Scanning tunneling microscopy on unpinned GaN(1100) surfaces: Invisibility of valence-band states

We investigated the origins of the tunnel current in scanning tunneling microscopy (STM) and spectroscopy experiments on GaN(1100) surfaces. By calculating the tunnel currents in the presence of a tip-induced band bending for unpinned n-type GaN(1100) surfaces, we demonstrate that only conduction-band states are observed at positive and negative voltage polarities independent of the doping concentration. Valence-band states remain undetectable because tunneling out of the electron-accumulation zone in conduction-band states dominates by four orders of magnitude. As a result band-gap sizes cannot be determined by STM on unpinned GaN(1100) surfaces. Appropriate band-edge positions and gap sizes can be determined on pinned surfaces.