Transition metal nitride films for optical applications

The three transition metal nitrides TiN, ZrN and HfN have remarkably high stability due to their bonding: a mixture of covalent and ionic contributions. The optical properties of these nitride compounds are free-electron like to a surprisingly large extent, in particular in comparison with the corresponding carbides. It is argued that the interband optical excitations of the d-electrons are restricted by selection rules, resulting in a Drude like behaviour of these d-electron compounds. Hitherto, one of the main optical application has been as opaque, wear-resistant replacements for gold-coatings. This review includes the efforts to study, understand and enhance the optical selectivity of group IVB transition metal nitrides as the selective surface in high temperature thermal solar absorbers, as the metal layer in LE-coatings on energy-efficient or solar control windows and as a Langmuir probe coating. The reflectance edge is not in the optimum position for absorber applications and attempts to shift it with alloying were unsucessful. Recent development of inhomogenous, cermet-type nitrides hold some promise. The LE-coatings will not reach as high selectivity as the current noble metal based multilayers, but may find use in aggressive environments because of their excellent stability.