Monolayer Doping of Silicon through Grafting a Tailored Molecular Phosphorus Precursor onto Oxide-Passivated Silicon Surfaces
Monolayer doping (MLD) of silicon substrates at the nanoscale is a powerful method to provide controlled doses of dopants and defect-free materials. However, this approach requires the deposition of a thick SiO2 cap layer to limit dopant evaporation during annealing. Here, we describe the controlled surface doping of thin oxide-passivated silicon wafers through a two-step process involving the grafting of a molecular phosphorus precursor containing a polyhedral oligomeric silsesquioxane (POSS) scaffold with silica-like architecture and thermal annealing. We show that the POSS scaffold favors the controlled formation of dopant-containing surface species with up to similar to 8 x 10(13) P atoms cm(-2) and efficiently avoids phosphorus evaporation during annealing for temperatures up to 800 degrees C. Silicon doping is demonstrated, in particular, by grafting the POSS phosphorus triester on SiO2/Si wafers with optimized surface preparation (thin SiO2 layer of 0.7 nm) and annealing temperature (1000 degrees C), which provides phosphorus doses of similar to 7 x 10(12) P atoms cm(-2) in the silicon substrates together with a decrease of their sheet resistance. A detailed study of the surface chemistry on SiO2 nanoparticles used as a high-surface-area model yields the grafting mechanism and the structure of the surface species. We show that the POSS scaffold is conserved upon grafting, that its size controls the final P-surface density, and that it behaves as a self-protecting ligand against phosphorus volatilization during the annealing step. We thus demonstrate that the use of custom-made dopant precursors with self-capping properties is a promising approach to tune medium to low doping doses in technologically relevant semiconductors.