ICMCTF2013 Session B7-2: Computational Design and Experimental Development of Functional Thin Films

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 E_g of at least 2.5 eV, and energy of formation E_f = –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 (E_f = –3.134 eV/atom). These calculations support the here proposed structure for tantalum pentoxide, which is formed under non-equilibrium conditions of sputtering.

[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.

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 V_0.5Mo_0.5N/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 V_0.5Mo_0.5N films do not crack. However, as there is no evidence of Cu-Pt ordering in the synthesized V_0.5Mo_0.5N 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 V_0.5Mo_0.5N, V_0.5W_0.5N, Ti_0.5Mo_0.5N and Ti_0.5W_0.5N 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-t_2g metallic states, which allows the selective response to tensile/shearing stresses, and explains the enhanced toughness confirmed experimentally for V_0.5Mo_0.5N films.

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

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

Zirconium oxide (ZrO_x) 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 ZrO_x 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.