ZnO—Ultrafast Generation and Decay of a Surface Metal

Band bending at semiconductor surfaces or interfaces plays a pivotal role in technology. Its control 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 band bending on ultrafast timescales would facilitate a drastic manipulation of the conduction, magnetic and optical properties of semiconductors for novel high-speed electronics. 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 the photoexcitation of deep surface defects. The resulting positive surface charges lead to downward band bending toward the surface. Above a critical excitation density, a metallic band below the equilibrium Fermi level is formed. This process is in analogy to chemical doping of semiconductor surfaces. Hence, it is not material-specific and presents a general route for controlling metallicity confined to semiconductor interfaces on ultrafast timescales.<br/><br/>[1] Nat. Commun. 12 978 (2021)