Oxide fixed charge

The carriers in tire channel of an enhancement mode device exhibit unusually high mobility, particularly at low temperatures, a subject of considerable interest. The source-drain current is carried by electrons attracted to tire interface. The ionized dopant atoms, which act as fixed charges and limit tire carriers mobility, are left behind, away from tire interface. In a sense, tire source-drain current is carried by tire two-dimensional (2D) electron gas at tire Si-gate oxide interface. 

Simple Models. The surface chemical properties of clay minerals may often be interpreted in terms of the surface chemistry of the structural components, that is, sheets of tetrahedral silica, octahedral aluminum oxide (gibbsite) or magnesium hydroxide (brucite). In the discrete site model, the cation exchange framework, held together by lattice or interlayer attraction forces, exposes fixed charges as anionic sites. 

The pH value at which the oxide surface carries no fixed charge, i.e. Oj = 0, is defined as the point of zero charge (PZC) . A closely related parameter, the isoelectric point (lEP), obtained from electrophoretic mobility and streaming potential data, refers to the pH value at which the electrokinetic potential equals to zero The PZC and lEP should coincide when there is no specific adsorption in the iimer region of the electric double layer at the oxide-solution interface. In the presence of the specific adsorption, the PZC and lEP values move in opposite directions as the concentration of supporting electrolyte is increased. ... 

The Si intermediate complexes generated from the oxidation of a part of the excess Si bonds in this transition region are responsible for fixed charge and fast surface states. They may be formed according to the reaction... 

Metal hydroxides in general are anion-selective in acid solution and turn to be cation-selective beyond a certain pH, called the point of the iso-selectivity, pHpjS it is pHpjS = 10.3 for ferric oxide and pHpis = 5.8 for ferric-ferrous oxide [72]. Adsorption of multivalent ions may also control the ion selectivity of hydrous metal oxides because of its effect on the fixed charge in the oxides. For instance, hydrous ferric oxide, which is anion-selective in neutral sodium chloride solution, turns to be cation-selective by the adsorption of such ions as divalent sulfate ions, divalent molybdate ions, and trivalent phosphate ions [70,73]. It is worth emphasizing that such an ion-selectivity change due to the adsorption of multivalent ions frequently plays a decisive role in the corrosion of metals. 

It has been consistently observed that wet oxidation has produced less fixed charge and fewer interface traps than dry oxidation [5-7,31]. The presence of H20 molecules appears to be critical to the formation of high quality SiC MOS capacitors [5,7]. Dry oxide on 3C-SiC is not quite stoichiometric Si02 [27]. 

The specific adsorption of H, OH, cations, and anions on hydrous oxides and the concomitant establishment of surface charge can be interpreted in terms of the formation of surface complexes at the oxide-water interface. The fixed charge of the solid surface and the pH of its isoelectric point can be measured experimentally by determining the proton balance at the surface (from alkalimetric titration curve) and by the analytical determination of the extent of adsorbate adsorption. Equilibrium constants established for the surface coordination reactions can be used to predict pHiEp, to calculate adsorption isotherms, and to estimate concentration-pH regions for which the hydrous oxide dispersions are stable from a colloid-chemical point of view. 

Experimental Measurement of Surface Charges. If the fixed charge of an oxide particle a (C m" ) arises from the specific adsorption of H, OH", cations, and anions by the hydrous oxide surface, it is possible to determine (in principle) its value by determining experimentally (analytically) the extent of adsorption of charged species ... 

Phosphate. The binding of phosphate to hydrous oxides, especially AI2O3 and FeOOH, is also characterized by a proton release and a shift of lEP to lower pH values. With goethite (a-FeOOH) dispersed in phosphate solutions, the fixed charge was computed as a function of pH from titration curves (surface proton balance) and from analytic information (phosphate adsorbed) (18) (Figure 8). Reasonable agreement with electrokinetic data was obtained. 

Figure 9. Exemplification of the method used in deriving the fixed charge of an oxide in absence of specifically adsorbable ions, surface acidity constants, and surface potential from alkalimetric-acidimetric titration curves.

Shift in lEP. The intrinsic constants may be used to estimate the composition of the oxide surface as a function of solution variables (pH, concentration of specifically adsorbable cations and anions). Without correcting for coulombic attraction or repulsion such calculations should give reasonable predictions only for surfaces that have a fixed surface charge of zero (or nearly zero). Hence, it should be possible to use intrinsic constants to predict shifts in lEP caused by specific cation and anion binding. Equation 20 gives the condition for zero fixed charge (lEP). 

The expression for the flat band barrier height is modified when there is a charge present in the interfacial oxide layer. Considering the case of the n-type semiconductor, if there is a fixed charge per unit area within the oxide layer then Equation [3.21] is replaced by ... 

Figure 9.4 Polarity change of fixed charges in pentane sulphonate/PPy membrane (left) and PVS/PPy membrane (right) for oxidized and reduced states, as determined by membrane potential (A ) measurements in a KCl concentration cell ( ) Membrane as prepared, ( ) reduced membrane at — l.OV vs SCE, (O) oxidized membrane at +2.0V. A negative value of AT indicates the existence of a fixed positive charge in the membrane.

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