Impurities donor

Resistivity measurements of doped, alpha-siUcon carbide single crystals from —195 to 725°C showed a negative coefficient of resistivity below room temperature, which gradually changed to positive above room temperature (45). The temperature at which the changeover occurred increased as the ionization of the donor impurity increased. This is beUeved to be caused by a change in conduction mechanism. 

For example, Heckelsberg and his associates (S3) have discovered that the introduction of a donor impurity (Al) into ZnO increases the reaction rate, while the addition of an acceptor impurity (Li) retards the reaction. 

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. 

The growth of the catalytic activity of Si02 with respect to the hydrogen-deuterium exchange reaction upon addition of a donor impurity to specimens has also been observed by Taylor and his colloborators (31). 

Holm and Clark (34) noted the increase of the activity of A1203 specimens with increasing amount of the Si02 impurity (donor impurity) further increase in impurity concentration, however, diminishes the activity. 

The introduction of donor impurities (Al, Ga) into zinc oxide increased both electrical conductivity and activity, while the addition of an acceptor (Li) lowers both conductivity and activity (30, 33). 

Thus, Kohn and Taylor (40) point out that the y irradiation of zinc oxide which speeds up the reaction of hydrogen-deuterium exchange lowers the magnitude of the effect when a donor impurity is introduced into the specimen. 

The introduction of an impurity into a crystal causes a displacement of the Fermi level both inside the crystal and, generally speaking, at its surface [in this case the Fermi level is displaced in the same direction both at the surface and in the bulk of the crystal, see reference (1) ]. This results, according to (63) and (5), in a change of g0. A donor impurity displaces the Fermi level upward, while an acceptor impurity shifts it in the opposite direction. The same impurity exerts diametrically opposite influences on the catalytic activity in acceptor and donor reactions. 

The great majority of experimental data (see Section III.A) indicate that the hydrogen-deuterium exchange reaction belongs to the class of acceptor reactions (i.e., reactions that are accelerated by electrons and decelerated by holes). This means that the experimenter, as a rule, remains on the acceptor branch of the thick curve in Fig. 8a, on which the chemisorbed hydrogen and deuterium atoms act as donors. Here a donor impurity must enhance the catalytic activity, while an acceptor impurity must decrease it. This is what actually occurs, as we have already seen (see Section III.A). 

Emphasis should here be placed on the observations of Holm and Clark (34), according to whom the reaction rate go passes through a maximum when the concentration of a donor impurity is monotonically increased. This maximum may be due, as shown in Fig. 8a, to the transition from the acceptor to the donor branch of the go = go ( [Pg.186]

The introduction of an impurity into a specimen (accompanied by a change in tv and es ) will transfer us from one point to another in Fig. 9. Suppose that when a donor impurity is introduced into the specimen (decrease in v and e8 ), we are transferred from the point A to the point B. This involves a decrease in K, as can be seen from Fig. 9. Such a decrease in the photocatalytic effect caused by the addition of donor impurities has been observed by Kohn and Taylor (40) who studied the photoreaction of hydrogen-deuterium exchange on zinc oxide exposed to y radiation. 

Suppose that the introduction of an acceptor impurity (increase of v and, -) transfers us from point A to point C, while in the case of addition of a donor impurity (decrease of v- and es ) we are transferred from the point A to B in Fig. 9. In this case, as is evident from Fig. 9, the acceptor impurity will enhance, and the donor impurity, weaken the photocatalytic effect. This is what has been observed by Romero-Rossi and Stone (11) on ZnO specimens and by Ritchey and Calvert (58) on Cu204 using Li, S, and Sb as acceptor impurities and Cr as a donor impurity. 

Note that the impurities exert opposite influences on the reaction in the dark. The oxidation of CO, like any acceptor reaction, is retarded by acceptor impurities (increase of tB ) and accelerated by donor impurities (decrease of e3 ). As a matter of fact, according to Parravano s data (61)... 

At the same time Markham and Laidler (70) and also Veselovsky and Shub (71, 72) have shown that the photocatalytic activity of zinc oxide diminishes as a result of the calcination of specimens at high temperatures (around 1000°C) in the reduced atmosphere (such pretreatment results in an increase of the concentration of superstoichiometric zinc in the specimen). In other words, a donor impurity (zinc in excess of stoichiometry) retarded the reaction. 

HYDROGEN NEUTRALIZATION OF SHALLOW-DONOR IMPURITIES IN ARSENIC-DOPED EPILAYERS ON SILICON... 

Figure 1.6 (a) Donor impurity (PS ) in a silicon crystal. (b) Donor energy levels below the... 

Another defect problem to which the ion-pair theory of electrolyte solutions has been applied is that of interactions to acceptor and donor impurities in solid solution in germanium and silicon. Reiss73>74 pointed out certain difficulties in the Fuoss formulation. His kinetic approach to the problem gave results numerically very similar to that of the Fuoss theory. A novel aspect of this method was that the negative ions were treated as randomly distributed but immobile while the positive ions could move freely. 

Extrinsic Semiconductors are materials that contain donor or acceptor species (called doping substances) that provide electrons to the conduction band or holes to the valence band. If donor impurities (donating electrons) are present in minerals, the conduction is mainly by way of electrons, and the material is called an n-type semiconductor. If acceptors are the major impurities present, conduction is mainly by way of holes and the material is called a p-type semiconductor. For instance in a silicon semiconductor elements from a vertical row to the right of Si of... 

Fig. 2-26. Localized electron levels of lattice defects and impurities in metal oxides Mi = interstitial metal ion Vm = metal ion vacancy V = oxide ion vacancy D = donor impurity A = acceptor impurity.

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. 

It follows from Equations (30), (32), and (33a,b) that acceptor reactions are accelerated by a donor impurity and retarded by an acceptor impurity, and vice versa in the case of donor reactions. Thus, a given impurity on a given catalyst may be a promoter for one reaction and a poison for another. This is frequently observed in practice. For example, the addition of LijO to ZnO promotes the reaction of dissociation of NjO (71) and at the same time poisons the reaction of oxidation of CO (60). 

For an n-type semiconductor, if the donor impurity concentration is much greater than the intrinsic carrier concentration, Nd >nj, then no Nd. Equation (3.4.9) can then be written as... 

A 50 mV drop in the potential of a ro = 6 nm colloidal Ti02 particle correlates to an ionized donor impurity concentration of 5 x lO cm , correlating to a Debye length Ld on the order of 10 m thus satisfying the condition ro Ld. 

The presence of a hydrogen donor impurity in the aluminum chloride was confirmed by the observation that HD was formed when deuterium alone was contacted with the catalyst. 

We next consider the case when impurities are present. The total number of electrons available for distribution among the various energy cells is then N t + Yji Nui, where the subscript D denotes a donor impurity or defect. 

Quite a different approach has been developed in some other works149, l52). It has been shown by extrapolation of the dependence of the initial reaction rate on the proton-donor concentration in the system to the zero concentration of the proton-donor that the initial reaction rate is expected to be equal to zero. The authors attempted to carry out precision vacuum cleaning and drying of the reagents and reaction vessels to remove proton-donor impurities. The results of this investigation are given in Fig. 14. As can be seen, even trace amounts of moisture have a great effect on the... 

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