Intrinsic bandgap of cleaved ZnO(112¯) surfaces

The existence of intrinsic surface states, the position of the Fermi level, and the size of the surface bandgap of the non-polar ZnO(112¯0) cleavage surfaces were investigated by scanning tunneling microscopy and spectroscopy. The comparison of spectroscopic measurements performed on atomically flat and stepped surfaces reveals the absence of intrinsic surface states within the fundamental bulk bandgap, but shows the occurrence of step-induced gap states. These states lead to a pinning of the Fermi level at the surface within the bandgap and generate a significant defect-related tunnel current, narrowing the measured apparent bandgap.