Surfaces band bending

In several particular cases the solution of equation (1.15) enables one to express the value of the volume charge through the degree of the surface band bending qUg, followed (through applying the condition of electric neutrality to the whole absorbate-adsorbent system) by deduction of adsorbent s SCR characteristics of interest. 

Solution of equations (1.26) and (1.27) for above two limiting cases differing in the value of the surface concentration of adsorption particles brings about different dependencies of the value of the surface band bending as a function of parameters of the absorbate-adsorbent system. Thus, in case of adsorption of acceptors we obtain from (1.26) that... 

This expression indicates that increase in the surface band bending ( > 0) is accompanied by decrease in conductivity, which attains the minimum under value = n N f / br ). Above this value the... 

In general, the peculiarities of the surface effects in thin semiconductors, for which application of semi-infinite geometry becomes incorrect were examined in numerous papers. As it has been shown in studies [101, 113, 121 - 123] the thickness of semiconductor adsorbent becomes one of important parameters in this case. Thus, in paper [121] the relationship was deduced for the change in conductivity and work function of a thin semiconductor with weakly ionized dopes when the surface charge was available. Paper [122] examined the effect of the charge on the temperature dependence of the work function and conductivity of substantially thin adsorbents. Papers [101, 123] focused on the dependence of the surface conductivity and value of the surface charge as functions of the thickness of semiconductor and value of the surface band bending caused by adsorption and application of external field. 

In the domain of interest Ng < Ngi where the recharging of BSS takes place we arrive to the following expression for the value of the surface band bending... 

The theoretical models of effects of recharging of the surface on the band diagram in the surface-adjacent domain of semiconductor adsorbent accompanying adsorption have been developed. The effect of the surface band bending in semiconductor adsorbent on its electrophysical characteristics caused by transition phenomena have been studied. The theories of adsorption-caused response of above characteristics were derived for both ideal monocrystalline adsorbent [4] and monocrystal with... 

To understand the role of the noble metal in modifying the photocatalysts we have to consider that the interaction between two different materials with different work functions can occur because of their different chemical potentials (see [200] and references therein). The electrons can transfer from a material with a high Fermi level to another with a lower Fermi level when they contact each other. The Fermi level of an n-type semiconductor is higher than that of the metal. Hence, the electrons can transfer from the semiconductor to the metal until thermodynamic equilibrium is established between the two when they contact each other, that is, the Fermi level of the semiconductor and metal at the interface is the same, which results in the formation of an electron-depletion region and surface upward-bent band in the semiconductor. On the contrary, the Fermi level of a p-type semiconductor is lower than that of the metal. Thus, the electrons can transfer from the metal to the semiconductor until thermodynamic equilibrium is established between the two when they contact each other, which results in the formation of a hole depletion region and surface downward-bent band in the semiconductor. Figure 12.6 shows the formation of semiconductor surface band bending when a semiconductor contacts a metal. 

Figure 12.6 Plot showing the formation of semiconductor surface band bending when a semiconductor contacts a metal (Ec, the bottom of conduction band Ev, the top of valence band EF, the fermi energy level SC, semiconductor M, metal Vs, the surface barrier). (From Liqiang, J. et al., Solar Energy Mater. Solar Cells, 79, 133, 2003.)...

Anpo and coworkers (Anpo, 2004 Anpo and Che, 1999) investigated the PL behavior of Ti02 photocatalysts in the presence of various kinds of reactants. The dependence of the PL intensity on the nature of the atmosphere was explained in terms of surface band bending of Ti02 particles, its extent depending on the electronegativity or electroaffinity of the reactant molecules. Furthermore, such an effect on the PL intensity was found to be reversible after elimination of the reactant molecules by... 

The effects are due to surface band bending induced by charge from the adsorbed molecules. It is known that water acts in this way, with... 

Fig. 9.14. Illustration of the mechanism of surface band bending by adsorbed water molecules which act as electron donors.

Fig. 61. Schematic description of the surface band bending of ZnO (top) and TiOa (bottom). 1, after degassing in vacuo (n-type semiamductors) 2, after adsorption of H O (decrease of band bending by formation of positively charged species on the surface) 3, after adsorption of O2 (increase of band bending by the formation of negatively charged species on the surface) [reproduced with permission from Anpo et a . (224)].

Jung, M.Y.L., Gunawan, R., Braatz, R.D. and Seebauer, E.G. (2004b) Effect of Near-Surface Band Bending on Dopant Profiles in Ion-Implanted Silicon./. Appl. Phys., 95, 1134-1139. 

The driving force behind this reaction is the difference between the Fermi level Ep in diamond and the chemical potential ge of electrons in the aqueous adsorbate layer. As long as ge < Bp. electrons exit from the diamond surface in the course of reaction (Eq. 6.1) taking place. Once ge = Bp, equilibrium is achieved. Thus a space charge is generated in the uppermost layer of the diamond film, which leads to the so-called surface band bending (Figure 6.31). 

Figure 6.31 The surface conductivity of diamond films arising from surface band bending ( APS 2000).

Chemiavskaya, O. et al.. Photoionization of individual CdSe/CdS core/sheU nanocrystals on silicon with 2-nm oxide depends on surface band bending. Nano Lett. 3, 497-501, 2003. 

The classical, state-of-the-art preparation technology for SMOX-based gas sensors - thick, porous sensing layers - is not the best choice for p-type materials. In their case, the direct readout of the changes in the surface band bending would be more efficient in the case of a resistive readout, thin, compact films with electrodes deposited on the top would be more appropriate. 

The dependence of the conductance on the surface band bending in the case of the accumulation layer can therefore be described by ... 

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