We report experimental results on the dependence of conductivity on thickness in hydrogenated amorphous silicon and hydrogenated amorphous silicon carbon thin film alloys. The analysis of multilayer structures indicates that band bending at the vacuum-him interface accounts for most of the conductivity changes. The film-substrate interface seems to play a minor role in the conductivity experiments. The behavior of the sub-gap absorption coefficient of thin films shows that the material near the interfaces and in the bulk has the same defect density.