We have carried out combined measurements of dark ESR, light-induced ESR and low photon energy absorption by Constant Photocurrent Method (CPM) for various thickness a-Si:H, a-Si1-x-.Nx Cx.:H and a-Si1-x-.Nx:H films. As a result, both the surface density of dangling bonds in the disordered surface layer and the density of dangling bonds in the bulk region were obtained discriminating the neuttal and charged dangling bonds. In a-Si:H, it is found that a large fraction of dangling bonds is charged ones. The incorporation of C decreases and the incorporation of N increases the fraction of charged ones. It is concluded from the present result that charged defects in a-Si1-xNx,:H are pairs of N+ 4and Si- 3. It is possible that charged defects in a-Si:H originate from a small amount of N contamination.