Structure of thin-film oxides: an ab initio study of TiC/Alumina

Oxides and oxide films play a major role in present-daytechnologies. Identification and analysis of their atomic andelectronic structure are important to develop new functionalmaterials. At the same time, the ionic character, complexity,and structural flexibility make accurateatomic structure determinationsdifficult, both from the experimental and the theoreticalpoint of view.This licentiate thesis presents an efficientand general method to identify promisingcandidates for oxide thin films and to study theirstructural elements using ab initiodensity functional theory calculations.Thin films generated from building blocksof the complex bulk structure of (meta-)stableoxides form a well-defined network of initial configurations.Strong atomic relaxations are predicted andcharacterized by ab initio calculations.We order the resulting,relaxed thin-film candidatesaccording to variations in ab initiototal energy and ab initio thermodynamic Gibbsfree energy. The relaxed structures for the mostfavorable films provide insight on the atomicconfiguration of the truly stable and metastable films.The method is illustrated and testedfor thin-film alumina (Al2O3)on a titanium carbide substrate (TiC).The TiC/alumina system plays an importantrole for wear-resistant coatings grown bychemical-vapor deposition (CVD).The bulk structures of the stable alpha-and metastable kappa-Al2O3lead to the identification of 38 initial thin-filmconfigurations for a given film thickness n,including three different stoichiometric compositions,Al_{4n-4}O_{6n}, Al_{4n-2}O_{6n}, and Al_{4n}O_{6n}.The results of extensive density functional theorystudies show that the energetically most favorablethin films differ heavily from theirinitial structures and possess up to 60 %tetrahedrally coordinated Al ions.This is considerably more than in the mainbulk-alumina crystal structures. The method iscapable of catching thin film candidates withstructural building blocks that are not explicitlyincluded in the network of initial configurations.The results of this licentiate thesis can haveimplications for the theory description of CVD growth ofwear-resistant TiC/alumina multilayer coatings. We find thatthe thermodynamically favored TiC/Al_{4n-4}O_{6n} systems,stable in the physically relevant range of the oxygen chemicalpotential, separate into a tightly bound O-monolayer onTiC (TiC/O) and a weakly bound Al_{4(n-1)}O_{6(n-1)}overlayer. Strongly binding filmsare predicted to be stabilized onlyat extreme UHV conditions or by Ti defects between the tightlybound O-monolayer and the alumina overlayer. The thesissuggests that the nonequilibrium nature of the CVD growthenvironment plays an important role in securing the necessarystrength of the TiC/alumina binding and the optimal aluminanucleation