An efficient vacuum ultraviolet light source of large diameter has been developed for use in photoexcited processes. It has a novel structure in which microwaves propagating through a sapphire window break down Xe gas enclosed in a disk‐shaped volume and thereby produce intense emission from Xe at a wavelength of 147 nm. By optimizing the pulse width and the repetition rate, we obtained an illuminance of more than 3 mW/cm2at the window surface. When this lamp was used as a source for photoinduced chemical vapor deposition of hydrogenated amorphous silicon films, a deposition rate of 5 nm/min was attained.