Effects of nitrogen flow rate on the preferred orientation and phase transition for niobium nitride films grown by direct current reactive magnetron sputtering

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.