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.