Surface traps in vapor-phase-grown bulk ZnO studied by deep level transient spectroscopy

Deep level transient spectroscopy, current-voltage, and capacitance-voltage measurements are used to study interface traps in metal-on-bulk-ZnO Schottky barrier diodes (SBDs). c-axis-oriented ZnO samples were cut from two different vapor-phase-grown crystals, and Au- and Pd-SBDs were formed on their (0001) surfaces after remote oxygen-plasma treatment. As compared to Au-SBDs, the Pd-SBDs demonstrated higher reverse-bias leakage current and forward-bias current evidently due to higher carrier concentrations, which might have been caused by hydrogen in-diffusion through the thin Pd metal. The dominant traps included the well-known bulk traps E(3) (0.27 eV) and E(4) (0.49 eV). In addition, a surface-related trap, E(s) (0.49 eV), is observed but only in the Pd-SBDs, not in the Au-SBDs. Trap E, is located at depths less than about 95 nm and shows an electron capture behavior indicative of extended defects. A possible correspondence between trap E(s) and the well-known 2.45 eV green band is suggested by depth-resolved cathodoluminescence spectroscopy on the same samples, which reveals an increase in the intensity of this band within similar to 100 nm of the Pd/ZnO interface. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2978374]