Impact of Plasma Ashing on Mixed Monolayer Doping of Silicon

Mixed monolayer doping (MMLD) is an attractive strategy for controllable low-dose, low-energy silicon doping via surface chemistry, yet the influence of carbon removal and surface effects on electronic properties remains unclear. In this work, silicon is doped using mixed monolayers of allyldiphenylphosphine (ADP) and 1-undecene, followed by O₂ plasma ashing, SiO₂ capping, and rapid thermal annealing (RTA). The plasma treatment is systematically investigated, identifying conditions that effectively remove carbon while largely preserving phosphorus in the grafted layer, as confirmed by x-ray photoelectron spectroscopy (XPS). Kelvin probe force microscopy (KPFM) reveals a pronounced decrease in work function (WF) with increasing ADP molar fraction, and the SiO₂ deposition method plays a key role: evaporated SiO₂ combined with O₂ plasma leads to a WF consistent with enhanced n-type activation, whereas sputtered SiO₂ induces strong work-function pinning, independent of ADP concentration. Four-point probe and Hall measurements confirm increasing conductivity and sheet carrier density with ADP fraction, but show no significant dependence on plasma treatment, indicating that carbon-related effects are confined to the near-surface region. These results demonstrate that surface-sensitive WF changes do not necessarily reflect bulk dopant activation, and highlight the need to combine complementary characterization techniques for a reliable assessment of MMLD processes.