Polarity-dependent pinning of a surface state

We illustrate a polarity-dependent Fermi level pinning at semiconductor surfaces with chargeable surface states within the fundamental band gap. Scanning tunneling spectroscopy of the GaN (10 (1) over bar0) surface shows that the intrinsic surface state within the band gap pins the Fermi energy only at positive voltages, but not at negative ones. This polarity dependence is attributed to arise from limited electron transfer from the conduction band to the surface state due to quantum mechanically prohibited direct transitions. Thus, a chargeable intrinsic surface state in the band gap may not pin the Fermi level or only at one polarity, depending on the band to surface state transition rates.