Three photoluminescence (PL) bands at 340—370,400—430, and 740 nm were observed at room temperature in a-Si∶H∶O films fabricated by plasma enhanced chemical vapor deposition without any post-processing. The violet-blue emission is very strong and stable, and its intensity is closely related to the oxygen content in the films, which can be controlled by the applied dc biases on the sample substrates during deposition. The first two PL peaks are ascribed to oxygen-related colour centers in the a-Si∶H∶O matrix, and the last one is ascribed to the quantum size effect of the nanocrystallites embedded in a-Si∶H∶O matrix and the colour centers in the crystallites interfaces.