The uniform and reproducible activation of silicon, ion-implanted directly into GaAs substrates, is often difficult to achieve. Epitaxial GaAs buffer layers have been used as an alternative implantation media which offers improved electrical characteristics. The characteristics of Si implanted GaAs buffer layers grown by the metalorganic chemical vapor deposition technique are presented here. The influence of the gas phase stoichiometry, a key determinant in the electrical properties of the layer, on the characteristics of the Si implanted and capless annealed layers was studied over the implantation dose range of 3×1012 to 1×1014 cm-2 at an implant energy of 150 keV. The electrical activation, mobility, deep-level concentration, and impurity distribution both prior to and after the implant and anneal were determined through optical and electrical characterization techniques. Undoped GaAs grown by this technique provides reproducible high levels of implant activation. Implantation into thick buffer layers allows the study of the implant and anneal processes free from the complicating influence of the GaAs substrate.