The effect of surface proximity on electron transport through ultra-shallow image-doped layers in silicon

We perform a systematic study of the electron transport properties of δ-doped phosphorus layers in silicon as a function of encapsulation thickness. We find that as the encapsulation thickness is reduced from 24 to 0 nm, the carrier density decreases, dropping off rapidly below 8 nm encapsulation. This translates to a corresponding reduction in the mobility as the ionized dopants are not screened as effectively at low densities. We observe good transport through the δ-doped layers down to 4 nm encapsulation. However layers with 2.5 nm encapsulation or less were prohibitively resistive.