The subgap absorption spectra for hydrogenated amorphous silicon carbide/silicon (a‐Si1-x Cx :H/a‐Si:H) and silicon nitride/silicon (a‐SiN:H/a‐Si:H) alternatively multilayered films were measured by photothermal deflection spectroscopy and the defect density at the heterojunction was evaluated from the increase in absorption shoulder with the number of layering periods. The defect density at the a‐Si0.66 C0.34 :H/a‐Si:H heterojunction increased with increasing a‐SiC:H layer thickness, while that at the a‐Si0.50N0.50 H/a‐Si H heterojunction showed no dependence on the a‐SiN:H layer thickness and was 1.4×1011 cm-2 . The defect density at the a‐Si1-x Cx :H/a‐Si:H heterojunction increased to 1×1011 cm-2 with increasing carbon content in the 0.1–0.5 range. Hydrogen dilution of the starting gases of the a‐SiC:H layer was effective to decrease the interface defect density to less than 2×1010 cm-2 . A graded composition layer at the a‐SiC:H/a‐Si:H interface did not decrease the defect density.