Infrared and spectroscopic ellipsometric studies have been done on thin films of hydrogenated amorphous silicon (a-Si:H), germanium (a-Ge:H) and silicon---germanium alloy of a typical composition (a-Si28Ge72:H) prepared by reactive ion beam sputtering (RIBS). Films deposited at increasing H2:Ar flow ratio generally show enhanced dihydride and polyhydride bonding and a decreased value of 2max. A correlation of the two results reveals that the increase in hydrogen content affects the growth process considerably and makes the films density deficient and inhomogeneous by increasing the volume fraction of voids, network strains and vacancies. The a-Si28Ge72:H films deposited at increasing H2:Ar flow ratio show a preferential attachment of hydrogen to silicon rather than to germanium. It has been found that RIBS-prepared films are more density deficient as compared to films deposited by glow discharge, owing mainly to the presence of dihydride species.