Semiconductors enrichment layer

The variation of the electrostatic potential surface region entails a bending of the bands, since the potential contributes a term —eo4>(x) to the electronic energy. Consider the case of an n-type semiconductor if the value s of the potential at the surface is positive, the bands band downwards. We set 4> = 0 in the bulk of the semiconductor and the concentration of electrons in the conduction band is enhanced (see Fig. 7.4). This is called an enrichment layer. If cj)s < 0, the bands bend upward, and the concentration of electrons at the sur-... 

Mutatis mutandis the same terminology is applied to the surface of p-type semiconductors. So if the bands bend upward, we speak of an enrichment layer if they bend downward, of a depletion layer. 

Figure 7.4 Band bending at the interface between a semiconductor and an electrolyte solution (a)-(c) n-type semiconductor (a) enrichment layer, (b) depletion layer, (c) inversion layer (d)-(f) p-t.ype semiconductor (d) enrichment layer, (e) depletion layer, (f) inversion layer.

From a chemical point of view a hole at the surface of a semiconductor entails a missing electron and hence a partially broken bond. Consequently semiconductors tend to dissolve when holes accumulate at the surface. In particular this is true for enrichment layers of p-type material. At the depletion layers of n-type materials the holes required for the dissolution can also be produced by photoexcitation. 

Significant depletion of In, enrichment in iodine and simultaneous reduction of selenium to Se° was observed by XPS for n-CuInSe2 photoelectrode surfaces in an I2—1 —Cu+—HI electrolyte [129]. Over a distance of 100 A the composition of the surface layer gradually changes to that of the substrate. With the help of UPS analysis the surface film, assumed to be CuISe3, was characterized as p-type semiconductor [130],... 

If the negative terminal of the voltage source is connected to the metal (Fig. 2.10, left), then positive holes are accumiflated at the semiconductor surface. With opposite biasing (Fig. 2.10, centre), the positive holes are displaced from the semiconductor. As a result, a depletion layer is formed. If, however, the magnitude of the positive potential becomes high enough, electrons start to enrich at the semiconductor surface. An inversion layer is formed. The width... 

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