Amorphous intrinsic silicon carbide (a-SiCx:H(i)) films and amorphous phophorous doped silicon carbide (a-SiCx:H(n)) films deposited by plasma enhanced chemical vapor deposition (PECVD) from silane/methane mixtures provide excellent electronic passivation of p-type c-Si. Effective surface recombination velocities (Seff) lower than 23 cm s-1 have been reported for a- SiCx:H(i) films and Seff < 11 cm s-1 for a-SiCx:H(n) films. The analysis of the dependence of Seff on the injection level indicates that the good electronic passivation is due to field-effect passivation resulting from a high fixed charge (Qf) created in the a-SiCx:H film. In this work the absorption of SiH bonds in infrared transmission spectra of a-SiCx:H films is quantitatively analysed resulting in about 30% smaller amount of SiH bonds in phosphorous doped films compared to intrinsic films. Furthermore, a strong reciprocal correlation of the hydrogen content in the films and the Qf created at the a-SiCx:H/c-Si interface is observed.