  An ultradense 2D electron system can be realized by adsorbing PH$_3$
precursor molecules onto an atomically clean Si surface, followed by epitaxial
Si overgrowth. By controlling the PH$_3$ coverage the carrier density of such
system can easily reach $\sim 10^{14}$ cm$^{-2}$, exceeding that typically
found in GaAs/AlGaAs structures by more than two-three orders of magnitude. We
report on a first systematic characterization of such novel system by means of
standard magnetotransport. The main findings include logarithmic temperature
dependence of zero-field conductivity and logarithmic negative
magnetoresistance. We analyzed the results in terms of scaling theory of
localization in two dimensions.
