Electronic states and mechanical properties in transition metal nitrides

Thin films of hard materials are of prime importance for wear-resistant, protective and decorative coatings. Besides adhesion, hardness is the most often quoted requirement, even if doubts remain on the experimental determination of the hardness values of thin films, on their theoretical interpretation and on their significance for wear protection. Transition metal interstitial compounds are extensively used because of their broad range of functional properties in the fields of machining, microelectronics, decoration, etc. This article presents a summary of recent relevant results on the structural, mechanical, electronic and optical properties of fee TIN, VN, CrN, NbN, W2N, hexagonal MoN, and some ternary nitrides in the form of sputtered thin films. The process parameters, e.g. the reactive gas partial pressure and the substrate bias, strongly influence the film properties. The composition and growth parameters influence the morphology, the stress state and other physical properties. The systematic investigation of the electronic density of states in valence and core states of comparable nitrides provides indications of the degree of covalency in the chemical bonding in relation to properties such as cohesive energy and hardness. For example, in molybdenum nitride the low stability of the cubic MoN phase is related to an increase in the charge transfer of Mo d electrons to nitrogen with increasing stoichiometric ratio N/Mo. Ellipsometric measurements of the dielectric function interpreted in relation to details of the band structure measure the variation of the density of conduction electrons. Vacancies and interstitials remove or add a specific number of electrons at the Fermi level. This analysis allows one to differentiate the types of defect at various compositions in, for example, TiNgamma films, for which the reported hardness values spread over a wide range.