The TiO2(100)||(0001)Al2O3 interface: A density-functional study

Maxim Popov

Research output: ThesisDoctoral Thesis

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Abstract

Materials made of alternating thin layers of Al2O3 and TiO2 are of high interest for industry and materials science. Present and potential applications of such materials include, e.g., optical coatings and multilayer optical windows, as well as hardness-enhanced multilayer films for high-temperature applications. Al2O3/TiO2 laminates are even considered as a possible candidate to replace the SiO2 films for gate dielectric applications in transistors. Despite the effort done to investigate TiO2/Al2 O3 interfaces using experimental methods, there is nothing reported in the existing body of literature about atomistic modeling of this interface. It is the primary goal of this work, thus, to fill this gap, and to stimulate further work in the atomistic investigation of TiO2 /Al2 O3 interfaces, especially using first principles methods. The methodological basis of this work consists of density functional theory (DFT) and linear elasticity theory. The former is the standard method in computational solid-state physics and materials science for dealing with matter at the level of atoms, that allows to calculate electronic structure and related properties. The latter is a well established framework for description of strain, stress, and elasticity of materials at a macroscopic level. Using experimental data on the phase composition of the film and the epitaxial relationships of TiO2 deposited on (0001) sapphire, a model of the interface is established. In order to cope with the lattice misfit between the substrate and the overlayer, the stress balancing method is introduced, that allows to minimize the total strain energy of a superlattice using linear elasticity theory. The local arrangement of atoms in the vicinity of the interface is obtained by atomic relaxation. The structural features of the optimized geometries are analyzed by means of radial- and angular-distribution functions. The values for the work of separation, for both the static and the relaxed case, are obtained. It is found that the maximal adhesion strength is achieved, when the stacking sequence that is intrinsic for TiO2 along [100] and for Al2O3 along [001] is preserved across the interface. The electronic properties, including the spatial charge distribution, and the total, partial, and local densities of electronic states are investigated in detail. In order to investigate the mechanical properties of the system, the bulk modulus (B), the Young’s modulus (E[001]), and the shear modulus (G(001)[010]) are alculated. The numerical values of these moduli are also estimated using the effective elastic constants within the framework of the Grimsditch-Nizzoli method.
Translated title of the contributionDie TiO2 (100) || (0001) Al2O3 Grenzfläche: Eine dichtenfunktionaltheoretische Untersuchung
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Puschnig, Peter, Assessor A (internal)
  • Draxl, Claudia, Supervisor (internal)
Publication statusPublished - 2013

Bibliographical note

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Keywords

  • TiO2/Al2O3 interface
  • ab initio
  • DFT
  • structure
  • electronic properties
  • the work of separation
  • mechanical moduli

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