Electron dynamics at the ZnO (10(1)over-bar0) surface

We use femtosecond time-resolved two-photon photoemission spectroscopy (TR-2PPE) to study the dynamics of electrons excited at the ZnO (10 (1) over bar0) surface. Efficient relaxation of hot electrons within the F valley of the bulk conduction band results in sub-30 fs lifetimes for electron energies greater than 0.1 eV above the conduction band minimum (CBM). These relaxation rates, which are among the fastest observed in any semiconductor over the same energy range, are consistent with the emission of longitudinal optical phonons resulting from strong Frohlich coupling. For energy at or below the CBM, the excited electron lifetime increases exponentially with decreasing energy to as long as 1 ps. Dynamics in this region can be described by electronic relaxation within a quasi-continuum of defect-derived surface states whose density decreases exponentially into the band gap. Deliberately increasing defects on the ZnO surface drastically decreases the lifetime of electrons in this energy region. Existence of these states is consistent with observed upward band-bending and Fermi level pinning at the (10 (1) over bar0) surface.