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Surface states are states caused by the incomplete covalent bonds at the surface of a semiconductor. They result in electron energy levels within the energy bandgap. These states are also called Tamm-Shockley states. The density of such bonds at a silicon surface is about 1015 cm-2, a rather large number considering that the density of donors in a 0.1 mm depletion region in a material with a doping density of 1017 cm-3 is only 1012 cm-2.
Surface states can be donor states or acceptor states depending on whether or not they are neutral when occupied. While surface states are present at any semiconductor surface they can also be caused by impurities such as oxygen. For example arsenic oxide has been linked to the surface states in Gallium Arsenide located at about one third of the energy bandgap above the valence band. These surface states cause the Fermi energy to be pinned: electrons from the valence band fall into the surface states until the Fermi energy coincides with the level to which the surface states are filled. This causes a natural surface depletion in gallium arsenide and a potential across the semiconductor of about 0.6 V. Contacting gallium arsenide with different metals yields a barrier height around 0.8 V independent of the workfunction of the metal.
© Bart Van Zeghbroeck 1998