A method for improving the ion beam quality in an ion implanter is disclosed. In some ion implantation systems, contaminants from the ion source are extracted with the desired ions, introducing contaminants to the workpiece. These contaminants may be impurities in the ion source chamber. This problem is exacerbated when mass analysis of the extracted ion beam is not performed, and is further exaggerated when the desired feedgas includes a halogen. The introduction of a diluent gas in the ion chamber may reduce the deleterious effects of the halogen on the inner surfaces of the chamber, reducing contaminants in the extracted ion beam. In some embodiments, the diluent gas may be germane or silane.