Wide-gap a-Si:H films with device quality (Tauc’s optical gap > 1.9eV, σ
ph under AMI.5, 100mW/cm2 illumination ≥ 10−5, Ω−1cm−1, a σ
ph/σ a≥106) have been fabricated. These films are deposited at low substrtate temperatures (TS≤80°C ) either by diluting SiH4 with H2 or optimizing the plasma parameters in a capacitively–coupled RF plasma–CVD reactor. Reduction in the SiH2 bond density and the ESR spin density are also observed. In this study, good film quality is always accompanied by a small deposition rate. Furthermore, σ
ph is nearly the same if the deposition rate and T
is the same, regardless of other deposition parameters. This suggests that the surface reactions or structural relaxations at the film-growing surface can produce high–quality a–Si:H films even at low Ts
, if the deposition rate is low. Results in thermal annealing, light exposure, and solar cell performance confirm that these films have device quality and wide bandgap.