Invited Paper SE-MoA1
Control of Nanostructure Evolution in Metastable Transition Metal Nitride Layers using High-Flux, Low-Energy Ion Irradiation during Growth
We use high-flux, low-energy ion irradiation during low-temperature growth to control the kinetic pathways of phase separation in metastable transition-metal nitride layers in order to produce novel nanostructures composed of self-organized nanolamellae in δ-TaN@sub x@ or nanocolumns in Ti@sub 1-x@Ce@sub x@N. Coherent δ-TaN(111)/γ-Ta@sub 2@N(0002) nanolamellae form spontaneously in reactively-sputter-deposited TaN@sub x@ layers when using ion energy E@sub i@ ~ 50 - 65 eV with an ion-to-metal flux ratio J@sub i@/J@sub Me@ ~ 11; at lower E@sub i@ the films are phase-pure metastable δ-TaN @sub x@. The nanolayers are coherent platelets of alternating metastable cubic δ -TaN@sub x@ and thermodynamically-stable hexagonal γ-Ta@sub 2@N phases which are lattice-matched along their hexagonal closed-packed δ-TaN(111) and γ-Ta@sub 2@N(0002) planes. The formation of γ-Ta@sub 2@N nanolamellae is attributed to ion-irradiation-induced ordering of N vacancies within the (111) N planes of δ-TaN that causes hexagonal stacking of the closed-packed Ta planes. The nanolamellar TaN layers exhibit superhardness values (H = 40-45 GPa). During reactive sputter-deposition of metastable Ti@sub 1-x@Ce@sub x@N alloys, we observe nanophase films with x>0.1. Under conditions of low ion-irradiation, i.e. grounded or floating substrates, the nanostructure consists of equiaxed grains which forms due to continuous renucleation induced by CeN segregation. This is analogous to the nanostructure to the one observed in crystalline/amorphous nanocomposites, e.g. TiN/Si@sub 3@N@sub 4@. In contradistinction, a novel nanocolumnar structure forms when the alloys are grown under intense ion-irradiation with J@sub i@/J@sub Me@ ~ 15 and Ei = 45 eV. The intense ion mixing in the near surface area allows sufficient adatom mobility to form local TiN- and CeN-rich areas that propagate along the growth direction.