Thin films of hydrogenated amorphous silicon deposited on glass and crystalline silicon substrates by rf PECVD at different rf power densities were studied using slow positron beam in order to verify the influence of that deposition parameter on the film defect structure. Other techniques, namely, constant photocurrent method (CPM), infrared spectroscopy (IR) and conductivity measurements, were used to determine the density of states (DOS), the relative concentration of the SiHn radicals and the photo-to-dark conductivity ratio of the films, respectively. It was found that mainly two kinds of vacancy type defects are present in the films: large vacancy clusters or voids and small vacancy type defects. Correlations observed between the positron results and the information obtained from the other techniques is discussed.