Epitaxial layers of GaAs grown by MBE or MOCVD and doped with silicon impurities have been studied using FTIR spectroscopy. Vibrational modes are observed for Si(Ga), Si(As), Si(Ga)-Si(As) and a defect labelled SI-X. This latter defect appears to be responsible for the compensation effects found in some highly doped material. The Si(Ga) mode is modified in AlGaAs but understood in terms of a simple statistical model. Epitaxial GaAs layers doped with silicon or beryllium are passivated by exposing the samples to an RF hydrogen plasma which leads to the formation of Si-H and Be-H close pairs.