Figure 4.

a) The trapping of electrons at the surface of ZnO results in the formation of a space-charge region. b) Photogenerated holes in the ZnO can recombine with trapped electrons, releasing adsorbed species and reducing the space charge and associated band bending at the surface. c) A thicker space-charge region is expected for the ZnO:N, which should enhance the de-trapping process and result in a greater reduction in the surface's work function upon UV illumination (as indicated by the change in surface potential, DSP).