ICMCTF2012 Session B7-2: Computational Design and Experimental Development of Functional Thin Films

Ti_1-xAl_xN, as one of the most studied hard coatings, is widely applied on cutting tools. The influence of the Ti to Al ratio on mechanical properties, phase stability and oxidation resistance has been investigated for stoichiometric TiAlN. As sub- and super-stoichiometric Ti_1-xAl_xN_y have been observed experimentally [1,2] the effect of the nitrogen concentration on the aforementioned properties has to be explored. Alling et al have studied the decomposition pattern for Ti_1-xAl_xN_y theoretically [3]. However, the effect of nitrogen concentration on unit cell volume and elastic properties of sub- and super-stoichiometric Ti_1-xAl_xN_y are yet to be determined.

Here, we study the material system Ti_1-xAl_xN_y both by ab initio calculations and experiments. We show, based on ab initio data, that metal vacancies are responsible for super-stoichiometric (y > 1) Ti_1-xAl_xN_y, rather than nitrogen interstitials. Furthermore, we report lattice parameters and bulk and Young’s modulus of Ti_1-xAl_xN_y. As experimental data are rare, a method of combinatorial reactive magnetron sputtering was developed that allows depositing Ti_0.5Al_0.5N_y films with a lateral nitrogen gradient in a single deposition run. The nitrogen content measured by energy dispersive X-ray spectroscopy varies from 25 at% to 50 at% and thus enables studying Ti_1‑xAl_xN_y films over a broad composition range. In combination with experimental details on the combinatorial synthesis route, structural results from X-ray diffraction and elastic properties measured by nanoindentation will be presented and compared to ab initio calculations.

1: J. Bujak et al., Surf. Coat. Tech. 180-181 (2004), 150.

2: T. Zhou et al., Vacuum 83 (2009), 1057.

3: B. Alling et al., Appl. Phys. Let. 92 (2008), 071903.

Equation of state for chromium nitride has been debated in the literature in connection with a proposed collapse of its bulk modulus at the pressure induced transition from the paramagnetic cubic phase to the antiferromagnetic orthorhombic phase [F. Rivadulla et al., Nat Mater 8, 974 (2009); B. Alling et al., Nat Mater 9, 283 (2010)]. Experimentally the measurements are complicated due to the low transition pressure, while theoretically the simulation of magnetic disorder represent a major challenge. Here a first-principles method is suggested for the calculation of thermodynamic properties of magnetic materials in their high temperature paramagnetic phase. It is based on ab-initio molecular dynamics and simultaneous redistributions of the disordered but finite local magnetic moments. We apply this disordered local moment molecular dynamics method to the case of CrN and simulate its equation of state both at high and low temperature. In particular the debated bulk modulus is calculated in the paramagnetic cubic phase and is shown to be very similar to that of the antiferromagnetic orthorhombic CrN phase.

A significant problem in the atomistic simulation of materials is that molecular dynamics simulations are limited to microseconds, while important reactions and diffusive events often occur on much longer time scales. Although rate constants for infrequent events can be computed directly, this requires first knowing the transition state. Often, however, we cannot even guess what events will occur. In this talk, I will discuss the accelerated molecular dynamics approach, which we have been developing over the last decade, for treating these complex infrequent-event systems. The idea is to directly accelerate the dynamics to achieve longer times without prior knowledge of the available reaction paths. In some cases, we can achieve time scales with these methods that are many orders of magnitude beyond what is accessible to molecular dynamics. I will briefly introduce the methods and discuss some examples of applications to surface diffusion and thin film growth.

Epitaxial stabilization of various crystal structures by the template effect is the topic of many research activities (e.g. superlattice studies) due to the outlook to control the properties of coatings by engineering the structures at the nanoscale. Multilayers that consist of two nanoscale layered materials with the same crystal structure and a small lattice mismatch may grow hetero-epitaxially. In this study we aim at understanding the fundamental aspects of the phase stability due to the fully or partially coherent interfaces combined with the effect of crystallographic and mechanical properties of a substrate. As a model system we investigated two bi-layer configurations: TiN/AlN and CrN/AlN. AlN has two crystal structures, a stable wurtzite structure (w) with hexagonal symmetry and a metastable NaCl structure with cubic (c) symmetry. The elastic energy stored in TiN/AlN and CrN/AlN bi-layer systems with different substrates were investigated by finite element method (ABAQUS) in addition to ab initio calculations. A three dimensional model was simulated with 500 nm thick substrate and bi-layer film thicknesses varied in the nanometer range for the FEM studies. 8-node linear bricks with elastic and anisotropic behavior for different crystallographic orientations were employed. The results showed that for fully epitaxial TiN/AlN bi-layer system, initially the c-AlN phase is preferred as it allows for a lesser internal energy (chemical energy and strain energy) in the range of 0.17 (Si) to 0.32 nm (Sapphire) AlN thickness when grown on c-TiN with different substrates. For thicker AlN layer thicknesses the wurtzite structure possesses smaller energy. The c-AlN is predicted to vary between 0.6 (Si) to 1.0 nm (Sapphire) for the CrN/AlN bi-layer system, while w-AlN is energetically favorable for thicker films. In addition, the effect of not fully coherent interface between c-TiN and w-AlN (i.e. an interface containing misfit dislocations) will be addressed. In conclusion, we demonstrate using a combined FEM and ab initio modeling and rationalize the trends in substrate-induced mechanisms of stabilizing c-AlN when grown on c-TiN or c-CrN, as well as the effect of the quality of the interface, which can be further used for a targeted coating design.

First-principles pseudopotential calculations of the lattice constants and of the single-crystal elastic constants for Al_1-xCr_xN (0 ≤ x ≤ 1) alloys considering the cubic B1-rocksalt structure were first carried out. These calculations were performed using density functional perturbation theory (DFPT) and the supercell method (SC) for the ordered alloys. For the exchange-correlation potential we used the generalized gradient methods (GGA). The calculated equilibrium lattice parameters exhibit a positive deviation from Vegard’s rule corresponding to a positive bowing parameter while the calculated single-crystal stiffness, namely C₁₁ and C₄₄, gradually increases or decreases, respectively from AlN to CrN phases. In a second stage, we have estimated by homogenization methods in the frame of anisotropic elasticity, the averaged stiffnesses < C_ij>, direction dependent Young's moduli and Poisson's ratios of polycrystalline Al_1-xCr_xN (0 ≤ x ≤ 1) alloys considering a {002}-fiber texture. Finally, comparisons are made for 0.4 ≤ x ≤ 1 with the shear elastic modulus G_yz=G_xz and the out-of-plane longitudinal elastic constant C₃₃ measured by Brillouin light scattering and picosecond ultrasonics, respectively.

Keywords

Ab initio, VCA, DFPT, supercell, nitride materials, elasticity, Brillouin light scattering, picoseconds ultrasonics.

The knowledge of the structure and geometry of the SiN_x tissue phase in TiN/SiN_x nanocomposites is crucial for understanding their favorable mechanical properties. Perhaps the most promising experimental techniques for obtaining information about bonding in the tissue phase and between the tissue phase and the TiN grains, are x-ray photoelectron spectroscopy (XPS), x-ray absorption near-edge structure (XANES) and related spectroscopies. Unfortunately, the results of these experiments are difficult to interpret on their own for these complex interface structures, governed and created through non-equilibrium processes.

In this work we present first-principles theoretical calculations of spectroscopical properties of different complex TiN/SiNx/TiN interface structures, proposed, but yet not proven to be of relevance for this material system. By comparing the computed spectra from known structures with the measured spectra of the real films, it is possible to gain insights not accessible with either theory or experiments alone.

Substitutional A_xB_1−xC alloys are extensively used in numerous applications from soft piezoelectric devices [1], such as cell phones, satellites etc. to protecting hard coating of cutting or machining tools [2]. Prediction of anisotropic tensorial materials properties of substitutional alloys from first principles is still a challenging and highly required issue in computational materials science [3] Here we present a joint theoretical and experimental study on the elasticity in TiAlN alloys. We discuss the observed significant anisotropy [4] with implications on the spinodal decomposition (microstructure evolution) in TiAlN. We also present a supplementary theoretical approach to justify the applicability and analyze the performance of the special quasirandom structure (SQS) model in predicting non-scalar, anisotropic materials constants of alloys with certain symmetry. The approach is general enough to apply it for elastic tensors of alloys with other symmetry.

[1] F. Tasnádi, B. Alling, C. Höglund, G. Wingqvist, J. Birch, L. Hultman, and I. A. Abrikosov, Phys. Rev. Lett. 104, 137601 (2010).

[2] A. Hörling, L. Hultman, M. Odén, J. Sjölén and L. Karlsson, Surf. Coat. Technol. 191, 384 (2005). I. A. Abrikosov, A. Knutsson, B. Alling, F. Tasnádi, H. Lind, L. Hultman and M. Odén, Materials 4, 1599 (2011).

[3] A. van de Walle, Nature Mater. 7, 455 (2008).

[4] F. Tasnádi, I. A. Abrikosov, L. Rogström, J. Almer, M. P. Johansson and M. Odén, Appl. Phys. Lett. 97, 231902 (2010).

alloying based on self-organization on the nanoscale via a formation of metastable

intermediate products during the spinodal decomposition. We predict theoretically and demonstrate

experimentally that quasi-ternary (TiCrAl)N alloys decompose spinodally into (TiCr)N and

(CrAl)N-rich nanometer sized regions. The spinodal decomposition results in age hardening, while

the presence of Cr within the AlN phase delays the formation of a detrimental wurtzite phase

leading to a substantial improvement of thermal stability compared to the quasi-binary (TiAl)N

or (CrAl)N alloys.

By their enormous variability and potential low-cost fabrication, organic electronics attract significant commercial and scientific interest in recent years. In particular, pentacene was studied quite extensively as organic molecular semiconductor in various thin film applications. Although pentacene is currently among the organic materials with the highest charge carrier mobility, it is believed that there is still much room for improvement if the p-p interaction can be enhanced. To this aim, functional substitution can induce modified packing structures and electronic properties. For example, pristine pentacene shows a slipped 1D packing, while (6,13)-bis(triisopropyl-silylethynyl)-pentacene (TIPS pentacene) realizes a brickwork 2D packing. By introducing a trifluoromethyl group on the TIPS pentacene backbone (TIPS-CF3 pentacene), the system becomes soluble in common organic solvents. Moreover, the bulky side groups interrupt the herringbone pattern and induce a regular columnar stacking of the acene planes.

We study the effect of functional substitution and of the induced packing on the electronic and optical properties of pentacene, TIPS pentacene and TIPS-CF3 pentacene in comparison to each other in order to evaluate the influence of the functional group. The results are also compared to experimental data. Our calculations are based on density functional theory, using the full potential linearized augmented plane wave method. Exchange and correlation effects are treated within the local density approximation.

An enhanced difference between the band gaps of the molecule and the crystal is found for TIPS pentacene. The frequency dependent dielectric functions and absorption spectra are calculated and analyzed in terms of the transitions between the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals. It turns out that C and Si states from atoms in the chain which connects the side group to the pentacene account for the main contribution to the optical transitions. The calculated dielectric functions agree well with experimental data. Moreover, the experimentally observed red shift from the molecule to the crystal is confirmed.