| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2018) | |
| Thin Films | Tuesday Sessions |
| Session TF-TuM |
| Session: | Innovations in the Development of Multifunctional Thin Films |
| Presenter: | Joe Greene, Linköping University, Sweden, University of Illinois at Urbana-Champaign |
| Correspondent: | Click to Email |
Transition-metal (TM) nitrides exhibit an enormous range of properties and offer a smorgasbord of opportunities for materials scientists. Cubic TM nitrides have wide single-phase compound fields that can be exploited. We show results for vacancy hardening in 3d Group-IV TiNx(001) and Group-V VNx(001); the hardness H (and resistivity ρ) of epitaxial layers increases, while the elastic modulus E and the relaxed lattice constant decreases linearly, as x is decreased from 1.0 to 0.67 and 0.80, respectively. In contrast, H(x), E(x), and ρ(x) for 5d Group-V TaNx(001) remain constant due primarily to the presence of isoelectronic antisites. Strong electron/phonon coupling in VNx results in thermal conductivity at room temperature and above being dominated by electronic contributions.
All Group-IV TM nitrides, TiN, ZrN, and HfN, are very good metallic conductors with room-temperature resistivities of 12-14 μΩ-cm. 3d Group-III ScN(001) is a transparent semiconductor with an indirect Γ-X gap of 1.3 eV. Reflectivity measurements from Sc1‑xTixN(001) layers show TiN is strongly reflecting up to the reflectance edge at ћωe = 2.3 eV, while ScN is transparent, and ωe α x0.5 for the alloy. ZrN is intermediate with ћωe = 3.04 eV. Thus, hard decorative coatings can be obtained with a wide palette of colors.
Superconducting transitions Tc for the Group-IV TM nitrides range from 10.4 K for ZrN to 9.18 K for HfN to 5.35 K for TiN. For comparison, superconductivity is not observed for the Group-IV rare-earth (RE) nitride CeN. These results are consistent with electron/phonon coupling parameters of 1.11 (ZrN), 0.82 (HfN), 0.73 (TiN), and 0.44 (CeN). The acoustic phonon modes soften monotonically with increasing cation mass; optical mode energies remain approximately constant for the TM nitrides, but are significantly lower for the RE nitride due a lower interatomic force constant.
The extreme range of materials properties available in TM nitrides and related systems can be enhanced through the formation of self-organized superhard nanostructures consisting of commensurate nanolamellae, nanocolumns, nanospheres, and nanopipes.
An issue with hard ceramic films, however, is that they are typically brittle, leading to failure by crack formation and propagation. We show several approaches to obtaining TM nitride layers that are both hard and ductile (i.e., tough). IV-VI and V-VI alloys, e.g. Ti1‑xWxN and V1‑xMoxN, exhibit dramatic delocalization of electron density leading to a more ductile response to shear stress while exhibiting increased hardness under tensile and compressive loading. Vacancy-induced toughening is also observed in under-stoichiometric (V,Mo)Nx alloys.