Effects of ultra-low ethylene partial pressure on microstructure and composition of reactively sputter-deposited Ta–C thin films

Abstract

We report on the effects of low ethylene (C2H4) partial pressures (6.7 × 10−5 ≤ Pc ≤ 6.7 × 10−2 Pa) on the composition and microstructure of Ta–C thin films grown on MgO(001) substrates via ultra-high vacuum dc magnetron sputtering of a TaC compound target using 2.7 Pa argon/ethylene gas discharges at 1123 K. The as-deposited layer crystallinity and composition are determined using a combination of X-ray diffraction, transmission electron microscopy coupled with X-ray energy dispersive spectroscopy, and X-ray photoelectron spectroscopy. We find that the layers sputter-deposited in pure Ar (i.e., Pc = 0) and Ar/C2H4 gas mixtures with Pc = 6.7 × 10−5 Pa are polycrystalline trigonal-structured Ta2C (a = 0.310 nm and c = 0.492 nm). At Pc = 6.7 × 10−4 Pa, we obtain cubic, 002-textured TaC0.76 (a = 0.442 nm) porous thin film with facetted surface. Interestingly, we also observe 111-oriented twins due to four crystals rotated with respect to each other. Film grown at higher Pc = 6.7 × 10−2 Pa is relatively dense with smoother surface and composed of a two-phase mixture of nanocrystalline TaC and amorphous carbon. Our results demonstrate compositional and microstructural tunability during sputter-deposition of transition-metal carbide thin films using small amounts of the reactive gas.