Thermal Degradation in Ultrathin Films Outperforms Dose Control of n-Type Polymeric Dopants for Silicon

Polystyrene samples with different molar mass and narrow polydispersity index were prepared by nitroxide-mediated polymerization using N-tert-butyl-N-[1-diethylphosphono(2,2-dimethylpropyl)] nitroxide (SG1) as a radical controller and phosphorus containing moiety, and their doping performances were studied. In contrast to what is observed for conventional "grafting to" reactions, the thickness of the grafted layer is observed to rapidly increase with time, reaching a thickness plateau value that is significantly lower than 2 times the radius of gyration of the corresponding polymer chain. In addition, the plateau thickness initially increases as the molar mass of the grafting polymer increases and then reaches a somewhat limiting thickness. Interestingly, the phosphorus areal dose on the silicon surface is equal to the density of the grafted chains estimated from the thickness of the brush layer. The overall data indicate that during the high-temperature thermal treatment two parallel and competing reactions are operating, namely, the "grafting to" reaction and a degradation of the SGI moiety that leads to fragments, as confirmed by direct exposure probe analysis. This combination results in a fine-tuning of the phosphorus dose at the substrate surface with a substantial outperforming of the dopant dose control, achieved by using conventional self-assembled monolayers.