Variations in the colour of group IV B nitride

The colour of TiN and HfN films made by activated vapour deposition methods is found to vary not only with composition but also with lattice expansion and distortion. An ionic model is discussed in which the colour of all the group IV B nitrides depends on the location of the plasma reflection edge. This is at about 3 eV and can be moved by changes in the charge carrier concentration caused by composition or lattice effects. The predictions of the model are in good qualitative agreement with experimental observations. The effect of intentional impurities is also discussed. (author)