Adsorption of acceptor particles

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 

Accounting for the fact that according to the definition of barrier. 1 for (f) we have 

Here and in what follows N( and stand for the values reduced to the product NpLo- 

Substituting (1.66) into expression (1.64) leads to the fact that far from the equilibrium position owing to either full occupation of adsorption surface states or with leveling off in Ea with the Fermi level of adsorbent Ep solution of the latter can be written as 

Note that the large amount of experimental data makes it possible to assume that processes related to the transfer of the charge to the surface states formed during adsorption of acceptors on oxidated oxides develop much slower than the process of formation of the proper adsorption surface states and, therefore, they are the limiting stage of the process of charging of the surface [18, 20], Thus, in this case one can consider that Nfit) = Nt = const and expression (1.67) can be written as 

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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. 

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... 

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 ... 

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... 

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]. 

Let us dwell on existing key models describing chemisorption induced response of electric conductivity in semiconductor adsorbent. Let us consider both the stationary values of electric conductivity attained during equilibrium in the adsorbate-adsorbent system and the kinetics of the change of electric conductivity when the content of ambient atmosphere changes. Let us consider the cases of adsorption of acceptor and donor particles separately. In all cases we will pay a special attention to the issue of dependence of the value and character of signal on the structure type of adsorbent, namely on characteristics of the dominant type of contacts in microcrystals. 

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... 

All the aforesaid on the adsorption of acceptor atom and molecular particles lends support to the general conclusion of suitability of the designed sintered semiconductor oxide films (mainly ZnO) as highly sensitive semiconductor sensors meant to quantitatively detect extremely low concentrations of atom and molecular acceptor and donor particles in a concentration range of 10 -10 atoms per cubic centimetre in the volume adjacent to a semiconductor sensor. 

Physics of energy levels in metals and semiconductors Surface chemistry of intermediate radicals on surface and adsorption Spectroscopy of acceptor particles, gives energy levels for electrons Hydrodynamics of flow of solution, transports ions to surface... 

As for equilibrium values of as and P they are mainly dependent on relations between such parameters of the systems as initial electric conductivity of adsorbent, concentration of chemisorbed particles, reciprocal position of the energy levels of absorbate and adsorbent. Thus, during acceptor adsorption in case of small concentration of adsorption particles one can use (1.82) and (1.84) to arrive to expressions for equilibrium values of ohmic electric conductivity and the tangent of inclination angle of VAC ... 

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). 

Therefore, the activation energy of quasi-equilibrium conductivity changes as a logarithm of concentration of adsorption particles which, when the linear dependence between Nt and P is available, corresponds to situation observed in experiment [155]. We should note that due to small value m function (1.91) satisfactorily approximates the kinetics oit) A - B n(i + t/t>) observed in experiments [51, 167, 168]. Moreover, substantially high partial pressures of acceptor gas, i.e. at high concentrations of Nt expression (1.81) acquires the shape ait) Oait/toc) it,Nty " when t>toc>. This suggests that for... 

Accounting for BSS recharging having high importance for adsorption of the donor particles proper [84, 98], which can be done similarly to acceptor case yields... 

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. 

Fig. 2.1. Plausible mechanisms of influence of metal particles deposited on adsorbent on adsorption-caused response of its electrophysical characteristics a - chemical sensitization the dope particles cause the activation and spill-over of adsorbate b - electronic sensitization the dope particles become donors or acceptors of electrons in dependence on the conditions in gas phase.

Adsorption of molecular acceptor particles. Molecular oxygen... 

Electric effects detected in semiconductor oxide films during chemi-sorbtion of atom particles have been also thoroughly studied for chemi-sorbtion of various free radicals CH2, CH3, C2H5, C6H5OH2, OH, NH, NH2, etc. [41]. It was discovered that all of these particles have an acceptor nature in relation to the electrons of dope conductivity in oxide semiconductors their adsorption, as a rule, being reversible at elevated temperatures. It is clear that we deal with reversibility of electron state of the oxide film after it has been heated to more than 250-300°C in... 

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