Ultrafast generation and decay of a surface metal

TL;DR

This paper demonstrates an ultrafast, low-flux photoinduced transition of ZnO surface to a metallic state via band bending and exciton Mott transition, offering a new method for rapid interface property control.

Contribution

It reveals a general, low-flux optical method to induce and control surface metallization in semiconductors through exciton dynamics and band bending.

Findings

Ultrafast metallic surface generation in ZnO upon photoexcitation.

Surface metallization driven by exciton Mott transition at critical density.

Mechanism analogous to chemical doping, applicable to other semiconductors.

Abstract

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.