Binary Si: H alloy films having a wide optical band gap have been prepared by r.f. glow discharge of disilane. The optical band gap and the infrared absorption spectrum have been measured for those binary alloy films. The infrared absorption strength for 850 cm -1 peak and the vibrational freqcencies for all absorption peaks are increased with an increase in the optical band gap. This sugests the (SiH 2) n group formation in a wide optical gap film.