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Monolayer Doping of Silicon through Grafting a Tailored Molecular Phosphorus Precursor onto Oxide-Passivated Silicon Surfaces

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journal contribution
posted on 20.05.2016, 00:00 by Thibault Alphazan, Laurent Mathey, Martin Schwarzwälder, Tsung-Han Lin, Aaron J. Rossini, Raphaël Wischert, Virginie Enyedi, Hervé Fontaine, Marc Veillerot, Anne Lesage, Lyndon Emsley, Laurent Veyre, François Martin, Chloé Thieuleux, Christophe Copéret
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 ∼8 × 1013 P atoms cm–2 and efficiently avoids phosphorus evaporation during annealing for temperatures up to 800 °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 °C), which provides phosphorus doses of ∼7 × 1012 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.