Band bending at oxide surfaces induced by chemical doping or electric fields is known to create metallic surfaces with properties not found in the bulk, such as high electron mobility, magnetism or superconductivity. In this talk I show that photoexcitation of the ZnO(10-10) surface acts in direct analogy with chemical doping and induces band bending on ultrashort timescales. Using time- and angle-resolved photoelectron spectroscopy, I demonstrate that this “photodoping” effect results from the photoexcitation of deep donor-type defects at the ZnO surface. At low photon flux, surface-confined excitons are formed. Above a critical exciton density, a Mott transition occurs and the surface is metallized. Several properties that have been up to now only observed in the context of chemical doping are observed on the ultrafast time scale, including critical behaviour of the effective mass close to the transition point. These results form a great part of my PhD thesis and have recently been published in ref. [1].
[1] L. Gierster, S. Vempati & J. Stähler Ultrafast generation and decay of a surface metal, Nat. Commun. (accepted), https://arxiv.org/abs/2005.13424

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