“Pinning” and Fermi level movement at GaAs surfaces and interfaces

Band bending at GaAs interfaces with either metals or insulators is of considerable importance. Recently a literature has developed, which shows for both metals and insulators, that this band bending can be modified, i.e., the Fermi level position E_flat the interface changes. Of the five metals studied in detail, Ag, Au, Al, Ti, and Cr, three (Au, Al, and Ti) have been shown to produce Fermi level changes on the order of 0.1 eV (i.e., change in the Schottky barrier height <p) on thermal annealing. For ft-type GaAs φdecreases for Au, and increases for Al and Ti. These changes of E_fican be explained in terms of the generation of excess As or Ga at the interface due to chemical reactions between the metal and the GaAs. The Fermi levels move toward the conduction-band minimum (CBM) when the As/Ga ratio is increased and toward the valence-band maximum (VBM) when it decreases. The changes in As/Ga ratio have been confirmed independently. No movement is seen for Ag, which is unique in having little reaction with GaAs, or for Cr, which bonds to both As and Ga. The Fermi level position can also be changed at the GaAs insulator interface. Na₂S H₂G and (NH₄) ₂S treatments of the GaAs surface reduce surface recombination. Recent studies show that such treatments increase rather than decrease the band bending on ft-GaAs. All of these results agree with a model in which As_Craand Ga_Asantisites dominate the interface and Fermi level changes are explained in terms of changes in the relative numbers of these defects.