Epitaxial MoCx: Competition between cubic δ-MoCy (111) and hexagonal β-Mo2C(0001)

Molybdenum carbide thin films, 63-97 nm thick, are deposited by reactive DC magnetron sputtering onto Al2O3(0001) substrates at 1200 degrees C in 5 mTorr Ar - CH4 gas mixtures with a varying CH4 fraction f(CH4) = 0-10%. X-ray diffraction theta-2 theta scans, omega-rocking curves, phi-scans, and reciprocal space maps in combination with electron backscatter diffraction phase maps reveal that f(CH4) = 7-8% leads to epitaxial delta-MoCy(111) grains with [11 (2) over bar](delta-MoC) parallel to [11 (2) over bar0](Al2O3) and biaxial textured beta-Mo2C(0001) with a preferential [10 (1) over bar0](beta-Mo2C) parallel to [10 (1) over bar0](Al2O3) in-plane orientation. The two phases nucleate epitaxially on the substrate and/or on top of each other, followed by a competitive growth mode which results in a dominant cubic delta-MoCy(111) or hexagonal beta-Mo2C(0001) phase at f(CH4) = 7 or 8%, respectively, and a reduction in the layer density measured by X-ray reflectivity which suggests the formation of amorphous C clusters above the layer nucleation stage. Deposition at lower f(CH4) <= 6% leads to polycrystalline beta-Mo2C and/or bcc Mo phases, while higher f(CH4) >= 10% yields nanocrystalline delta-MoCy embedded in an amorphous C matrix. The increase in f(CH4) also causes a 3-fold decrease in the Mo deposition rate measured by Rutherford backscattering spectrometry and an 18% increase in the discharge voltage which is attributed to adsorbed CH4 and carbide formation on the target surface.

Automated summary

Molybdenum carbide thin films were deposited on Al2O3 substrates using reactive DC magnetron sputtering, revealing that an optimal CH4 fraction of 7-8% leads to epitaxial delta-MoCy and biaxial textured beta-Mo2C, while lower or higher CH4 fractions yield polycrystalline or nanocrystalline structures.