We have developed an ultrahigh vacuum plasma‐enhanced chemical‐vapor deposition system, and deposited high‐purity device‐quality hydrogenated amorphous silicon films. High sensitivity secondary ion mass spectrometry measurements show that impurity contents in the bulk of the present films are reduced to 2×1015 cm−3 for O, 7–10×1015 cm−3 for C, and 5×1014 cm−3 for N; these impurities are normally present at fairly high levels. Nevertheless, extensive light soaking of the films resulted in a defect density as high as 5×1017 cm−3, which is well above the impurity content. This result excludes those models of photoinduced degradation that postulate one‐to‐one correlation between light‐induced defects and O, C, or N impurity atoms. © 1996 American Institute of Physics.