Relatively low temperature synthesis of hexagonal tungsten carbide films by N doping and its effect on the preferred orientation, phase transition, and mechanical properties

Abstract

Articles you may be interested inEffect of silver incorporation in phase formation and band gap tuning of tungsten oxide thin films Effects of substrate bias on the preferred orientation, phase transition and mechanical properties for NbN films grown by direct current reactive magnetron sputtering J. Appl. Phys. Structural and chemical phase transitions in tungsten carbide films evidenced by the analysis of their stiffness tensorsThe authors deposited N-doped tungsten carbide thin films on Si͑100͒ substrates at 500°C using direct-current reactive magnetron sputtering in a mixture of CH 4 / N 2 / Ar discharge and explored the effects of N doping on the preferred orientation, phase transition, and mechanical properties of the films by using x-ray diffraction, x-ray photoelectron spectroscopy, and nanoindentation measurements. They found that N doping significantly influenced the compressive stress, which led to a pronounced change in the preferred orientation, phase structure, and hardness for the tungsten carbide film. A phase transition from ␤-WC to ␣-WC occurred when N doping was in the range of 2.9 and 4.7 at. %, meaning that ␣-WC can be obtained at relatively low temperature ͑500°C͒. To reveal the relationship between the stress and phase transition, as well as preferred orientation, the density-functional theory based on first principles was used to calculate the elastic constants and shear modulus for tungsten carbide with a structure of ␤-WC or ␣-WC. The calculated results showed that the preferred orientation depended on the competition between strain energy and surface energy, as well as the grains competitive growth, and the phase transition can be attributed to a decrease in the strain energy. The hardness of ␣-WC was harder than ␤-WC because the shear modulus for ␣-WC was larger than that of ␤-WC, whereas the bulk modulus for ␣-WC was almost equal to that of ␤-WC.

Reactive magnetron sputtering of tungsten target in krypton/trimethylboron atmosphere

“…The H varies in the range 23 to 31 GPa for films deposited with different TMB flows and at temperatures ≤ 600 o C; a closer inspection shows the lowest H values for the films deposited at temperatures of 200, 300 and 400 o C, with H = 23, 25, and 24 GPa, respectively.Our H values are comparable or even slightly higher than the reported HV of d-WC with 22 GPa for a 0001-oriented single-crystal[2] as well as to those reported for more carbon and boron rich W-B-C films by Alishahi et al[26] and Debnárová et al[27] with values in the range ~24 to ~29 GPa and ~23 GPa, respectively. In contrast, our H values are lower than the ~37 to ~47 GPa reported by Su et al[24] for d-WC films alloyed with ~4 to ~7 at. N as well as the ~45 GPa determined by Liu et al for W-B-C films reactively sputtered from a WB2 target[25].The H values measured by Su et al support the superior mechanical properties of d-WC films compared to WC1-x films in particularly when d-WC is alloyed with N. For the W-B-C films reported by Liu et al it is important to firstly note the higher boron content in their films with ~50 at% compared to our films with 6-8 at.% B. Secondly, the phase distribution in their…”

“…Our H values are comparable or even slightly higher than the reported HV of d-WC with 22 GPa for a 0001-oriented single-crystal [2] as well as to those reported for more carbon and boron rich W-B-C films by Alishahi et al [26] and Debnárová et al [27] with values in the range ~24 to ~29 GPa and ~23 GPa, respectively. In contrast, our H values are lower than the ~37 to ~47 GPa reported by Su et al [24] [14]. From this study we note the higher H value of 34.5 GPa measured for an epitaxial W film with 7 at.% C, suggesting that the H depends on film orientation.…”

“…The most applied gaseous reactants have been C2H2 (acetylene), [11] [16] [17] [18] [19] [20] and CH4 (methane) [21] [22], but with report from reactive processing from other hydrocarbons such as ethene (C2H4) [12] and benzene (C6H6) [23]. As experienced from CVD, there are difficulties in depositing the d-WC phase with only a few reports on reactive sputtering of phase-pure films, using alloying with N2 [24] or growth on carbon substrates at temperatures > 900 o C [12].…”

“…XPS spectra from W 4f in a) and C 1s in b) of the W-B-C films deposited at 500 o C and with TMB flows from bottom to the top: 1, 2.5, 5, 7.5 and 10 sccm. Literature binding energies for W-W[38] and C-W[24,39] are indicated by the vertical dotted lines in a) and b), respectively.…”

Enhanced mechanical properties of HfO2 film by nitrogen doping

“…It seems that the nitrogen content and accordingly high stress contribute the enhanced hardness. Indeed, several reports have showed the similar correlation between stress and hardness in TiN x O y films, 33 N doped WC films 34 and TiN x C 1− x films. 35 The incorporated nitrogen in films could contribute the strengthened hardness.…”