Donor incomplete ionization and mobility enhancement in ultra-thin silicon-on-insulator films doped by phosphorus end-terminated polymers

Ex situ doping of ultra-thin silicon-on-insulator (SOI) substrates is performed by using polymers terminated with a doping containing moiety. The injection of P impurity atoms is investigated confining the same P dose of ∼ 1 × 10¹³ cm⁻² in a progressively thinner device layer, with thickness values (H_SOI) from 6 to 70 nm. The dopant concentration is determined by Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) measurements. Sample resistivity (ρ), carrier concentration (n_e) and mobility (μ) are determined combining sheet resistance and Hall measurements in van der Pauw configuration. Almost complete activation and full ionization of the injected dopants is observed at room temperature in the samples with H_SOI ≥ 30 nm. The ionization fraction progressively drops to 5% when reducing the thickness of the device layer. Dopant incomplete ionization is accompanied by an increase in electron mobility, with values significantly larger than those reported for bulk Si. In the SOI samples with H_SOI > 20 nm, the fraction of ionized P atoms at room temperature is perfectly described by the 3D bulk model of Altermatt et al. For H_SOI ≤ 20 nm, the bulk model must be corrected to account for the effect of interface states and dielectric mismatch between Si and surrounding SiO₂.