Interface state density between a-Si:H and an insulating film (a-SiN 1.7 :H or a-SiO 2.0) was measured by photothermal deflection ͑PD͒ spectroscopy and electron spin resonance. While the interface state density in a-SiN 1.7 :H on a-Si:H structure was smaller than the free surface state density on a-Si:H, that in a-SiO 2.0 :H on a-Si:H structure was larger than the free surface state density of a-Si:H. The difference in the surface state density between these specimens was discussed in terms of plasma surface reaction. The effect of plasma reaction was examined by treating the surface of the a-Si:H layer by the plasma of NH 3 or N 2 O gas which were dominant constituents of the source gases used to deposit the insulating layers. The PD spectral shape of a-SiO 2.0 on a-Si:H was similar to N 2 O plasma-treated a-Si:H and that of the a-SiN 1.7 :H on a-Si:H structure was similar to NH 3 plasma-treated a-Si:H. These results indicate that the interface defects in the a-SiO 2.0 on a-Si:H structure were induced by the plasma reaction of the source gas with the surface of a-Si:H at the initial stage of deposition. The interface state densities in a-Si:H on a-SiN 1.7 :H and a-Si:H on a-SiO 2.0 interfaces were smaller than those of a-SiN 1.7 :H on a-Si:H and a-SiO 2.0 on a-Si:H, respectively. These differences in the interface defect density were due to the difference in the precursor used to deposit the upper layer. Chemical reaction on the surface of a-Si:H with a source gas induces interface defects at the initial stage of the deposition of the insulating layer. This surface chemical reaction was investigated by varying the condition of plasma species at the surface of a-Si:H.

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