Impurity conduction

To dissociate molecules in an adsorbed layer of oxide, a spillover (photospillover) phenomenon can be used with prior activation of the surface of zinc oxide by particles (clusters) of Pt, Pd, Ni, etc. In the course of adsorption of molecular gases (especially H2, O2) or more complex molecules these particles emit (generate) active particles on the surface of substrate [12], which are capable, as we have already noted, to affect considerably the impurity conductivity even at minor concentrations. Thus, the semiconductor oxide activated by cluster particles of transition metals plays a double role of both activator and analyzer (sensor). The latter conclusion is proved by a large number of papers discussed in detail in review [13]. The papers cited maintain that the particles formed during the process of activation are fairly active as to their influence on the electrical properties of sensors made of semiconductor oxides in the form of thin sintered films. 

It was shown in a number of works [29] that impurity conductivity of thin zinc oxide films are extremely sensitive to adsorption of atoms of various metals (see Chapters 2 and 3). Using this feature of oxide films, we first employed the sensor method to study evaporation of superstechiometric atoms of metals from metal oxide surfaces, zinc oxide in particular [30]. 

The above results once again make clear the reason for relaxation of impurity conductivity of a zinc oxide film alter termination of a beam of metal particles used for doping the surface of these films. Additionally, these results contain earlier suppositions of molecular particles of metals being electrically inactive. 

Miller A, Abrahams E (1960) Impurity conduction at low concentrations. Phys Rev 120 745... 

Impurity conduction can also be studied in compensated semiconductors, i.e. materials containing acceptors as well as donors, the majority carriers (or the other way round). For such materials, even at low concentrations, activated hopping conduction can occur (Chapter 1, Section 15), some of the donors being unoccupied so that an electron can move from an occupied to an empty centre. Here too a metal-insulator transition can be observed, which is certainly of Anderson type, the insulating state being essentially a result of disorder. 

Impurity conduction metal-insulator transitions in impurity bands... 

The phenomenon now known as impurity conduction was first observed by Hung and Gleismann (1950) as a new conduction mechanism predominant at low... 

Summary of Features of Impurity Conduction in Silicon and Germanium... 

At the early stages the photoconductivity of solid solutions of the leucobase of malachite green in various organic media was investigated [285]. In these systems, carrier transport occurs by direct interaction between the leucobase molecules. No direct participation of the organic matrix in the charge transfer was observed. A model was proposed which links charge transfer in these systems with impurity conduction in semiconductors. 

The presence of the region of weak dependence of the conductivity of alloyed semiconductors on temperature can be explained by tunneling of electrons from one impurity centre to another, unoccupied centre. The necessary condition of the impurity conductivity is the partial filling of the impurity levels. At low temperatures this conduction can be maintained only by semiconductor compensation, i.e. by the simultaneous presence of donor and acceptor impurities. In the case, for instance, of the n-type semiconduc-... 

The process by which the semiconductor carriers reach the surface to react with surface states must be considered. The case of greatest importance under photoexcitation is with the semiconductor biased to depletion as shown in Figure 1. While it is possible for semiconductor carriers to reach the surface of the semiconductor through tunneling, or impurity conduction processes, these processes have not been shown to be important in most examples of photoexcited semiconductor electrodes. Consequently, these processes will be ignored here in favor of the normal transport of carriers in the semiconductor bands. Furthermore, only carriers within a few kT of the band edges will be considered, i.e., "hot" carriers will be ignored. 

There is a growing tendency to invoke surface states to explain electron transfer at semiconductor-electrolyte interfaces. Too frequently the discussion of surface states is qualitative with no attempt to make quantitative estimates of the rate of surface state reactions or to measure any of the properties of these surface states. This article summarizes earlier work in which charge transfer at the semiconductor-electrolyte interface is analyzed as inelastic capture by surface states of charge carriers in the semiconductor bands at the surface. This approach is shown to be capable of explaining the experimental results within the context of established semiconductor behavior without tunneling or impurity conduction in the bandgap. Methods for measuring the density and cross section of surface states in different circumstances are discussed. 

Extrinsic semiconductors ate those in which the carrier concentration, either holes or electrons, are controlled by intentionally added impurities called dopants. The dopants are termed shallow impurities because their energy levels lie within the band gap close to one or other of the bands. Because of thermal excitation, -type dopants (donors) are able to donate electrons to the conduction band and p-type dopants (acceptors) can accept electrons from the valence band, the result of which is equivalent to the introduction of holes in the valence band. Band gap widening/narrowingmay occur if the doping changes the band dispersion. At low temperamres, a special type of electrical transport known as impurity conduction proceeds. This topic is discussed in Section 7.3. 

Another characteristic feature of disordered systems is the variable range hopping (VRH) mechanism to the electrical conduction, which is observed on the nonmetallic side of the M-NM transition at low temperamres. This arises from the hopping of charge carriers between localized states, or impurity centers. Hence, the phenomenon is also known as impurity conduction. Experimentally, it is indicated by characteristic temperature dependency to the d.c. conductivity. Eor three-dimensional systems with noninteracting electrons, the logarithm of the conductivity and are linearly related, in accordance with the equation given by Mott (Mott, 1968)... 

Meyer T. J. (1983), Excited-state electron transfer , Prog. Inorg. Chem. 30, 389 40. Meyer T. J. and Newton M. D. (1993), Electron Transfer, North-Holland, Amsterdam. Miller A. and Abrahams E. (1960). Impurity conduction at low concentrations , Phys. Rev. 120, 745-755. 

The peak value of Z exists at the peculiar temperature in the impurity conduction range in general. 

With regard to chemical and physical-chemical specifications (contents of additives and impurities, conductivity, acid conductivity and pH) more and more sophisticated, reliable and sensitive systems have been developed [1]. 

For normal semiconductors n = 1, but the form of impurity conduction found in bolometer material is better characterized by =0.5 (Summers and Zwerdling [3.37], Redfield [3.38]). From (3.21) the resistance temperature coefficient a is... 

The electronic conductivity of pure, stoichiometric ZnO is still unknown. The concentration of foreign admixtures in undoped crystals is of the order of 10 -10 cm . Since Eg opt = 3.2eV and impurity ionization energies are about 0.01-0.1 eV at temperatures below 900 K, impurity conduction is always observed. At temperatures above 900 K, dissociation of the intrinsic material occurs. 

When the mean free path I is on the order of the distance between atoms, the free-electron approximation (i.e., Drude model) breaks down. This occurs particularly in noncrystaUine materials and for impurity conduction. In this situation, we make use of the Kubo-Greenwood formula [1176], which has universal validity. This formula uses elementary quantum mechanics to calculate the conductivity of a metal at frequency transition probabilities from occupied states below Epio empty states above Ep and then letting w tend to 0 for dc conductivity [1125, 1156, 1157]. The main steps in the calculation are as follows we suppose that an electric field F cos (ot acts on a specimen of volume fl. Since the quantity o mean rate of loss of ener per unit volume, the conductivity o(ti>) is given by the product of the following factors ... 

---