Dopants diffusion rate

Among the alkali metals, Li, Na, K, Rb, and Cs and their alloys have been used as exohedral dopants for Cgo [25, 26], with one electron typically transferred per alkali metal dopant. Although the metal atom diffusion rates appear to be considerably lower, some success has also been achieved with the intercalation of alkaline earth dopants, such as Ca, Sr, and Ba [27, 28, 29], where two electrons per metal atom M are transferred to the Cgo molecules for low concentrations of metal atoms, and less than two electrons per alkaline earth ion for high metal atom concentrations. Since the alkaline earth ions are smaller than the corresponding alkali metals in the same row of the periodic table, the crystal structures formed with alkaline earth doping are often different from those for the alkali metal dopants. Except for the alkali metal and alkaline earth intercalation compounds, few intercalation compounds have been investigated for their physical properties. 

The diffusion of dopants into, and out of, conducting polymers is important for possible applications in batteries, and as conductors or semiconductors. For conductors or semiconductors, the chief requirements are that the material can be doped in a reasonable time, but that it will then not lose dopant over periods of years. This is particularly important in determining junction stability in devices. In the case of batteries, on the other hand, rapid and reversible uptake and loss of dopants is needed, since the diffusion rate controls charging and discharging rates. In addition, the accessibility of the structure to oxygen, and other degradants, will be a factor in the stability of the polymer. 

Shinohara et al.52n suggested that the diffusion rate of dopant ions in polypyrrole films depends upon the size of the anion used as counter-ion in the electrochemical synthesis so that films prepared with CI counter-ions were impermeable to larger ions, at least in the time-scale of a cyclic voltammetry experiment. Tietje-Girault et al. 522) made two-layer polypyrrole films in which a layer was prepared with a large... 

B is one of the most commonly used p-type dopants in SiC. Diffusion of B in SiC was found to be activated by a kick-out mechanism, in which a B substitutional impurity at a Si lattice site is displaced by a nearby Si interstitial [49]. The displaced B takes an interstitial site that can then diffuse through the crystal. Rurali et al. [49, 50] also determined the lowest energy diffusion path of the interstitial B impurity, going from a trigonal site to the next one via C and Si interstitialcies, with a barrier of 0.65 eV. B interstitials therefore diffuse easily through the SiC crystal until they recombine with an existing vacancy or they reach the surface. The difficulty observed in the experiment is therefore associated with the activation of the B interstitial, which must proceed via a Si self-interstitial, rather than the diffusion process itself. This explains the experimental observations of an enhanced diffusion rate in the presence of intrinsic defects [54]. 

Diffusion in a solid is possible only if the lattice is not perfect but has a certain degree of disorder and contains vacancies and interstitials. The main defects that help diffusion were listed in the previous section. Nonstoichiometric compounds (see Table 10.3 for examples) have high concentrations of defects such as ion vacancies and dopants and these concentrations strongly affect diffusion rates. Figure 10.11... 

One of the first characteristics of a CV of a CP film that one searches for is the dependence of the peak current (ip) on the scan rate (v). According to well-established electrochemical treatments, for a behavior dominated by diffusion effects, ip is proportional to whilst for a material localized on an electrode surface, such as a CP film, ip is proportional to v. For most CP films, the latter case obtains, thus indicating surface-localized electroactive species.. For the P(Py) system of Fig. 4-L ip is proportional to v. As more detailed analysis shows, however, this is so only for CP films that are not inordinately thick (which most are not), not inordinately compact (which most are not), and not doped with very large or sluggish dopant ions which have inordinately small diffusion coefficients (which most dopants do not). If any of the latter conditions prevail, however - i.e. wherever dopant diffusion effects can predominate - ip can be proportional to as the case of Poly(p-amino diphenyl amine) discussed below shows. Intermediate or transitional behaviors are also possible. 

Fig. 4-2. the peak/scan rate dependence, also shows the effect of dopants on voltammetric behavior. The slope differs for different dopants. A lower slope usually implies slower dopant diffusion in and out of the CP, and broader peaks. The effect... 

Returning to the question of whether the system will be in equilibrium as the Fermi level is changed by doping, note that insertion of vacancies will occur at the same time as insertion of dopants by any technique. Therefore the compensation process usually is not limited by diffusion rates, as vacancies will move at least as fast as the impurities causing doping. The question is what is the temperature establishing the equilibrium It would normally be at or near the doping temperature. 

In the oxidation process, a layer of dopant is apphed to the surface of sihcon and patterned sihcon dioxide for subsequent thermal diffusion into the sihcon. The masking property of the Si02 is based on differences in rates of diffusion. Diffusion of dopant into the oxide is much slower than the diffusion into the sihcon. Thus, the dopants reach only the sihcon substrate. Oxide masks are usually 0.5—0.7 p.m thick. 

The diffusion of H and D atoms in the molecular crystals of hydrogen isotopes was explored with the EPR method. The atoms were generated by y-irradiation of crystals or by photolysis of a dopant. In the H2 crystals the initial concentration of the hydrogen atoms 4x 10 mol/cm is halved during 10 s at 4.2 K as well as at 1.9 K [Miyazaki et al. 1984 Itskovskii et al. 1986]. The bimolecular recombination (with rate constant /ch = 82cm mol s ) is limited by diffusion, where, because of the low concentration of H atoms, each encounter of the recombinating partners is preceded by 10 -10 hops between adjacent sites. 

Pai Vemeker and Kannan [1273] observe that data for the decomposition of BaN6 single crystals fit the Avrami—Erofe ev equation [eqn. (6), n = 3] for 0.05 < a < 0.90. Arrhenius plots (393—463 K) showed a discontinuous rise in E value from 96 to 154 kJ mole-1 at a temperature that varied with type and concentration of dopant present Na+ and CO2-impurities increased the transition temperature and sensitized the rate, whereas Al3+ caused the opposite effects. It is concluded, on the basis of these and other observations, that the rate-determining step in BaN6 decomposition is diffusion of Ba2+ interstitial ions rather than a process involving electron transfer. 

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