The bilayer structures composed of hydrogenated amorphous silicon (a‐Si:H) and hydrogenated amorphous silicon nitride (a‐Si3N4:H) are prepared on fused‐quartz substrates and their interfacial Si dangling bonds (DBs) are evaluated by electron spin resonance (ESR). The ESR study reveals that the bottom nitride (BN) structure (a‐Si:H/a‐Si3N4:H/fused quartz) has a smaller amount of neutral DBs in the interface than the top nitride (TN) structure (a‐Si3N4:H/a‐Si:H/fused quartz). The photoconductivity measurements also support this finding. A larger amount of charged DBs in the interface, however, exist in the BN structure than in the TN structure. The larger amount of interfacial neutral DBs in the TN structure is likely to be caused by the UV irradiation from the plasma during the deposition of a‐Si3N4:H layer. © 1995 American Institute of Physics.