Diffusion doping dependence

Fig. 16. Panorama of values in the literature for diffusion coefficients of hydrogen in silicon and for other diffusion-related descriptors. Black symbols represent what can plausibly be argued to be diffusion coefficients of a single species or of a mixture of species appropriate to intrinsic conditions. Other points are effective diffusion coefficients dependent on doping and hydrogenation conditions polygons represent values inferred from passivation profiles [i.e., similar to the Dapp = L2/t of Eq. (95) and the ensuing discussion] pluses and crosses represent other quantities that have been called diffusion coefficients. The full line is a rough estimation for H+, drawn assuming the top points to refer mainly to this species otherwise the line should be higher at this end. The dashed line is drawn parallel a factor 2 lower to illustrate a plausible order of magnitude of the difference between 2H and H.

There is already a large number of different conductive polymers. A typical monomer is 3-methylthiophene, which can be electrically polymerized to a polymer coupled by the 2-and 5-positions of the monomer. In the oxidized form, usually called doped , the chains contain positive charges at about every fourth monomer unit. In order to keep the polymer layer electrically neutral, also counter anions should be present in the polymer matrix. It is analytically interesting that the diffusion rate of these counter anions controls the rate of oxidation and reduction of the polymer, and the diffusion rate depends on the size, degree of solvation etc. of the anion. Hence, by a suitable choice of the polymer, it should be possible, at least in principle, to tailor-make sensors for different anions. In addition, it has been shownthat electrically neutral polymers can be incorporated from the solution into the polymer matrix during the polymerization process. This of course extends enormously the possibilities for developing selective sensors without undue efforts to synthesize new electrically polymerizable monomers. 

A more complex example is La2Cu04. The doping dependence of oxygen tracer diffusion coefficients in La2Cu04 and their anisotropy are illustrated in Fig. 6.19. Whilst the behaviour at low x-values is in accordance with simple defect chemistry, the interactions and structmral changes at high doping concentrations lead to deviations from the ideal mass action laws (see Fig. 5.55). 

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

Nanoparticles of Mn and Pr-doped ZnS and CdS-ZnS were synthesized by wrt chemical method and inverse micelle method. Physical and fluorescent properties wra cbaractmzed by X-ray diffraction (XRD) and photoluminescence (PL). ZnS nanopatlicles aniKaled optically in air shows higher PL intensity than in vacuum. PL intensity of Mn and Pr-doped ZnS nanoparticles was enhanced by the photo-oxidation and the diffusion of luminescent ion. The prepared CdS nanoparticles show cubic or hexagonal phase, depending on synthesis conditions. Core-shell nanoparticles rahanced PL intensity by passivation. The interfacial state between CdS core and shell material was unchan d by different surface treatment. 

Contrary to silicon, very little work has been done in germanium regarding quantitative hydrogen diffusion or electric field drift studies. Such experiments may be complicated by the fact that ultra-pure germanium becomes intrinsic already at temperatures near 200 K. It would be worthwhile to explore the possibility of using lightly doped germanium for such studies in order to explore Fermi level dependent effects. 

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