Impurity: also acceptor

Calcium carbonate comes from natural sources such as bone, limestone, shells, or it can be made synthehcally. It can be pure enough to take as medicine (anhacid), or it can be ground up rock with many impurities (also used as anhacid). It may be the most widely used filler. (A similar claim can be made for carbon black or carbon in ah its many forms.) Calcium carbonate is widely used in PVC as an acid acceptor, in polypropylene as a filler, and in many thermosets and thermoplashcs as an inexpensive filler. 

The presence of impurities also will affect the concentrations of charged native defects. Consider an elemental crystal A in which vacancies act as acceptors ... 

In n type semiconductors, electrons are tire majority carriers. Holes will also be present tlirough accidental incoriioration of acceptor impurities or, more importantly, tlirough tlie intentional creation of electron-hole pairs. Holes in n type and electrons in p type semiconductors are minority carriers. 

Electrochemical reactions at semiconductor electrodes have a number of special features relative to reactions at metal electrodes these arise from the electronic structure found in the bulk and at the surface of semiconductors. The electronic structure of metals is mainly a function only of their chemical nature. That of semiconductors is also a function of other factors acceptor- or donor-type impurities present in bulk, the character of surface states (which in turn is determined largely by surface pretreatment), the action of light, and so on. Therefore, the electronic structure of semiconductors having a particular chemical composition can vary widely. This is part of the explanation for the appreciable scatter of experimental data obtained by different workers. For reproducible results one must clearly define all factors that may influence the state of the semiconductor. 

Molinari and Parravano (30) have also noted that the incorporation of a donor impurity (Al, Ga) into ZnO specimens promotes the exchange reaction, while an acceptor impurity (Li) slows it down. 

Suppose now that the introduction of an acceptor impurity (increase of ev and es ) brings us from the point A to the point C (Fig. 9). This involves an increase in K, as seen from Fig. 9. This is in agreement with the results obtained by the same authors (41), who observed an increase in the photocatalytic effect on silica gel when acceptor impurities were added to the catalyst, and also with the data of Lunsford and Leland (42) who found that the effect was enhanced on MgO with increasing concentration of V-centers (acceptors). 

Thus, Romero-Rossi and Stone (11) have found that the effect is enhanced on ZnO when an acceptor impurity (Li) is introduced into the specimen. The increase of the effect on Cu20 upon the introduction of acceptor impurities (S and Sb) has also been observed by Ritchey and Calvert (58). The addition of a donor (Cr) to ZnO, as reported (11), lowers the magnitude of the effect. 

Most of the other metal-related deep levels in Si are also passivated by reaction with hydrogen (Pearton, 1985). Silver, for example, gives rise in general to a donor level at Ee + 0.54 eV and an acceptor level at Ec - 0.54 e V (Chen and Milnes, 1980 Milnes, 1973). These levels are very similar to those shown by Au, Co and Rh and raise the question of whether Au might actually be introduced into all of the reported samples or a contaminant, or whether as discussed by several authors there is a similar core to these impurity centers giving rise to similar electronic properties (Mesli et al., 1987 Lang et al., 1980). This problem has not been adequately decided at this time. It has been... 

Impurity substitution that is effectively neutral, that is, neither donor nor acceptor, can also lead to significant changes in properties that are utilized in NTC thermistors. For example, the replacement of Ga3+ in the spinel MgGa204 by Mn3+ involves no apparent donor or acceptor action. The conductivity in the system MgGa2 Jt.MnJt.04 evolves from insulating (conductivity about 10-9 0 1m 1) for the parent phase with x = 0, to a conductivity approximately equal to that of germanium (10 10-1 m-1) in the compound MgGaMn04, in which x = 1. The resistivity decreases markedly with temperature and the compounds display typical NCT behavior. 

Besides the effect of solvent polarity, the C=C rotation in many push-pull ethylenes is sensitive to acid catalysis (143). This is probably explained by protonation of the acceptor groups, for example, the oxygen atoms in C=0 groups (16), which increases their acceptor capacity. Small amounts of acids in halogenated solvents, or acidic impurities, may have drastic effects on the barriers, and it is advisable to add a small quantity of a base such as 2,4-lutidine to obtain reliable rate constants (81). Basic catalysis is also possible, but it has only been observed in compounds containing secondary amino groups (38). 

Two types of impurities should be distinguished namely, acceptor and donor impurities which play the part of traps (i.e., localization centers) for the free electrons and the free holes, respectively. It should be especially stressed that foreign particles dissolved in the crystal may act as acceptors or donors depending not only on their nature, but also on whether they enter the lattice (interstitially or substitutionally). For example, the interstitial Li atoms in the NiO lattice are donors, but the same Li atoms when replacing the Ni atoms act as acceptors. In the case of a substitutional solution, the foreign atoms of a given type may be either acceptors or donors depending on the lattice in which they are dissolved. For example, Ga atoms are donors in the ZnO lattice and acceptors in the Ge lattice. Thus, if the adsorbed particles are, say, acceptors, the same particles when dissolved in the volume of the crystal may act as donors, and vice versa. 

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. 

We depart briefly from our discussion of SI GaAs to consider an example that better illustrates some of the features of temperature-dependent Hall measurements. This example (Look et al., 1982a) involves bulk GaAs samples that have sc — F — 0-15 eV. We suppose, initially, that the impurity or defect controlling the Fermi level is a donor. Then any acceptors or donors above this energy (by a few kT more) are unoccupied and any below are occupied. Also, p n for kT eG. From Eq. (B34), Appendix B, we get... 

Energy levels corresponding to electrons localized near the surface may also be present. These are termed surface states. For example, adsorbed ions are one type of surface state they may be of the form of donors, such as hydrogen, which yield electrons to the material, or in the form of acceptors, such as oxygen, which accept, or trap electrons from the material. In Fig. 1, surface traps of the acceptor type are shown, and it is indicated that there are two possible levels present. Other possibilities are impurity atoms at the surface, which are introduced in the preparation of the sample or diffuse from the interior during heat treatment, or nonstoichiometry of the surface layers of the compound. Surface... 

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