Researchers from Ecole Polytechnique Federale de Lausanne used plasma treatment to help dope silicon surfaces. They mixed phosphorus molecules with others to tune the dopant levels. The study, published in Small Methods, showed that this method helps control near-surface properties, which is important for building better nanoscale and quantum devices.
Controlling dopant concentration near the silicon surface is increasingly important for ultrashallow junctions and nanoscale devices. Mixed monolayer doping (MMLD) offers a surface-chemistry-based approach in which dopant-containing molecules are mixed with nondopant molecules to tune the amount of dopant available at the silicon surface before thermal diffusion.
In a new study, researchers from Ecole Polytechnique Federale de Lausanne doped silicon using mixed monolayers of the phosphorus-containing molecule allyldiphenylphosphine (ADP) and 1-undecene. The team investigated the use of O₂ plasma ashing after molecular grafting to remove residual carbon while preserving phosphorus at the surface. X-ray photoelectron spectroscopy (XPS) confirmed that the plasma treatment reduced carbon content while largely retaining phosphorus.
Kelvin probe force microscopy (KPFM) revealed a systematic decrease in silicon work function with increasing ADP concentration. The results, published in the journal Small Methods, also showed that the SiO₂ capping method strongly influenced the measured surface response: evaporated SiO₂ combined with O₂ plasma treatment produced the clearest evolution toward n-type behavior, whereas sputtered SiO₂ resulted in strong work-function pinning.
Electrical characterization using four-point probe and Hall-effect measurements confirmed increasing conductivity and carrier concentration with increasing ADP content. However, no significant difference was observed between plasma-treated and untreated samples in these bulk-integrated measurements. This highlights the importance of distinguishing between near-surface electronic properties and bulk electrical transport when evaluating monolayer-doped silicon.
Overall, the work demonstrates plasma-assisted MMLD with small molecules as a promising, tunable approach to silicon doping while emphasizing the importance of combining surface-sensitive and electrical characterization techniques. Such control of near-surface doping is particularly relevant for nanoscale and emerging quantum-device applications.