  Band bending at semiconductor surfaces induced by chemical doping or electric
fields can create metallic surfaces with properties not found in the bulk, such
as high electron mobility, magnetism or superconductivity. Optical generation
of such metallic surfaces via BB on ultrafast timescales would facilitate a
drastic manipulation of the conduction, magnetic and optical properties of
semiconductors for high-speed electronics. Here, we demonstrate the ultrafast
generation of a metal at the (10-10) surface of ZnO upon photoexcitation.
Compared to hitherto known ultrafast photoinduced semiconductor-to-metal
transitions that occur in the bulk of inorganic semiconductors, the
metallization of the ZnO surface is launched by 3-4 orders of magnitude lower
photon fluxes. Using time- and angle-resolved photoelectron spectroscopy, we
show that the phase transition is caused by photoinduced downward surface band
bending due to photodepletion of donor-type deep surface defects. At low photon
flux, surface-confined excitons are formed. Above a critical exciton density, a
Mott transition occurs, leading to a partially filled metallic band below the
equilibrium Fermi energy. This process is in analogy to chemical doping of
semiconductor surfaces. The discovered mechanism is not material-specific and
presents a general route for controlling metallicity confined to semiconductor
interfaces on ultrafast timescales.
