Epitaxial Mocx: Competition Between Cubic Delta-Mocy (111) And Hexagonal Beta-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.