ICMCTF2013 Session B7-1: Computational Design and Experimental Development of Functional Thin Films

Wednesday, May 1, 2013 8:00 AM in Room Royal Palm 1-3

Wednesday Morning

Time Period WeM Sessions | Abstract Timeline | Topic B Sessions | Time Periods | Topics | ICMCTF2013 Schedule

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8:00 AM B7-1-1 Nanoengineered Oxide and Nitride Thin Films with Unique Functionalities
Haiyan Wang, JoonHwan Lee, Aiping Chen, Michelle Myers, Chen-Fong Tsai, Qing Su, Yuanyuan Zhu, Li Chen, Liang Jiao, Jie Jian, Wenrui Zhang, Fauzia Khatkatty, Clement Jacob (Texas A&M University, US); Quanxi Jia (Los Alamos National Laboratory, US); Judith Driscoll (University of Cambridge, UK); Jian Gan, James Cole (Idaho National Lab, US)
The talk focuses on various nanostructured functional oxide and nitride thin films for high temperature superconductors (HTS), solid oxide fuel cells, transparent conducting oxides, nitride radiation tolerant materials and others. Wires that carry electrical current without resistance are fabricated by coating metal substrates with the HTS YBa2Cu3O7-δ (YBCO). Using nanolayer architecture and nano-particle doping approaches, YBCO coated conductors with dramatically enhanced transport properties can be achieved. Another way of nanoengineering is to process self-assembled vertical-aligned nanocomposite (VAN). We have successfully demonstrated several VAN systems. These VAN systems show interesting lattice epitaxial relationship along the vertical grain boundaries, which enables highly strained films with thickness higher than 100nm. Using vertically aligned nanopore structure, highly efficient thin film cathodes with superior ionic conductivity can be made in thin film solid oxide fuel cells. Enhanced low field magnetotransport properties have been observed in various VAN systems. Nitride coatings with single layer or nanolayers have been demonstrated as superior diffusion barrier and corrosion resistant coatings for fuel claddings in advanced nuclear reactors. Detailed microstructural characteristics of these nanostructured ceramic thin films will be discussed and correlated with their unique functionalities. Conventional TEM studies are coupled with in situ TEM and high resolution STEM analysis.
8:40 AM B7-1-3 Modeling Amorphous Materials from First Principles
Erik Holmstrom, Raquel Lizarraga (Instituto de Fisica, Universidad Austral de Chile, Valdivia, Chile)
We present a stochastic quenching (SQ) approach to obtain theoretical amorphous structures. The quenching is performed by using an ab initio code following a path that minimizes the forces between the atoms until they are close to zero. For a large enough cell, the resulting amorphous structure shows the same

pair-correlation distribution as obtained by a standard molecular dynamics (MD) simulation. However, the calculations are considerably speeded up compared to an ab intio MD simulation. Once an amorphous structure that describes the system at hand is obtained, state-of-the-art DFT methods can be applied to calculate, for instance, theoretical XPS and NMR spectra. Results will be presented for monatomic amorphous metals, bulk metallic glasses, amorphous oxides, hard coating materials as well as amorphous graphene.

9:20 AM B7-1-5 Advanced Modelling of Amorphous Ceramics
Jiri Houska (University of West Bohemia - NTIS, Czech Republic)

In this paper we discuss algorithms for ab-initio modelling of structures, electronic structures and properties of amorphous ceramics. All materials simulated were also prepared experimentally. We focus on the possibilities and limitations of the modelling, and on the comparison of representative examples of calculated and experimental results.

First, we describe the simulation algorithm which allows one to predict atomic structure of amorphous materials. We show examples of bonding structure analysis in C:H (simple case where the key quantity is "only" the CC bond order), SiBCN (complex case due to different bonding preferences of individual elements, and the necessity to distinguish between bonding valence electrons and electronic lonepairs) and Zr(Si)BCN (even more complex due to a mixture of covalent and metallic bonding). [1-3]

Second, we go through special cases, such as (i) presence of implanted Ar ions in sputtered materials (which can e.g. affect their homogeneity due to segregation of Si around Ar in SiBCN), (ii) formation of isolated gas molecules (N2 or H2; which cannot diffuse out due to the periodic boundaries) during the simulation or (iii) intentional presence of isolated molecules in a material. [4]

Third, we summarize quantities which control (or can be used to predict) selected functional properties of amorphous materials. In the case of electrical conductivity this includes not only the band gap but also weights of electronic states on individual elements and bonds (particularly around the Fermi level) and electronic mobility (which depends on localization of electronic states). In the case of thermal stability this includes not only the static amorphous network characteristics (bond types, coordination numbers) but also temperature-dependent dynamic quantities (formation rate of N2 molecules, rate of bond-breaking processes). [5]

[1] Surf. Coat. Technol. 203, 3770 (2009); [2] J. Vac. Sci. Technol. A 25, 1411 (2007); [3] J. Phys.: Condens. Matter 23, 025502 (2011); [4] J. Phys.: Condens. Matter 18, 2337 (2006); [5] Acta Materialia 59, 2341 (2011)
9:40 AM B7-1-6 Dynamics of Ti, N, and TiNx (x = 1 - 3) Admolecule Transport on TiN(001) Islands and Surfaces
Daniel Edström, Davide Sangiovanni, Valeriu Chirita, Lars Hultman (Linköping University, IFM, Thin Film Physics Division, Sweden); Ivan Petrov, Joseph Greene (University of Illinois at Urbana-Champaign, US)
We use the Modified Embedded Atom Method (MEAM) to perform realistic large scale classical Molecular Dynamics (MD) of the typical processes occurring in the initial nucleation of TiN thin film growth. The simulations are carried out at 1000 K, a reasonable growth temperature, and follow the dynamics of Ti, N, and TiNx (x = 1 - 3) admolecules, on TiN(001) islands. We perform statistically independent MD runs of 2 and 5 ns, for total simulation times of several microseconds, which allow the quantification of average residence times on islands and a detailed analysis of the corresponding descent mechanisms on the (001) terrace. Results show significant differences, in terms of diffusion on islands and descent mechanisms, for each adspecies. We find that Ti adatoms descend from islands exclusively via push-out exchange with island edge atoms and reside on islands for significantly shorter times than N adatoms. This effect is primarily due to the much lower diffusion rates on islands of N adatoms, which ultimately descend onto the terrace either via direct hopping or push-out exchange with island edge and corner atoms. The dynamics of TiNx (x = 1 - 3) admolecules are considerably more complicated, due to the additional rotational degrees of freedom engaged in the transport of these adspecies. Thus, TiN dimers exhibit the most complex descent mechanisms, which involve combinations of push-out exchange with edge atoms and direct hops over island edges. Surprisingly, we find TiN2 trimers to have the shortest residence times on islands and to descend onto the terrace only via direct hops over island edges. TiN3 tetramers, however, are essentially stationary on the TiN(001) islands, and no descent events are recorded in this case. These results demonstrate that tetramers lead directly to island-on-island growth on TiN(001) and have a critical effect on TiN thin film growth modes.
10:00 AM B7-1-7 Ab Initio Study of the Effect of Al Addition on Surface Kinetics of Ti, Hf and Zr Nitrides
Christopher Tholander, Björn Alling, Ferenc Tasnadi (Linköping University, Sweden); Ivan Petrov, Joseph Greene (University of Illinois at Urbana-Champaign, US); Lars Hultman (Linköping University, Sweden)

Understanding of the diffusion properties in nitrides helps us to find ways to improve the growth of materials, e.g. thin films. Using different surfactants during growth could, for example, increase or decrease adatom mobility by modifying the diffusion barriers , with consequences for the growth modes.

In our work we have investigated the changes in surface kinetics due to Al substitution in low indexed surfaces of TiN, HfN and ZrN using first principles calculations. The adatom diffusion paths for Ti, Al, Hf, Zr and N have been investigated using both magnetic and non-magnetic nudged elastic band DFT calculations. We also present adatom energy surfaces for selected adatoms on TiN, HfN and ZrN.

The substitution of Ti for Al has been shown to have significant effects on the diffusion paths. For example on TiN(001) the Ti adatoms bind harder to the hollow site positions around the Al substitution, decreasing their mobility. In other cases Al substitution decreases the diffusion barriers and increase the mobility of the adatoms.

10:20 AM B7-1-8 First-principles Study of Electronic, Elastic and Thermodynamic Properties of CrN
Liangcai Zhou (Vienna University of Technology and Montanuniversität Leoben, Austria); David Holec (Montanuniversität Leoben, Austria); Paul Mayrhofer (Vienna University of Technology, Austria)

We used Density Functional Theory with generalized gradient and local density approximations together with selectively applied on-site Coulomb interaction to study electronic, vibrational, thermodynamic and elastic properties of CrN in various configurations, including antiferromagnetic (AFM), nonmagnetic (NM), ferromagnetic (FM) and paramagnetic (PM) states. The analysis of the electronic structures reveals that nitrogen under-stoichiometry induced by nitrogen vacancies, interstitials or anti-sites in CrN contributes to the metallic behavior. The phonon density of states (PDOS) shows that orthorhombic and cubic AFM, cubic FM and cubic PM CrN are vibrationally stable phases at zero pressure, while cubic NM CrN exhibits non-zero PDOS for imagine frequencies for zero as well as under high pressure. The thermodynamic data obtained from the quasi-harmonic approach are used to discuss the transition from AFM orthorhombic to PM cubic phase. Finally, heat capacity, linear thermal expansion coefficient, bulk modulus, and elastic constants are calculated as functions of temperature and critically compared with the available experimental data.

10:40 AM B7-1-9 Importance of Finite Temperature Effects in AB INITIO Simulations of Materials for Hard Coating Applications
Igor Abrikosov, Peter Steneteg, Olle Hellman, Lasse Hultberg, Ferenc Tasnadi, Nina Shulumba, Olga Vekilova, Björn Alling (Linköping University, Sweden)
Ab initio electronic structure theory is known as a useful tool for prediction of materials properties, for their understanding, as well as for determination of parameters employed in higher-level modeling. However, majority of simulations still deal with calculations in the framework of density functional theory (DFT) with local or semi-local functionals carried out at zero temperature. In this talk we underline the importance of explicit treatment of finite temperature effects in ab initio simulations of materials for hard coating applications and present new methodological solutions, which go beyond traditional approach and allow us to calculate materials properties at elevated temperature. In particular, we study elastic properties of TiN within a wide temperature interval [1]. Single crystal elastic constants C11, C12 and C44 are calculated. In all cases strong dependence on the temperature is predicted, with C11 decreasing by more than 30% at 1800 K as compared to its value obtained in conventional static calculations at T=0K. We observe that the material becomes substantially more isotropic at high temperatures. Next, we suggest first-principles method for the calculation of thermodynamic properties of magnetic materials in their high-temperature paramagnetic phase [2]. It is based on ab initio molecular dynamics (AIMD) and simultaneous redistributions of the disordered but finite local magnetic moments. We apply this disordered local moments molecular dynamics method to simulate equation of state of CrN [2,3] and thermodynamic properties of Cr1−xAlxN alloy. Our results unambiguously demonstrate importance of taking into account finite temperature effects in theoretical calculations of thermodynamic and elastic properties of materials. [1] P. Steneteg, O. Hellman, O. Yu. Vekilova, N. Shulumba, F. Tasnadi, and I. A. Abrikosov, in preparation. [2] P. Steneteg, B. Alling, I. A. Abrikosov, Phys. Rev. B 85, 144404 (2012). [3] B. Alling, T. Marten, and I. A. Abrikosov, Nature Materials 9, (2010) 283; B. Alling, T. Marten, and I. A. Abrikosov, Phys. Rev. B 82, (2010) 184430.
Time Period WeM Sessions | Abstract Timeline | Topic B Sessions | Time Periods | Topics | ICMCTF2013 Schedule