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

Abstract

Molybdenum carbide thin films, 63–97 nm thick, are deposited by reactive DC magnetron sputtering onto Al2O3(0001) substrates at 1200 °C in 5 mTorr Ar - CH4 gas mixtures with a varying CH4 fraction fCH4 = 0–10%. X-ray diffraction θ-2θ scans, ω-rocking curves, ϕ-scans, and reciprocal space maps in combination with electron backscatter diffraction phase maps reveal that fCH4 = 7–8% leads to epitaxial δ-MoCy(111) grains with [112¯]δ-MoC || [112¯0]Al2O3 and biaxial textured β-Mo2C(0001) with a preferential [101¯0]β-Mo2C || [101¯0]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 δ-MoCy(111) or hexagonal β-Mo2C(0001) phase at fCH4 = 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 fCH4 ≤ 6% leads to polycrystalline β-Mo2C and/or bcc Mo phases, while higher fCH4 ≥ 10% yields nanocrystalline δ-MoCy embedded in an amorphous C matrix. The increase in fCH4 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.