Co-implantation schemes for sub-65 nm n-type extension formation have been explored. These included phosphorous (P) and arsenic (As) implants into crystalline Si as well as combinations using higher energy silicon pre-amorphization implant (PAI) steps together with low energy carbon (C) implantation. Dopant energies were varied between 500 eV and 1 keV, with C dose of 1 X 10(15) cm(-2) and P and As doses of 7 X 10(14) cm(-2). The anneal step was a fast ramp-up and ramp-down spike at 1050 degrees C. Comparisons with dopant-only implants or with PAI and dopant only showed these two schemes to give deeper, more gradual and less well activated junctions than when combined with C. The best result was obtained using a 25 keV Si PAI, 6 keV C and 1 keV P, resulting in a junction depth of 20.5 nm at 1 X 10(19) cm(-3), and R-s=318 Omega/rectangle and a 3 nm/dec trailing slope. (c) 2006 American Vacuum Society.