Nanostructural design of transition metal nitride thin films

Abstract: Early transition metal nitrides have found extensive use in a range of thin film applications. We are just beginning to understand their growth from vapor phase deposition and microstructure-property relationships. There are large fields to explore, not the least for the nanoand microstructural design in materials offered by a developed deposition process control and alloying by additional elements to grow, e.g., ternary nitrides or superlattices. The work in this thesis has been directed towards increasing the fondarnental understanding of the synthesis, characterization, and properties for some technologically relevant nitrides. Binary and ternary phases of transition metal nitrides, as well as artificial superlattice structures have been studied. In order to prepare materials of high purity, film deposition was performed by Ultra-High Vacuum Reactive Magnetron Sputtering.In this thesis single-crystal Ti2A1N(0001) thin films were synthesized by epitaxial growth onto MgO(111) substrates at elevated temperature and using a 2Ti:A1 compound sputtering target. This is the first thin film deposition process reported for a nitride of the so called Ma+1AXn phase family of compounds - an inherently nanolaminated material that is characterized by metallic conductivity and ductility with retained ceramic strength, high-temperature stability, oxidation, and corrosion resistance. Th2A1N is found to have a room-temperature resistivity of 39 μΩcm, Young's modulus of 16-17 GPa, and hardness of 210 GPa. It is also found that nitrogen-depleted deposition conditions yield the growth of equilibriwn phases TiA1, Ti3A1 and Ti3A1N. For overstoichiometric deposition conditions with respect to Ti2A1N, a phase mixture with TiN was obtained. A super-structure in the TiN phase was also observed to form along the [111] direction at a repetition distance of 7.34 Å, possibly related to A1 segregation.CrN/ScN superlattices were designed for use as soft x-ray mirrors and investigated with respect to thermal stability, hardness, and x-ray reflectivity properties. The combined performance of as-deposited superlattices films, with a compositional modulation period of 1.64 nm, both as a mirror and for thermal and mechanical stability was found to be far better than state of the art metallic Cr/Sc multilayers. In fact, the obtained reflectance of 6.95% at a wavelength of 3.1 nm is excellent, the structure is intact after annealing above 800 °C, and the hardness of 19 GPa makes the mirror effectively scratch-proof.Stress measurements in the TiN/TaN system were performed in-situ. The obtained results showed that a technique based on curvature measurements by laser deflection on the sample during thin film deposition works when employed at the elevated temperatures typically used for sputtering of nitrides. Findings from the in-situ measurements show a correlation between the stress and the film microstructure and the phase composition of TaN layers. It is also shown how the individual layers in an TiN/TaN artificial superlattice affects the stress with a sub-nm resolution. The contribution from thermal stress is also detected and the fine increase in temperature due to exposure of energetic particles from the plasma can be calculated from that stress.A multiphase region in nitrides was demonstrated to form in the NbxZr1-xN model system. The existence of such a multiphase or polytypic structure is predicted by first-principles density functional theory calculations that to occur in nitrides of compositional regions with valence electron concentrations that yield the same total energy for different crystal structures. Films with varying composition were grown and transmission electron microscopy studies revealed an increase in defect density for the x= 0.5 composition. Nanoindentation performed on such films showed an increase in hardness of - 20% compared to the binary nitrides. Analysis of the indents revealed that materials volumes had rotated away from the indenter, thus offering an alternative mechanism for plastic deformation compared to glide on preferred slip systems seen in the cubic binary nitrides. The materials design concept of such phase stability tuning for mechanical strengthening by a high density of phase interfaces is proposed to be expanded to other materials systems.