Cross-sectional scanning tunneling microscopy and spectroscopy of nonpolar GaN(11¯00) surfaces

GaN ( 1 1 ¯ 00 ) cleavage surfaces were investigated by cross-sectional scanning tunneling microscopy and spectroscopy combined with calculations of the tunnel current. The different contributions to the tunnel current and their origins are determined by comparison of the experimental results with calculated tunnel currents for both pinned and unpinned GaN surfaces. It is found that both the N and Ga derived intrinsic dangling bond surface states are outside of the fundamental band gap and lead to electron tunneling out of the valence and into the conduction band, respectively. The band edges of both intrinsic surface states are located at the Γ¯ point of the surface Brillouin zone. The observed Fermi level pinning 1.0 eV below the conduction band edge is attributed to the high defect density at the surface, but not to intrinsic surface states. The defects give rise to a current due to electrons tunneling out of the defect states within the band gap.