p‐polarized 633 nm light reflection wasinsitumeasured to detect a change, if any, in optical constants of hydrogenated amorphous silicon (a‐Si:H) thin films by the application of 2 MHz ultrasonic vibration to the substrate during their plasma chemical vapor deposition (CVD). Although the vibration scarcely changed the hydrogen content ina‐Si:H films under the reaction conditions of this study, it increased both the refractive index and absorption coefficient ofa‐Si:H. The results are attributable to the densification of amorphous network structure as well as to the increase of density of extended states in the valence and conduction bands. Films with improved photoconductivity were obtained by the ultrasonic vibration assisted plasma CVD. ©1995 American Institute of Physics.