Acceptor particles

We should note that adsorption of acceptor particles on oxide semiconductors of p-type influences their electric conductivity and work function in the opposite way. As for donor particles such as atmns of H, Na, K, Zn, Cd, Pb, Ag, Fe, Ti, Pt, Pd and many others, their adsorption at medium and low temperatures (when there is no notable diffusion of atoms proper into the crystal and, consequently, there is no substitution of atoms created, the latter obeying the Vervey rule) is always accompanied by increase in electric conductivity and decrease in the work function for semiconductor adsorbent of -type, the opposite being valid in case of p-type adsorbent. 

Fig. 1.5. The energy diagram illustrating the surface-adjacent band bending caused by adsorption of acceptor particles on initially neutral surface...

Knowing the value of 5 enables one to obtain the profile distribution of the charge carriers through the depth of adsorbent. Thus, in case of adsorption of acceptor particles at small depths when were the condition > 1 holds the solution of equation (1.22) yields... 

Fig. 1.8. The dependence of the height of the surface barrier caused by adsorption of acceptor particles as a function of the density of BSS.

So far, we have focused our attention on adsorption of donor particles on semiconductor oxides. As for the effect of adsorption of acceptor particles on electrophysical characteristics, in concurrence with conclusions made none of adsorption phenomenon involving such characteristic acceptor particles as molecular and atom oxygen on -semiconductor, atoms of nitrogen and simplest alkyl and amine radicals brought about a non-monotonous change in characteristics of adsorbents, despite the fact that experiments had been conducted at various conditions. 

In case of adsorption of acceptor particles the kinetic equation describing the rate of change in density of ASS occupied by electrons can be written as... 

For different acceptor particle adsorption isotherms expressions (1.85) - (1.89) provide various dependencies of equilibrium values of <7s for a partial pressure P (ranging from power indexes up to exponential). Thus, in case when the logarithmic isotherm Nt InP is valid the expression (1.85 ) leads to dependence <75 P" often observed in experiments [20, 83, 155]. In case of the Freundlich isotherm we arrive to the same type of dependence of - P" observed in the limit case described by expression (1.87). 

We used polycrystalline films of ZnO and Sn02 as adsorbents. The films were deposited from the water suspension of respective oxides on quartz substrates. These substrates contained initially sintered contacts made of platinum paste. The gap between contacts was of about lO" cm. All samples were initially heated in air during one hour at T 500 C. We used purified molecular oxygen an acceptor particle gas. H and Zn atoms as well as molecules of CO were used as donor particles. We monitored both the kinetics of the change of ohmic electric conductivity and the tangent of inclination angle of pre-relaxation VAC caused by adsorption of above gases and the dependence of stationary values of characteristics in question as functions of concentrations of active particles. 

Having acknowledged a qualitative agreement in experimentally determined change in a and p during adsorption of acceptor particles with theoretical predictions we should comment the correct choice of parameter toe expressions (1.81) and (1.83) makes it feasible to obtain a... 

We should point out that up to now we have considered only polycrystals characterized by an a priori surface area depleted in principal charge carriers. For instance, chemisorption of acceptor particles which is accompanied by transition-free electrons from conductivity band to adsorption induced SS is described in this case in terms of the theory of depleted layer [31]. This model is applicable fairly well to describe properties of zinc oxide which is oxidized in air and is characterized by the content of surface adjacent layers which is close to the stoichiometric one [30]. 

Adsorption of molecular acceptor particles. Molecular oxygen... 

Assuming that interaction of acceptor particles (in our case oxygen with superfluous metal) provides the unique reason of the change of concentration of the latter in adsorption process we arrive at the following expression... 

Stationary concentration of adsorbed acceptor particles of O- and N-atoms on a film of zinc oxide is attained for the most part due to the competition between the chemisorbtion of particles and their interaction, i. e. mutual recombination on the adsorbent surface, and with free atoms attacking the adsorbed layer of the adsorbent from outside. 

Our comments on adsorption of oxygen and nitrogen atoms lead to conclusion that practically under all conditions the initial rate of variation of conductivity of zinc oxide film due to adsorption of acceptor particles discussed in this section is proportional to the concentration of particles in the space adjacent to the surface of oxide film. This is similar to the case of donor particles. This means that the following equation is applicable ... 

It should be noticed that a similar equation describes the effect on conductivity of ZnO film of such acceptor particles as atoms of nitrogen and oxygen, (see (3.12)). 

Similar expression is applicable to describe chemisorbtion of other acceptor particles (free atoms and radicals) [52], Major distinctions between the liquid and dry interfaces of sensors when detecting the above particles, provided that the experiment lasts long enough are ... 

Thus, it is quite natural to consider the properties of other acceptor particles, for example, atoms of nitrogen, aminoradicals, hydroxyl radicals, and many others, adsorbed on oxide semiconductors. However, the properties of these particles are not studied yet. As to adsorbed donor particles, it was found in our experiments that liquid media with different values of the dielectric constant do not have any influence on the properties of adsorbed atoms of hydrogen. 

Formulas (5) refer to the general case when the chemisorbed particles are both acceptors and donors. In the particular case of acceptor particles, putting... 

Let us now consider the case of acceptor particles. In this case, according to (7), ijo+ = 0. We limit ourselves to the region of ea values for which the following condition is satisfied ... 

Figure 7 pertains to the case of acceptor particles [formula (51a) ]. As to the donor particles [formula (51b)], the same pattern is observed, with the only exception that eY, es-, and ir must be replaced, respectively, by v+, e8+, and w+. 

Substituting (19) into the last relation and using (2) and taking into account that in our case vo+ = 0 (purely acceptor particles), one has... 

The appearance on the surface of any acceptor particles resulting in the negative charging of the surface and hence in the lowering of the Fermi level at the surface (in an increase of es- at v = const) must lead, according to (95) or (98), to the weakening of the photocatalytic effect. This is what actually occurs in the photosynthesis of hydrogen peroxide [jsee references (65-67, 71-73, 77)]. 

At each time step, and for a tunneling particle within each nuclear basis function, we check whether a tunneling event might be occurring. We first determine its current donor particle, defined as the current donor/acceptor particle to which it is closest. If this distance is shorter than RT, this particle... 

---