• grain boundaries;
  • zinc oxide;
  • varistors;
  • scanning tunneling microscopy;
  • tunneling

Scanning tunneling microscopy (STM) and spatially resolved tunneling spectroscopy (TS) were used to observe correlations between the geometric structure and electrical properties of polycrystalline ZnO surfaces under ultrahigh vacuum. Constant current images revealed crystallographic features at a range of length scales, including facets which are hundreds of nanometers long and monoatomic steps ≅0.5 nm (≅5 Å) in height. Tunneling spectroscopy was used to identify individual ZnO grains, grain boundaries, and surface impurities. Areas of reduced conductivity which extend 5 to 40 nm (50 to 400 Å) on either side of the grain boundaries are attributed to associated space charge regions. This paper demonstrates that, when used together, STM and TS are powerful techniques for the study of the structure and electrical properties of single interfaces and grain boundaries.