Ultrashort and metastable doping of the ZnO surface by photoexcited defects

Shallow donors in semiconductors are known to form impurity bands that induce metallic conduction at sufficient doping densities. The perhaps most direct analogy to such doping in optically excited semiconductors is the photoexcitation of deep electronic defects or dopant levels, creating defect excitons (DX) which may act like shallow donors. In this work, we use time- and angle-resolved photoelectron spectroscopy to observe and characterize DX at the surface of ZnO. The DX are created on a femtosecond timescale upon photoexcitation and have a spatial extent of few nanometers that is confined to the ZnO surface. The localized electronic levels lie at 150 meV below the Fermi energy, very similar to the shallow donor states induced by hydrogen doping [Deinert et al., Phys. Rev. B: Condens. Matter Mater. Phys., 2015, 91, 235313]. The transient dopants exhibit a multi-step decay ranging from hundreds of picoseconds to 77 μs and even longer. By enhancing the DX density, a Mott transition occurs, enabling the ultrafast metallization of the ZnO surface, which we have described previously [Gierster et al., Nat. Commun., 2021, 12, 978]. Depending on the defect density, the duration of the photoinduced metallization ranges from picoseconds to μs and longer, corresponding to the decay dynamics of the DX. The metastable lifetime of the DX is consistent with the observation of persistent photoconductivity (PPC) in ZnO reported in the literature [Madel et al., J. Appl. Phys., 2017, 121, 124301]. In agreement with the theory on PPC [Lany and Zunger, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 72, 035215], the deep defects are attributed to oxygen vacancies due to their energetic position in the band gap and their formation by surface photolysis upon UV illumination. We show that the photoexcitation of these defects is analogous to chemical doping and enables the transient control of material properties, such as the electrical conductivity, from ultrafast to metastable timescales. The same mechanism should be at play in other semiconductor compounds with deep defects.