Electronic Relaxation Dynamics at the ZnO (10-10) Surface

ZnO is a remarkably versatile material with applications in catalysis, optoelectronics, and photovoltaics. ZnO nanowires and nanoparticle films have shown promise as nanoscale light sources, photodetectors, optical switches, and anodes in solar cells. Because of the intrinsic anisotropy of the wurtzite unit cell, a large fraction of the surface area in these nanostructured devices belongs to the low-energy {10-10} family of planes. Consequently, an understanding of excited state electron dynamics at these surfaces is essential for realization of such exciting technologies.

In this talk, I will present a detailed study of electron relaxation dynamics at the ZnO (10-10) surface using femtosecond time-resolved two-photon photoelectron spectroscopy (TR-2PPE). Efficient emission of longitudinal optical phonons results in electron lifetimes shorter than 30 fs within the Γ valley of the bulk conduction band, in agreement with simple perturbation theory calculations. At or below the band minimum, dynamics are accurately described by electronic relaxation within a quasi-continuum of defect-derived surface states whose density decreases exponentially into the band gap. Existence of these states is consistent with observed upward (n-type) band-bending and Fermi level pinning at the (10-10) surface.