Two-equivalent and one-equivalent surface states

Two concepts are discussed for the control of semiconductor surface properties by chemical surface state additives. The first is that additives can be used which lead to both filled and empty states, and both are required to control the surface Fermi level. Chemically this is equivalent to adding the impurity in two different oxidation states (valences). It is shown that if one uses this approach, accurate control of the surface Fermi level should be possible for stable systems. The second concept is that it is important to distinguish between two-equivalent and one-equivalent species for use as surface states. The former are chemicals in which a relatively unstable oxidation state exists between the stable oxidation states of interest. The latter (one-equivalent) are chemicals with adjacent stable oxidation states, i.e. stable forms separated by only one electronic charge. Analysis shows that two-equivalent impurities often will behave like minority carrier trapping centers.