Doping level

Several factors detennine how efficient impurity atoms will be in altering the electronic properties of a semiconductor. For example, the size of the band gap, the shape of the energy bands near the gap and the ability of the valence electrons to screen the impurity atom are all important. The process of adding controlled impurity atoms to semiconductors is called doping. The ability to produce well defined doping levels in semiconductors is one reason for the revolutionary developments in the construction of solid-state electronic devices. 

As outlined above, electron transfer through the passive film can also be cmcial for passivation and thus for the corrosion behaviour of a metal. Therefore, interest has grown in studies of the electronic properties of passive films. Many passive films are of a semiconductive nature [92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102 and 1031 and therefore can be investigated with teclmiques borrowed from semiconductor electrochemistry—most typically photoelectrochemistry and capacitance measurements of the Mott-Schottky type [104]. Generally it is found that many passive films cannot be described as ideal but rather as amorjDhous or highly defective semiconductors which often exlribit doping levels close to degeneracy [105]. 

Yarmoff J A and McFeely F R 1988 Effect of sample doping level during etching of silicon by fluorine atoms Phys. Rev. B 38 2057-62... 

Some small lattice mismatch occurs as a result of high nitrogen doping level. 

Fig. 6. Calculated siUcon resistivity vs temperature for the impurity (doping) levels shown, where (—) is -type, (------), n-ty e. Left of the peaks is the...

Historically, materials based on doped barium titanate were used to achieve dielectric constants as high as 2,000 to 10,000. The high dielectric constants result from ionic polarization and the stress enhancement of k associated with the fine-grain size of the material. The specific dielectric properties are obtained through compositional modifications, ie, the inclusion of various additives at different doping levels. For example, additions of strontium titanate to barium titanate shift the Curie point, the temperature at which the ferroelectric to paraelectric phase transition occurs and the maximum dielectric constant is typically observed, to lower temperature as shown in Figure 1 (2). 

An ESR study for the K-doped CNTs with a doping level of 1-2% has been reported [35]. The comparison of spin susceptibilities x., between pristine and K-intercalated CNTs is shown in Fig. 7. A significant increase of the susceptibility... 

From an analysis of data for polypyrrole, Albery and Mount concluded that the high-frequency semicircle was indeed due to the electron-transfer resistance.203 We have confirmed this using a polystyrene sulfonate-doped polypyrrole with known ion and electron-transport resistances.145 The charge-transfer resistance was found to decrease exponentially with increasing potential, in parallel with the decreasing electronic resistance. The slope of 60 mV/decade indicates a Nemstian response at low doping levels. 

Polyacetylene in the doped state is sensitive to air and moisture. Other polymers (e.g., those of pyrrole, thiophene, and benzene) are stable in air and/or toward humidity in their doped and undoped states. Generally, when stored in the doped state, the polymers lose doping level by mechanisms not fully understood in most cases the loss is reversible. 

The most interesting properties are exhibited by thin films, where rapid changes in doping level, conductivity, and optical transmission can be obtained. Thicker films exhibit vastly slower kinetics, which is due to slow counterion transport and, to some extent, to slow conformation changes. 

Instabilities of doped conductive polymers are largely an environmental problem Some polymers when p-doped to the limit have oxidation potentials high enough to attack the most inert solvents. Most will attack water even at much lower doping levels. 

In the low-energy range a depends on the defect density, doping level, and details of the preparation process. Sensitive subbandgap spectroscopy is used to measure a and relate it to the defect density in the material [78, 79]. 

The second source of partitioning data is experimental equilibration of crystals and liquids followed by microbeam analysis of quenched run products. Starting materials can be natural rocks, or synthetic analogues. In either case it is customary to dope the starting material with the U-series element(s) of interest, in order to enhance analytical precision. Of course, doping levels should not be so high as to trigger trace phase saturation (e g.. 

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