Silicon Doping by Polymer Grafting: Size Distribution Matters

Phosphorus δ-layers in SiO2 have been prepared by means of poly(methyl methacrylate) (PMMA), terminated with a phosphorus-containing moiety acting as an anchoring group. In particular, grafting of two P-terminated PMMA samples with Mn = 7.5 kg/mol (D̵ = 1.14) and Mn = 17.8 kg/mol (D̵ = 1.23) onto 10 nm thick SiO2 films deposited on Si substrates has been investigated, focusing on the thickness evolution of the brush layer as a function of the processing parameters, that is, annealing temperature and time. Upon removal of the polymer chains and subsequent encapsulation into a SiO2 matrix, the concentration of phosphorus atoms into the P δ-layers has been monitored by time-of-flight secondary ion mass spectrometry. The effective P dose in the P δ-layer is mainly dictated by the molecular weight of the P-terminated PMMA, and the doping process results are highly reproducible, provided that tight control over the experimental protocol is granted. However, although the grafting density is expected to progressively increase as a function of annealing time with a linear correlation between grafting density and thickness, the measured P dose in the δ-layers is observed to follow the opposite trend. This effect has been accounted for by considering a distortion of the molecular weight distribution of the grafted species with respect to the initial molecular weight distribution of the polymer. The overall picture reveals important information about the mechanism and dynamics governing the "grafting to"process of P-terminated PMMA polymers onto nondeglazed Si substrates.