The subgap absorption spectra for hydrogenated amorphous silicon carbide/silicon (a‐Si1−xCx: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 thea‐Si0.66C0.34:H/a‐Si:H heterojunction increased with increasinga‐SiC:H layer thickness, while that at thea‐Si0.50N0.50@B:H/a‐Si@B:H heterojunction showed no dependence on thea‐SiN:H layer thickness and was 1.4×1011cm−2. The defect density at thea‐Si1−xCx:H/a‐Si:H heterojunction increased to 1×1011cm−2with increasing carbon content in the 0.1–0.5 range. Hydrogen dilution of the starting gases of thea‐SiC:H layer was effective to decrease the interface defect density to less than 2×1010cm−2. A graded composition layer at thea‐SiC:H/a‐Si:H interface did not decrease the defect density.