Layer enrichment

Figure 9.2. Constitutional supercooling in alloy solidification (a) phase diagram (b) solute-enriched layer ahead of the solid/liquid interface (c) condition for a stable interface (d) condition...

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. 

In the anoxic zone, heterotrophic respiration of particulate Mn02 and Fc203 or FeOOH causes manganese and iron to be reduced to Mn (aq) and Fe (aq). As dissolved ions, these trace metals diffuse through the pore waters. The ions that diffuse upwards will reenter the oxic zone, where they react with O2 to reform the oxyhydroxides. This produces a metal-enriched layer that lies just above the redox... 

Evidence also exists for a terrestrial source of the iridium enrichment as volcanic ejecta is enriched in this rare element. Thus, the enriched sediment layer could also have been caused by an abrupt and large increase in volcanic activity. Evidence for this is suggested by high levels of volcanic ash, soot, and shocked minerals in the iridium-enriched layer. Other geochemical characteristics of this sediment layer appear to have been caused by acid rain and tsunamis, both of which are by-products of volcanic activity. 

On this low current branch, concentration variations within the charged layers are small (of order Cq). The condition (4.3.17d) merely says that in order for the lower limiting current to exist, the total concentration gain er in the enrichment layers (i — 1,4), has to be smaller than the appropriate total concentration decrease in the depletion layers (i = 2,3). (According to (4.3.15), the total concentration variation within each layer is proportional to the passing current /, fixed charge density Ni, and the thickness A of the layer.) In particular, (4.3.17d) implies the existence of some critical iV4 (N 4r) beyond which the low current branch with saturation ceases to exist. 

The cross section for the 3H(maximum value at only 107 KeV incident deuteron energy. When thick ( 1 mg cm-2 thick deposit of titanium) titanium-tritium targets are used, however, the neutron yield continues to increase even above 200 KV acceleration potential. This is due to increased penetration of the deuteron beam into the tritium enriched layer. Since the penetration of molecular deuterium ions is less than that for monatomic deuterium ions for the same acceleration potential, accelerators using Penning ion sources require extremely clean vacuum systems to minimize build-up of deuteron absorbing deposits on the surface of the target. 

The common theme of this article was hence the formation of surface enrichment layers in thin films and their effect on the phase transitions mentioned above, with emphasis on the accompanying interface formation i.e., the lamel-... 

For the polymer blend with symmetric walls, approaching the bulk critical temperature from the disordered region there occurs a completely smooth and gradual formation of surface enrichment layers on both walls (Fig. lc), without any phase transition. The unmixing phase transition involves formation of con-... 

J. F. DEWALD (Bell Telephone Laboratories) I do not believe that the effect of semiconductivity on catalysis is completely contained in either the Schottky or the Mott-Schottky approximations. Consider a supported metal catalyst. If one has an exhaustion layer on an n-type support then the total charge in the exhaustion layer would be very small. The potential, acting over so large a distance, would have very little effect on catalysis. If, however, you can cause an electron enrichment layer in the vicinity of the metal-n-type contact, you might expect very much larger catalytic effects, for there would be much larger electric fields near the point of contact. 

The multilayered character of acetonitrile adsorption creates a pseudo-stationary phase of significant volume on the surface, which acts as a suitable phase for the ion accumulation. In the low organic concentration region (from 0 to 20 v/v% of acetonitrile), studied ions show significant deviation from the ideal retention behavior (decrease in ion retention with increase in acetonitrile composition) due to the formation of the acetonitrile layer, and significant adsorption of the chaotropic anions was observed. This creates an electrostatic potential on the surface in which there is an adsorbed acetonitrile layer, which provides an additional retentive force for the enhancement of the retention of protonated basic analytes. When the dielectric constant is lower than 42 [167], this favors the probability of ion pair formation in this organic enriched layer on top of the bonded phase. 

Hellmann R. (1997a) The albite-water system Part IV. Diffusion modelling of leached and hydrogen-enriched layers. Geochim. Cosmochim. Acta 61, 1595—1612. 

The HP treatment of Si N prepared by nitrogen implantation with dose, D > 1X1 o cm, produces continuous buried insulating nitrogen-enriched layer. 

The MPF and TMGM prospecting methods are based on the use of metallo-organics (fulvates and humates of metals) and oxides of iron and manganese (metals bound in oxides and hydroxides of iron and manganese). These forms of metals are the result of the secondary fixation of the movable fomis in rocks and have features such as (1) increased concentration coefficient and (2) only a weak bond with their initial geological source (in comparison, for example, with the movable forms collected in CHIM and MDE). Samples for MPF are taken from the humus-enriched layer at a depth of 5-10 cm, and samples for TMGM are taken from the sand-clay layer at a depth of 15-20 cm. 

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