Epitaxial growth and properties of transition metal carbide thin films

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Fang, Peijiao
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Materials engineering
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Transition metal carbides and nitrides are of broad interest due to their thermal stability, chemical inertness, metallic to semiconducting conductivity, high hardness and wear resistance. In the past decades, transition metal carbides have been widely used in a wide range of technological applications including wear-resistant and decorative coatings, components for cutting and drilling tools, electrical contacts and diffusion barriers in microelectronics, and electrodes and catalysts for electrochemical storage and conversion systems. However, compared to numerous studies of transition metal nitrides on its intrinsic electrical, optical, optoelectrical, piezoelectrical, electrochemical and mechanical properties, much less is known for its transition metal carbide counterparts. To develop more fundamental understandings of transition metal carbide, specifically the early transition metal carbides, I have studied four representative transition metal carbide and carbonitride systems including WC, MoC, TiC and TiCN.Rocksalt WCy(001) layers are deposited onto MgO(001) single crystal substrates using DC magnetron sputtering method. A CH4 fraction of fCH4 = 0.4% - 6%, yields total C-to-W ratios x = 0.57 - 1.25. The C-to-W ratio y in the cubic WCy phase is smaller than x, ranging from y = 0.47 to 0.68, as determined from lattice constant ao measurements in combination with first-principles calculations that predict an increasing ao = (0.4053 + 0.0295y) nm for y = 0.3-1.0. This suggests that the cubic phase is stabilized by carbon vacancies and that the layers contain amorphous C with a volume fraction increasing from 4% - 26% for fCH4 = 0.4% - 6%. Structural analyses confirm the growth of epitaxial rock-salt structure WCy(001) layers with a cube-on-cube epitaxial relationship with the substrate: (001)WC
(001)MgO and [100]WC
[100]MgO. The measured XRD out-of-plane coherence length of 8 – 14 nm is nearly independent of the film thickness d = 10 or 600 nm, suggesting that growth beyond d = 10 nm leads to an epitaxial breakdown and the nucleation of misoriented hexagonal or orthorhombic W2C grains for fCH4 ≤ 1% and cubic nanocrystalline WCy grains for fCH4 > 1%. Molybdenum carbide, which are formed by the same group transition metal as the tungsten carbide, are also investigated in this study. Molybdenum carbide layers are grown on Al2O3(0001) substrates with a varying CH4 fraction fCH4 = 0-10%. Structural analyses reveal that fCH4 = 7-8% leads to epitaxial -MoCy(111) grains with ["11" "2" ̅]-MoC
["11" "2" ̅"0" ]Al2O3 and biaxial textured -Mo2C(0001) with a preferential ["10" "1" ̅"0" ]-Mo2C
["10" "1" ̅"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 -MoCy(111) or hexagonal -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 -Mo2C and/or bcc Mo phases, while higher fCH4 ≥ 10% yields nanocrystalline -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. Titanium carbide are deposited onto MgO(001) by reactive DC magnetron sputtering in Ar/CH4 mixtures at 1100 C using a varying CH4 fraction fCH4 = 0.4-8% that yields C-to-Ti ratios x = 0.08-1.8. Structural analyses indicate epitaxial TiCx(001) growth for x = 0.08-1.5, but incorporation of secondary Ti and C impurity phases for x ≤ 0.24 and x  1.5, respectively. First-principles calculations of the formation energy of Ti1-yCy confirms the phase separation of hcp Ti and titanium carbide at low carbon contents. The relaxed lattice constant increases from 0.4304 nm to 0.4325 nm and the measured strain decreases from ε = 0.3% to 0.1 % for phase pure epitaxial TiC0.5 and TiC1.0 layers. Nanomechanical properties include hardness H and elastic modulus E shows a monotonically increasing trend with increasing carbon incorporation from H = 8.7 GPa and E = 143 GPa for TiC0.08 to H =31.2 GPa and E = 462 GPa for TiC1.0, while further increasing x leads to a rapid drop till H = 13.5 GPa and E = 201 GPa for TiC1.8. The measured resistivities at 298 K range from 83-598 cm for TiCx layers with the lowest resistivity of 83 cm for near-stoichiometric epitaxial TiC1.0 layer. The resistivities at 77 K are only 10-26 cm lower than at 298 K, indicating defect scattering dominance. The electrical and nanomechanical properties of titanium carbide can be tuned by alloying with titanium nitride, which has a 6-fold decrease in the electrical resistivities and an fine regulation of valence electron concentration. For this purpose, titanium carbonitride films are sputter-deposited onto MgO(001) in Ar/CH4/N2 mixtures at 1100 C using a varying N2 partial pressure pN2 = 0.2-0.8 mTorr. The compositional analysis yields Ti0.44C0.39N0.17¬, Ti0.44C0.3N0.26 and Ti0.47C0.21N0.32 for pN2 = 0.3, 0.6 and 0.8 mTorr respectively. X-ray diffraction results indicate epitaxial TiCN(001) growth. The relaxed lattice constant decreases from 0.4296 nm to 0.4266 nm from pN2 = 0.2 to 0.8 mTorr, indicating N replacement of C into the carbonitride lattices. Nanomechanical properties include hardness H and elastic modulus E shows an initial increasing trend with increasing nitrogen incorporation from H = 26 GPa and E = 299 GPa for pN2 = 0.2 mTorr to H =28.3 GPa and E = 495 GPa for pN2 = 0.3 mTorr, while further increasing pN2 drops till H = 25.6 GPa and E = 456 GPa for pN2 = 0.8 mTorr. The measured resistivities at 298 K shows monotonically decreasing trend from 40 to 26 cm as pN2 increasing from 0.2 to 0.8 mTorr.
December 2021
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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