Band bending (BB) at semiconductor surfaces or interfaces plays a pivotal role in technology, ranging from field effect transistors to nanoscale devices for quantum technologies. The control of BB via 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 metallic surfaces via BB on ultrafast timescales would facilitate a drastic manipulation of the conduction, magnetic and optical properties of semiconductors for novel high-speed electronics. Here, we demonstrate the ultrafast (20 fs) generation of a metal at the (10-10) surface of ZnO upon photoexcitation. This semiconductor is widely used in optoelectronics due to its transparency for visible light and its ease of nanostructuring. Compared to hitherto known ultrafast photoinduced semiconductor-to-metal transitions (SMTs) that occur in the bulk of inorganic semiconductors, the SMT at the ZnO surface is launched by 3-4 orders of magnitude lower photon fluxes; also, the back-transition to the semiconducting state is at least one order of magnitude faster than in previous studies of other materials. Using time- and angle-resolved photoelectron spectroscopy, we show that the SMT is caused by photoinduced downward surface BB due to photodepletion of deep surface defects. The resulting positive surface charges pull the conduction band below the equilibrium Fermi level, similar to chemical doping. The discovered mechanism is not material-specific and presents a general route for controlling metallicity confined to semiconductor interfaces on ultrafast timescales.
The original publication is available by link DOI: ?