A combinatorial investigation of sputtered Ta–Al–C thin films

• The Ta–Al–C phase space was explored using combinatorial sputtering.
• The ternary phases Ta5Al3C and Ta3Al2C were observed.
• Resputtering is not supported as a factor limiting Tan + 1AlCn growth.
• Slow surface diffusion of Ta is proposed as a factor limiting Tan + 1AlCn growth.

We describe a combinatorial experiment investigating the Ta–Al–C material system, conducted with the aim of determining why the tantalum-containing Mn + 1AXn phases have so far proved to be not amenable to thin-film synthesis. Samples were deposited onto (0001) Al2O3 wafers at 850 °C and characterized by X-ray diffraction wafer maps, scanning electron microscopy, and surface optical scattering. Elemental Ta, the binary phases TaC, Ta2C, and TaAl3, and the ternary phases Ta3Al2C and Ta5Al3C were identified. The morphology, phase composition and preferred orientation of the films deposited were found to be highly sensitive to the Ta fraction of the incident flux during deposition. No MAX phase material was observed, indicating that the Ta-containing MAX phases do not form under the deposition conditions investigated. Explanations associated with inadequate coverage of stochiometries, preferential sputtering, and thermodynamic instability have been ruled out. An explanation based on reduced surface diffusion of Ta during growth is proposed. A substantially higher substrate temperature during deposition is likely to be required to synthesize Ta-containing MAX phases.