  We have studied the effect of nitriding on the humidity sensing properties of
hydrogenated amorphous carbon (a-C:H) films. The films were prepared in two
stages combining the techniques of physical deposition in vapor phase
evaporation (PAPVD) and plasma pulsed nitriding. By deconvolution of the Raman
spectrum we identified two peaks corresponding to the D and G modes
characteristic of a-C:H. After the N$_2$-H$_2$ plasma treating, the peaks
narrowed and shifted to the right, which we associated with the incorporation
of N into the structure. We compared the sensitivity to the relative humidity
(RH) of the films before and after the N$_2$-H$_2$ plasma treatment. The
nitriding improved the humidity sensitivity measured as the low frequency
resistance.
  By impedance spectroscopy we studied the frequency dependence of the AC
conductivity $\sigma$ at different RH conditions. Before nitriding
$\sigma(\omega)\sim A \omega^s$, it seemed to have the universal behaviour seen
in other amorphous systems. The humidity changed the overall scale $A$. After
nitriding, the exponent $s$ increased, and became RH dependent. We associated
this behaviour to the change of the interaction mechanism between the water
molecule and the substrate when the samples were nitriding.
