A theoretical study of mass transport processes on TiN(001) and mechanical properties of TiN- and VN-based ternaries

Sammanfattning: This thesis concerns computer simulations, using classical molecular dynamics, of transport processes related to TiN(001) growth. It is motivated from the challenge to understand transport processes at the atomic scale responsible for crystal and film growth and their different growth modes. Not even the most advanced experimental techniques are capable of resolving the sub ps time and sub-Ångström length-scales required. TiN belongs to an important class of transition metal nitrides, and is chosen here as a model system for such fundamental studies of surface transport. The simulations show that on terraces, Ti adatoms exhibit much higher migration rates than N adatoms. For TiNx complexes, as x increases from 1 to 3, rotation becomes increasingly more prevalent than translation. This leads to surprisingly high mobilities of TiN2 trimers, higher than that of N adatoms. On islands, Ti adatoms experience a significant funneling effect, resulting in short residence times. TiN dimers and TiN2 trimers exhibit surprisingly high diffusivities and residence times even shorter than Ti adatoms. TiN3 trimers, however, are essentially stationary on both terraces and islands and serve as nucleation clusters. Overall, Ti adatoms and TiN2 trimers are the most efficient carriers of Ti and N atoms with and between TiN(001) surface layers. These results indicate that Ti/N flux ratios close to one promote layer-by-layer TiN(001) growth, whereas lower ratios result in surface roughening. Understanding of these phenomena enables experimentalists to tune  the growth processes to optimize material properties.In this thesis I also carry out theoretical calculations to investigate the role of configurational order on the metallic sublattice in relation to toughness enhancement. My studies set out from the recent understanding that the toughness of transition metal nitrides can be enhanced by tuning the valence electron concentration. My results show that ordered alloys exhibit lower resistance to shear deformations than disordered alloys, and higher resistance to tensile deformation. The lower resistance to shear deformations is explained by the formation of fully bonding electronic states perpendicular to the applied stress. Using the Pugh-Pettifor criterion, it is shown that while configurational order has an effect on the ductility of the material, this is primarily governed by the valence electron concentration.