Surface Steps, Defects, Band Bending

Acceptor type surface states in the band gap trap electronic charge from the bulk. A band bending evolves accompanied by a depletion layer beneath the surface, (c) Donor type surface states donate charge to the conduction band. A band bending evolves accompanied by an accumulation layer beneath the surface. 

The energy position of the surface state in the band gap then determines the position of the Fermi level. The density of surface states necessary to pin the Fermi level is pretty low, as compared to the atomic density at the surface. For instance, on GaAs, the density of atoms on the (110) plane amounts to 8.85 x 10 cm. The Fermi level pinning arises for a density of surface states as low as lO cm, which means less 

Some of the ionized bulk impurities localized in the atomic layers immediately beneath the surface are also visible in the STM image, as their positive charge appears as dark depressions in the STM image. Additional bright blobs in the STM image are local defects on the surface, for instance, adsorbates or point defects. 

Indeed, on As-terminated (2 x 4) reconstructed GaAs(OOl) surfaces, itwas shown that surface defect states are generated by kinks in the As dimer rows [116,117]. Low n-doped GaAs(OOl) thus shows a rather well-ordered As row structure, because few 

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On a real semiconductor surface, defects, impurities, steps, and Idnks occur, for instance, due to contamination or due to structural defects. They are associated with localized electronic states, distinct to the band structure of the reconstructed surface. Even if the concentration is low, they may have a large impact on the electronic properties of a semiconductor surface, as they may accumulate charge to the surface and induce a surface electronic band bending, which is quite an important aspect of semiconductor surfaces and interfaces [13, 27]. We will discuss this issue in Section 13.5.4. 

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