Deficit semiconductor

Figure 3.7 Typical p-type metal-deficit semiconductor NiO with cation vacancies and positive holes.

As discussed earlier, NiO is ap-type cation-deficit semiconductor and, therefore, the cations will migrate with electrons from the scale-metal interface to the scale-gas interface during oxidation. Correspondingly, there will be a flow of defects, cation vacancies and electron holes, in the opposite direction. Consequently, the driving force for the reaction will be reflected by the concentration gradient of cation vacancies across the scale. The nickel vacancies are formed according to Equation (4.2),... 

The wustite phase, FeO, is a p-type metal-deficit semiconductor which can exist over a wide range of stoichiometry, from Feo.950 to Feo.ggO at 1000 °C according to Engell. With such high cation-vacancy concentrations, the mobilities of cations and electrons (via vacancies and electron holes) are extremely high. 

Cobalt forms two oxides, CoO and C03O4, of NaCl and spinel structures, respectively CoO is a p-type cation-deficit semiconductor through which cations and electrons migrate over cation vacancies and electron holes. In addition to the usual extrinsic defects, due to deviations from stoichiometry above 1050 °C, intrinsic Frenkel-type defects are also present. The variations of oxidation-rate constant with oxygen partial pressure and with temperature are, therefore, expected to be relatively complex. Consequently, it is important to ensure that very accurate data are obtained for the oxidation reactions, over a wide range of oxygen pressure and temperature. 

FIGURE 30.15 Schematic representation of energy ieveis in a deficit semiconductor such as Cu2-xO. 

In the final chapter of this volume, Van de Walle reviews the theoretical information that is available on isolated, interstitial hydrogen and muonium in crystalline semiconductors. Given the limited direct experimental information available on isolated, interstitial hydrogen and the vital contributions that muonium studies have made in confronting this deficit, it is clear that theory is a particularly essential tool for progress on this topic. Van de Walle first reviews the principal calculational techniques... 

Most of the modern theories of the photoconductivity sensitization consider that local electron levels play the decisive role in filling up the energy deficit The photogeneration of the charge carriers from these local levels is an essential part of the energy transfer model. Regeneration of the ionized sensitizer molecule due to the use of the carriers on the local levels takes place in the electron transfer model. The existence of the local levels have now been proved for practically all sensitized photoconductors. The nature of these levels has to be established in any particular material. A photosensitivity of up to 1400 nm may be obtained for the known polymer semiconductors. There are a lot of sensitization models for different types of photoconductors and these will be examined in the corresponding sections. 

In the so-called semiconductors, such as ZnO, Cu20, etc., the bands are just filled for the perfectly pure substance at low temperatures. Conduction can only occur if the number of electrons is increased (excess conduction or N-type semiconductors, Fig. 28E), which extra electrons find a place in a free band, or if the number of electrons is decreased, whereby a hole is produced in the filled band (defect conduction or P-type semiconductors, Fig. 28F). Such a deficit is displaced in an electric field like an electron with a positive charge. Such a change in the number of electrons, more correctly in the number of electrons per lattice unit, is produced by deviations from the stoichiometric composition. 

Surface-localized redox reactions can be viewed as the transfer of an electron between one particular surface metal ion and an adsorbed molecule, with a change in the oxidation state of the metal ion. This is a reasonable description if electrons have no mobility in the mineral that is, if the mineral is an insulator. However, some minerals are semiconductors or conductors, in which case electron transfer might be better described as insertion of electrons into (or extraction of electrons from) the overlapping electronic orbitals of the solid. The resultant electron excess or deficit is then delocalized over the solid, not associated with one particular metal ion at one surface location. 

A similar treatment can be applied to the oxidation of cobalt to CoO. Cobalt monoxide is a metal deficit p-type semiconductor forming cation vacancies and electron holes according to Equation (3.59), where K <) = Cw Ch pX. ... 

Thus, it is a metal-deficit, p-type semiconductor in a wide range of sulfur activities. Only at very low pS2 near the Mn/MnS equilibrium it is a metal-excess, n-type semiconductor with doubly ionized interstitial cations and quasi-free electrons [57, 58]. The growth of MnS proceeds by outward diffusion of cations, being the rate-determining step of manganese sulfidation. The low nonstoichiometry is the reason the MnS growth is several orders of magnitude slower than that of other transition metal sulfides [59, 60]. 

Epitaphial effects of a scale can influence diffusivity as does any defect such as porosity, grain boundaries, cracks, dislocation substructures, etc. Impurity cations can have a great effect on diffusivity in the oxide depending on the valence of the impurity ion and the semiconducting properties of the scale. Common scales formed from oxides, sulfides, and nitrides can be classified as p-type, n-type, or amphoteric semiconductors. The p-type, metal-deficit scales are nonstoichiometric with cation vacancies present. Impurity ions with valencies greater than the p-type semiconductor will tend to increase the concentration of cation vacancies and, hence, diffusivity. Lower vacancy ions will have the opposite effect. Impurity ions with the same valence should have little effect on diffusion. The n-type semiconductors... 

Most metal oxides are nonstoichiometric., such as Feo. O instead of the ideal molecular formula FeO. This characteristic may be due to the different concentration of cations (Cc) and anions (Go). If Gc > Go, the metal oxide is an n-type semiconductor since there is metal-excess. On the other hand, if Gc < Go, then a p-type semiconductor occurs due to metal-deficit condition. 

To allow commercial applications of sensors at various potential emission sites, the construction of these sensors from solid materials is desirable, so as to minimize the size of the sensors and simplify the manufacturing process. To date, a number of small CO2 gas sensors have been developed, and these may be categorized by their sensing mechanism, whether based on optical cells, resistance/capacitance of semiconductors, or electromotive force (EMF)/current measurements based on solid electrolytes. However, such sensors continue to exhibit deficits, including low selectivity, poor chemical and physical stability, or high cost, and these problems must be mitigated to improve their usefulness. 

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