Plasma treatment helps reveal how molecular coatings dope silicon

Controlling dopant levels near silicon surfaces is crucial for creating ultrashallow junctions and nanoscale devices. Mixed monolayer doping (MMLD) is a surface chemistry technique that combines dopant-containing molecules with non-dopant molecules to precisely regulate the amount of dopant near the silicon surface before it diffuses into the bulk.

A recent study from Ecole Polytechnique Federale de Lausanne explored the use of mixed monolayers of allyldiphenylphosphine (ADP) and 1-undecene to dope silicon. After grafting the molecules onto the silicon, the team subjected the sample to oxygen plasma ashing to remove any leftover carbon while preserving the phosphorus. X-ray photoelectron spectroscopy (XPS) confirmed that the plasma treatment effectively eliminated the carbon while largely maintaining the phosphorus content.

Kelvin probe force microscopy (KPFM) showed that as the concentration of ADP in the mixed monolayer increased, the silicon work function decreased systematically. The scientists also found that the way silicon dioxide (SiO₂) was applied—whether evaporated or sputtered—significantly affected the observed near-surface properties. Evaporated SiO₂ combined with O₂ plasma treatment yielded the most pronounced shift towards n-type behavior, while sputtered SiO₂ led to strong pinning of the work function.

Electrical characterization using four-point probe and Hall-effect measurements confirmed that increasing ADP content led to higher conductivity and carrier concentration. However, these bulk-integrated measurements did not show a significant difference between plasma-treated samples and untreated control samples, emphasizing the need to separate near-surface electronic properties from bulk electrical transport when evaluating monolayer-doped silicon.

The research demonstrates that plasma-assisted MMLD with small molecules can be a tunable approach to silicon doping while underscoring the importance of combining surface-sensitive techniques like XPS and KPFM with electrical characterization methods. Such precise control of near-surface doping is especially valuable for applications in nanoscale and emerging quantum devices.