We deposit ternary WCxNy thin films on Si (100) substrates at 500°C using direct current (DC) reactive magnetron sputtering in a mixture of CH4/N2/Ar discharge, and explore the effects of substrate bias (Vb) on the intrinsic stress, preferred orientation and phase transition for the obtained films by virtue of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and selective area electron diffraction (SAED). We find that with increasing the absolute value of Vb up to 200V the carbon (x) and nitrogen (y) atom concentrations of WCxNy films keep almost constant with the values of 0.75 and 0.25, respectively. The XPS and SAED results, combined with the density-functional theory (DFT) calculations on the electronic structure of WC0.75N0.25, show our obtained WCxNy films are single-phase of carbonitrides. Furthermore, we find that the compressive stress sharply increases with increasing the absolute value of Vb, which leads to a pronounced change in the preferred orientation and phase structure for the film, in which a phase transition from cubic β-WCxNy to hexagonal α-WCxNy occurs as Vb is in the range of −40 to −120V. In order to reveal the relationship between the stress and phase transition as well as preferred orientation, the DFT calculations are used to obtain the elastic constants for β-WCxNy and α-WCxNy. The calculated results show that the preferred orientation is dependent on the competition between strain energy and surface energy, and the phase transition can be attributed to a decrease in the strain energy.

We investigated the effects of both bias voltage and annealing on the structure and mechanical properties of WC0.75N0.25 thin films, deposited on Si (100) substrates by a direct current reactive magnetron sputtering system, in which the negative substrate bias voltage (Vb) was varied from floating (−1.6V) to −200V, and the deposited films were annealed at 800°C for 2h. The X-ray photoelectron spectroscopy and selected area electron diffraction analyses, along with the density-functional theory (DFT) calculations on the electronic structure, showed that WC0.75N0.25 films were a single-phase of carbonitrides. After annealing, a significant decrease in hardness for the films was observed, being a result of point-defect annihilation as Vb was in the range of floating to −120V. However, when Vb was in the range of −160 to −200V, the hardness increased from ∼37GPa for the as-deposited film to a maximum of ∼43GPa for the annealed one. This increase in hardness after annealing might be attributed to age-hardening.

We deposit cubic 8-NbN/SiNx and hexagonal delta'-NbN/SiNx nano-multilayer films using reactive magnetron sputtering in discharge of a mixture of Ar and N-2 gas, and explore the effects of SiNx. layer thickness on the microstructure and mechanical properties for both cubic 8-NbN/SiNx. and hexagonal delta'-NbN/SiNx multilayer films by virtue of X-ray diffraction, electron diffraction, high-resolution transmission electron microscope, and nanoindentation measurements. We find that the structure for NbN layers is a critical factor in determining the hardness of NbN/SiNx. multilayer films. For cubic 8-NbN/SiNx. nano-multilayer films, as SiNx layer thickness is smaller than or equal to 0.4 nm, the cubic delta-NbN layers force SiNx layers to crystallize, accompanied with a remarkable increase in hardness with maximum hardness of 32 GPa. With an increase in thickness, SiN. layers become amorphous, and correspondingly the hardness decreases. For hexagonal delta'-NbN/SiN. nano-multilayer films, both the stress and hardness decrease with increasing the SiN. layer thickness and the nano-multilayer films do not show any enhancement in hardness, compared to delta'-NbN single layer film. (C) 2009 Elsevier B.V. All rights reserved.

We deposit niobium nitride (NbN) thin films on Si(1 0 0) substrates using direct current (dc) reactive magnetron sputtering in discharging a mixture of N-2 and Ar gas, and explore the effects of nitrogen flow rate (F-N2) on the preferred orientation (PO), phase transition and mechanical properties for the obtained films by virtue of x-ray diffraction and nanoindentation measurements. We find that with increasing F-N2 the biaxial compressive stress increases, which leads to a pronounced change in the PO and phase structure for the film. A phase transition from delta (face-centred cubic) to delta' (hexagonal) occurs in the film as the biaxial stress increases to a critical value of about 3.9 GPa, which can be attributed to a decrease in the strain energy. With a further increase in the biaxial stress, the fraction of the d delta' phase in the film increases, and finally the film only has a single phase, delta'-NbN, resulting in a consistent increase in hardness.

We have calculated the mechanical properties of cubic δ-NbN and hexagonal δ′-NbN with density functional theory (DFT). It is found that the calculated ideal strength of δ′-NbN is higher than that of δ-NbN, which is consistent with the experimental findings. The tensile strength perpendicular to the polar plane in δ’-NbN is comparable to the weakest bonding direction in diamond, which provides huge potential technological and industrial applications. The electronic origins of mechanical properties are discussed.

The authors deposited N-doped tungsten carbide thin films on Si(100) substrates at 500°C using direct-current reactive magnetron sputtering in a mixture of CH4∕N2∕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 β-WC to α-WC occurred when N doping was in the range of 2.9 and 4.7at.%, meaning that α-WC can be obtained at relatively low temperature (500°C). 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 β-WC or α-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 α-WC was harder than β-WC because the shear modulus for α-WC was larger than that of β-WC, whereas the bulk modulus for α-WC was almost equal to that of β-WC.

NbN films are deposited using direct current reactive magnetron sputtering in discharge of a mixture of N2 and Ar gas, and the effects of substrate bias (Vb) on the preferred orientation, phase transition, and mechanical properties for NbN films are explored by x-ray diffraction, selective area electron diffraction, and nanoindentation measurements. It is found that Vb has a significant influence on the stress in NbN films, leading to a pronounced change in the preferred orientation, phase structure, and hardness. As the substrate is at voltage floating, the stress is tensile. In contrast, as negative Vb is applied, the stress becomes compressive, and increases with increasing the absolute value of negative Vb. It is observed that a phase transition from δ (face-centered cubic) to δ′ (hexagonal) for NbN films occurs as Vb is in the range of −80to−120V, which can be attributed to a decrease in the strain energy for NbN films. In order to explore the relationship between the stress and phase transition as well as preferred orientation, density-functional theory based on first principles is used to calculate the elastic constants and shear modulus for NbN with a structure of δ or δ′. The calculated results show that the shear modulus for δ′-NbN is larger than that for δ-NbN, whereas the bulk modulus for δ′-NbN is almost equal to that for δ-NbN, resulting in a difference in hardness for δ- or δ′-NbN single crystal.