An ultrahigh vacuum electron cyclotron resonance (ECR) plasma source has been used to deposit a:Si, Si3N4, SiN(x), SiO(x)N(y), and SiO2 dielectric thin films on InP and related compound semiconductors for optoelectronic applications. Films are deposited without substrate heating using mixtures of SiH4, N2, O2, and Ar. Following deposition, thermomechanical properties have been studied by annealing to 450-degrees-C in N2. High temperature film stability is strongly influenced by the addition of Ar to the gas mixture, indicating an important role of ion bombardment during film growth by ECR plasma chemical vapor deposition. In addition, for applications as optical coatings for photonic devices, film refractive index can be accurately controlled by the N2/O2/Ar flow rate ratios.