Abstract
Thin films of NbSi
y
N
x
have been deposited by reactive magnetron sputtering from confocal Nb and Si targets in mixed Ar/N
2 atmosphere, at a substrate temperature of 250 °C. The total pressure, the nitrogen partial pressure and the current on the Nb target were kept constant, while the current on the Si target was varied in order to obtain Si concentrations between 1 and 34 at.%. For Si contents below 11 at.%, X-ray diffraction (XRD) reveals that the films are crystalline and have a fcc
δ-NbN structure. For higher Si concentrations, the films exhibit a multiphase structure consisting of
δ-NbN nanocrystallites and an amorphous matrix. The texture and the crystallite size depend on the Si content. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) measurements allowed to identify the amorphous phase as Si
3N
4. Transmission electron microscopy (TEM) observations reveal that the microstructure of films is columnar, and that each column is formed by an agglomerate of crystallites. Amorphous regions were observed on the high-resolution transmission electron microscopy (HRTEM) images of the films with
C
Si=11 and 20 at.%.
Nanoindentation measurements show that the hardness increases with increasing Si content up to 5 at.%, and reaches a maximum of 34 GPa. Above 11 at.% of Si, it decreases to the value comparable to that reported for amorphous Si
3N
4 (22 GPa). The hardness variation of the NbSi
y
N
x
films appears to be related to the formation of the amorphous Si
3N
4 phase and its volume fraction within the film. This behavior is similar to that observed in nc-MeN/a-Si
3N
4 nanocomposites (Me=Ti, W, V).