti t2g-states, as well as anti-bonding Ti eg-states are of maximum intensity at room temperature as the random distribution of Al and Ti on the metal lattice is still kept. This proves the strong configurational sensitivity of the electronic structure of TiAlN that was previously suggested by Alling et al. [2]. As temperature is raised, ordering into cubic Al-rich and Ti-rich domains decreases the unfavorable antibonding states. The N p-Ti eg, and to some extent the N p-Ti t2g overlap remain almost constant with temperature, whereas hybridisation between N p and Al p increases considerably. A similar trend was found at the Al and Ti K-edges. This observation is in agreement with previous phase field simulations [3]. The N p spectra calculated from DFT were in good agreement with spectra calculated by the G0W0-approach. These results should be of use in the in-depth understanding of structural changes in TiAlN and provide proof from the electronic structure to experimental tests reported in the past.

[1] J. L. Endrino, C. Århammar, A. Gutierréz, R. Gago, D. Horwat, L. Soriano, G. Fox-Rabinovich, D. Martín y Marco, J. Guo, J-E. Rubensson, J. Andersson, Acta Materialia 59 (16), 6287-6296 (2011)

[2] B. Alling, Dissertation No. 1334, Department of Physics, Chemistry and Biology, Linköping University, Sweden.

[3] J. Ullbrand, LIU-TEK-LIC-2012:30, Department of Physics, Chemistry and Biology, Linköping University, Sweden.

2:50 PM B7-2-3 Lattice Ordering Effects on Toughness Enhancement in Transition Metal Nitride Thin Films
Davide Sangiovanni, Daniel Edström, Valeriu Chirita, Lars Hultman (Linköping University, IFM, Thin Film Physics Division, Sweden)

Enhanced toughness in hard and superhard thin films is a primary requirement for present day ceramic hard coatings, known to be prone to brittle failure during in-use conditions, in modern applications. In our previous Density Functional Theory (DFT) investigations, we have predicted significant improvements in the hardness/ductility ratio of several pseudobinary B1 NaCl structure transition-metal nitride alloys, obtained by alloying TiN or VN with NbN, TaN, MoN and WN [1, 2]. The initial calculations, which were carried out on model, highly ordered configurations with Cu-Pt ordering on the cation sublattice, reveal that the electronic mechanism responsible for toughness enhancement stems from the high valence electron concentration (VEC) of these alloys, which upon shearing, leads to the formation of alternating layers of high and low charge density oriented orthogonal to the applied stress, and ultimately allows a selective response to tetragonal and trigonal deformations.

Recently, these results have been validated experimentally [3]. Single-crystal V0.5Mo0.5N/MgO(001) alloys, grown by dual-target reactive magnetron sputtering together with VN/MgO(001) and TiN/MgO(001) reference samples, exhibit hardness >50% higher than that of VN, and while nanoindented VN and TiN reference samples suffer from severe cracking, the V0.5Mo0.5N films do not crack. However, as there is no evidence of Cu-Pt ordering in the synthesized V0.5Mo0.5N films, here we present new DFT results, which address the issue of lattice ordering effects on the mechanical properties of these pseudobinary alloys. Our investigations concentrate on V0.5Mo0.5N, V0.5W0.5N, Ti0.5Mo0.5N and Ti0.5W0.5N alloys obtained by alloying TiN and VN with WN and MoN, which are all predicted to have significantly enhanced toughness. Our calculations, carried out for structures with increasing levels of disorder, reveal that while the degree of electronic structure layering, i.e. the formation of alternating layers of high and low charge density upon shearing, becomes less pronounced in disordered configurations, the overall VEC effect is not affected. The essential feature in the disordered alloys, as initially predicted for highly ordered configurations, remains the increased occupancy of electronic d-t2g metallic states, which allows the selective response to tensile/shearing stresses, and explains the enhanced toughness confirmed experimentally for V0.5Mo0.5N films.

[1] D. G. Sangiovanni et. al. Phys. Rev. B 81 (2010) 104107.

[2] D. G. Sangiovanni et. al. Acta Mater. 59 (2011) 2121.

[3] H. Kindlund et. al, submitted to Nature Materials.
3:10 PM B7-2-4 Oxygen-deficient Zirconia Thin Films Synthesized by Reactive Magnetron Sputtering
Stephanos Konstantinidis, Giles Geumez, Tanguy Van Regemorter, Jérôme Cornil, Rony Snyders (University of Mons, Belgium)

Zirconium oxide (ZrOx) thin films were synthesized by DC reactive magnetron sputtering. A 3 cm- in diameter zirconium target was sputtered in Ar/O2 atmospheres (10mTorr). During the deposition process, the oxygen flow was controlled by means of a Plasma Emission Monitoring (PEM) device (speedflo, Gencoa Ltd.) by monitoring the Zr I lines. PEM allowed growing ZrOx films inside the so – called metal-oxide transition.

X-Ray Photoelectron Spectroscopy data revealed that the films synthesized in the transition region are oxygen-deficient: ~10% of oxygen vacancies are incorporated in these films. The X-Ray Diffraction (XRD) patterns of these films exhibit reflections related to the high-temperature tetragonal and/or cubic phase of ZrO2. In contrast, XRD patterns of the stoichiometric films deposited in the fully oxidized regime present reflections emanating from the low-temperature stable monoclinic phase. Our data reveal that the film chemistry, especially the incorporation of oxygen vacancies, is a key feature for controlling the phase constitution of zirconia thin films. Quantum-chemical calculations based on the Density Functional Theory method are consistent with these experimental observations. As oxygen vacancies are introduced in the ZrO2 cell, the cubic phase is stabilized (by 20 meV/at. with ~10% of O vacancies in the cell) at the expense of the monoclinic structure. Finally, the oxygen-deficient 100 nm-thick films were annealed in air for 2 hours. It was found that the tetragonal/cubic phase was preserved up to 600°C.