Substitutional dopants

B. G. Sumpter, J. S. Huang, V. Meunier, J. M. Romo-Herrera, E. Cruz-Silva, H.Terrones, M. Terrones, A theoretical and experimental study on manipulating the structure and properties of carbon nanotubes using substitutional dopants., International Journal of Quantum Chemestry, vol. 109, pp. 97-118, 2009. 

Calculations of the fractional interstitialcy components for B, P, As, and Sb are shown in Table I (33, 39-42). A significant spread in the values of/, is obtained. The value of / has been correlated with the amount of energy required to make a substitutional dopant atom become interstitial. Energies of interstitial formation in Si are shown in Table II. The larger the energy... 

A completely different approach for the doping of a-Si H has been followed by Beyer and Fischer (1977). They showed that n-type doping is also obtained by interstitially incorporated lithium brought into the films by in-diffusion as well as by ion implantation. The possibility of introducing substitutional dopants in a-Si H by ion implantation has been proved by Muller et al. (1977). Subsequently, these authors demonstrated the doping effect of most elements of the groups I, III, and V of the periodic table by ion implantation experiments (Kalbitzer et al., 1980). 

The new transfer doping mechanism produces conductive material with a lower density of gap states than phosphorus-doped material of comparable resistivity, where the substitutional dopant always introduces extra defect states. Evidence for the lower density of defects comes from the magnitude of the low energy shoulder in the photoconductivity response spectrum shown in Fig. 14, where the absorption of the layered material at photon energies below 1.4 eV is more than an order of magnitude lower than the phosphorus-doped material of comparable dark resistivity. Furthermore, the photoconductivity of the transfer-doped material is large (10 Q cm ) compared with the photoconductivity achievable in heavily P-doped material under similar illumination. 

Defect exarriDles V Iron vacancy in e.g. FejO Vp Oxygen vacancy in a metal oxide Zn" Zinc interstitial in e.g. ZnO Al( Al substitutional dopant in e.g. SrTIOj Defect reaction reouirements 1. Conservation of mass 2. Conservation of lattice site stoichiometry 3. Conservation of charge... 

Substitutional dopants Dissolved foreign atoms or ions replacing lattice atoms. If aliovalent they may introduce vacancies 0... 

Substitution of a dopant for an element of the perfect crystal leads to a distortion of the perfect lattice from which electrons can scatter. If that substitutional dopant is ionized, the electric field of that ion adds to the scattering. Impurities located at interstitial sites (i.e., between atoms in the normal lattice sites) also disrupt the perfect crystal and lead to scattering sites. Crystal defects (e.g., a missing atom) disrupt the perfect crystal and appears as a scattering site in the free space seen by the electron. In useful semiconductor crystals, the density of such scattering sites is small relative to the density of sihcon atoms. As a result, removal of the silicon atoms through use of the effective mass leaves a somewhat sparsely populated space of scattering sites. 

Consulting a Periodic Table and Table of Atomic Radii, what atoms would be suitable (a) interstitial dopants and (b) substitutional dopants within a Mn lattice Show your calculations and rationale. 

The PTCR effect is complex and not fully understood in terms of the grain boundary states and stmcture. Both the PTCR effect and room temperature resistivities are also highly dependent on dopant type and ionic radius. Figure 11 (32) illustrates this dependence where comparison of the PTCR behavior and resistivity are made for near optimum concentrations of La ", Nd ", and ions separately substituted into BaTiO. As seen, lowest dopant concentration and room temperature resistivity are obtained for the larger radius cation (La " ), but thePTCR effect was sharpest for the smallest radius cation (Y " ), reflecting dual site occupancy of the Y " ion. 

Sohd solutions of ceria with trivalent ions, eg, Y and La, can readily be formed. The ions substitute for the tetravalent Ce and introduce one oxygen vacancy for every two ions. The dopant ions and the oxygen vacancies form charge associates. The resulting defect-fluorites have good oxide... 

Significant variations in the properties of polypyrrole [30604-81-0] ate controlled by the electrolyte used in the polymerization. Monoanionic, multianionic, and polyelectrolyte dopants have been studied extensively (61—67). Properties can also be controlled by polymerization of substituted pyrrole monomers, with substitution being at either the 3 position (5) (68—71) or on the nitrogen (6) (72—75). An interesting approach has been to substitute the monomer with a group terminated by an ion, which can then act as the dopant in the oxidized form of the polymer forming a so-called self-doped system such as the one shown in (7) (76—80). 

The interest of physicists in the conducting polymers, their properties and applications, has been focused on dry materials 93-94 Most of the discussions center on the conductivity of the polymers and the nature of the carriers. The current knowledge is not clear because the conducting polymers exhibit a number of metallic properties, i.e., temperature-independent behavior of a linear relation between thermopower and temperature, and a free carrier absorption typical of a metal. Nevertheless, the conductivity of these specimens is quite low (about 1 S cm"1), and increases when the temperature rises, as in semiconductors. However, polymers are not semiconductors because in inorganic semiconductors, the dopant substitutes for the host atomic sites. In conducting polymers, the dopants are not substitutional, they are part of a nonstoichiometric compound, the composition of which changes from zero up to 40-50% in... 

Other single-crystal x-ray diffraction studies of transition element dopants in jS-rh boron are based on the results of a refinement of the /3-rh boron structure that establishes the occurrence of four new low-occupancy (3.7, 6.6, 6.8 and 8.5%) B positions in addition to the earlier known ones. The dopant elements studied, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Hf and Ta, do not enter B positions in the framework, but they enter the Al, A2, D and E positions. In some cases the doping elements have been studied at several concentrations for each element and for different cooling rates. The percentage occupancies of certain positions are eorrelated with the atomie sizes of the dopants. The bond distances between the polyhedra are shorter than those within the polyhedra. The mechanism of doping for some cases is denoted displacive, rather than interstitial or substitutional, because of competing interactions between the six different partially occupied B positions and dopant atoms. 

The theoretical hmit of 5.4% (NaAlH4+2 mol% TiN) for the two subsequent decomposition reactions is in both cases only observed in the first cycle. The reason for the decrease in capacity is stiU unknown and litde is known about the mechanism of alanate activation via titanium dopants in the sohd state. Certainly, the ease of titanium hydride formation and decomposition plays a key role in this process, but whether titanium substitution in the alanate or the formation of a titanium aluminum alloys, i.e., finely dispersed titanium species in the decomposition products is crucial, is stiU under debate [41]. 

The process of substituting elements for the silicon is called doping, while the elements are referred to as dopants. The amount of dopant that is required in practical devices is very small, ranging from about 100 dopant atoms per million silicon atoms downward to 1 per billion. Dopants are usualty added to the silicon after the crystal growth process, when an integrated circuit is being formed on the surface of the wafer. 

Extrinsic Defects Extrinsic defects occur when an impurity atom or ion is incorporated into the lattice either by substitution onto the normal lattice site or by insertion into interstitial positions. Where the impurity is aliovalent with the host sublattice, a compensating charge must be found within the lattice to pre-serve elec-troneutality. For example, inclusion of Ca in the NaCl crystal lattice results in the creation of an equal number of cation vacancies. These defects therefore alter the composition of the solid. In many systems the concentration of the dopant ion can vary enormously and can be used to tailor specific properties. These systems are termed solid solutions and are discussed in more detail in Section 25.1.2. 

Indole differs from the more simple heterocycles in that when doped, the structure appears to contain one dopant anion to two monomer units, and its conductivity is considerably lower (by four orders of magnitude) when doped to a similar level [39]. From IR spectra, and the fact that /V-substituted indoles do not polymerise, the... 

Powder XR diffraction spectra confirm that all materials are single phase solid solutions with a cubic fluorite structure. Even when 10 mol% of the cations is substituted with dopant the original structure is retained. We used Kim s formula (28) and the corresponding ion radii (29) to estimate the concentration of dopant in the cerium oxide lattice. The calculated lattice parameters show that less dopant is present in the bulk than expected. As no other phases are present in the spectrum, we expect dopant-enriched crystal surfaces, and possibly some interstitial dopant cations. However, this kind of surface enrichment cannot be determined by XR diffraction owing to the lower ordering at the surface. 

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