  Unusual quantum phenomena usually emerge upon doping Mott insulators. Using a
combinatorial molecular beam epitaxy system integrated with cryogenic scanning
tunneling microscopy, we investigate the electronic structure of a
modulation-doped Mott insulator Sn/Si(111)-($\sqrt{3}\times
\sqrt{3}$)-R30$^\textrm{o}$. In underdoped regions, we observe a universal
pseudogap opening around the Fermi level, which changes little with the applied
magnetic field and the occurrence of Sn vacancies. The pseudogap gets smeared
out at elevated temperatures and alters in size with the spatial confinement of
the Mott insulating phase. Our findings, along with the previously observed
superconductivity at a higher doping level, are highly reminiscent of the
electronic phase diagram in the doped copper oxide compounds.
