Surface carrier density

Pore diameter and interpore spacing are determined by two large groups of factors those that affect carrier density on the surface of a pore bottom and those that affect only the distribution... 

The interaction of semiconductor with nanocarbon induces a modification of the intrinsic properties of semiconductor particles (band gap, charge carrier density, lifetime of charge separation, non-radiative paths, etc.) [1] as well as of the surface properties which were discussed in detail in the previous section. 

Where Vs is the potential value at the surface of the electrode. Then plotting the value of 1/Csc versus the applied potential E should yield a straight line whose intercept with the E axis represents the flat band potential, and the slope is used for the calculation of N, the charge carrier density in the semiconductor. A typical example of Mott-Schottky plot is given in Fig. 2 [7] in this graph, the extrapolated values of the fb potential are -1-0.8 V and —0.6 V vs. SCE for p-Si and n-Si respectively. 

At a semiconductor/solution interface, an n-type semiconductor (carrier density of 10 electrons cm A is in contact with a nonaqueous system using a redox system, i.e., no surface states. The capacity of this interface is 4 pF cm-2. Evaluate the potential differences within the semiconductor. (Bockris)... 

In metals, the concentration of mobile electrons is enormously high so that the excess charge is confined to a region very close to the surface, within atomic distances [14, 15]. In semiconductors with substantially less charge carrier density, on the other hand, a region of spatial charge distribution can be found [16, 17]. 

Table 6.3 Fermi surface dimensions and carrier densities of WC...

Localized states in the bulk of a semiconductor that have energies within the bandgap are known to capture mobile carriers from the conduction and valence bands.— The bulk reaction rate is determined by the product of the carrier density, density of empty states, the thermal velocity of the carriers and the cross-section for carrier capture. These same concepts are applied to reactions at semic ijiductor surfaces that have localized energy levels within the bandgap.— In that case the electron flux to the surface, F, reacting with a surface state is given by... 

Lastly a note on the chemical surface properties of 0. So far we have carried out only a few preliminary CDA experiments adding 5 vol.-% H2 to the N2. The observed decrease of the polarization and/ hence/ of the charge carrier density at the surface suggests that H2 consumes O , probably by way of oxidation H2 + 2 O" = H20 + O2". Further work will be required to study these reactions in more detail. 

In chemical terms, it may be proposed that the it electrons of the first graphite layer at the surface are localized at states which are separate from the bands of the bulk. Significant contribution of these states to the double layer capacitance necessitates an overpotential which is beyond the applied potential range (limited by water electrolysis). In terms of semiconductor theory, it may be assumed that the charge carrier density at the first and probably the second graphite layer is much lower than the bulk charge density value of 6 x 1018 carriers per cm3. 

In the presence of surface recombination, the minority carrier density at the surface is determined by the rate of their arrival from the collection region W + LP and the rate of their removal by the routes illustrated in Fig. 8.5. The concentration of holes accumulating at the surface can be expressed in terms of the equivalent surface concentration, ps (cm-2) = (Px=o since this allows a convenient formulation of the kinetic equations. Further simplification is achieved by considering the concentrations of redox species and majority carriers to be time invariant. This simplified scheme is illustrated for the case of an n-type semiconductor in Fig. 8.5. 

Thus, it can be seen that a study of the steady state photoelectrochemistry of colloidal semiconductors with the ORDE can provide information relating to the energy distribution of the particle surface states, the photogenerated carrier density and the quantum efficiency of carrier generation. The next section describes how to obtain information pertaining to intraparticle charge carrier dynamics from a study of the behaviour of transient photocurrents at the ORDE. 

This result is useful in understanding the variation of the field dependence of the TOF measured mobility from sample to sample, following the carrier density gradients (Fc dn/cbc). For example, the role of the diffusion carrier stream would explain the field dependence of jx in single crystals whenever their near-surface layer is strongly populated... 

Fig. 64. Calculation of surface recombination velocity vs. electrode potential. Fig. 65. Reciprocal surface recombination velocity vs, excess carrier density Snw.

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