Defect concentrations

The microstmcture and imperfection content of coatings produced by atomistic deposition processes can be varied over a very wide range to produce stmctures and properties similar to or totally different from bulk processed materials. In the latter case, the deposited materials may have high intrinsic stress, high point-defect concentration, extremely fine grain size, oriented microstmcture, metastable phases, incorporated impurities, and macro-and microporosity. AH of these may affect the physical, chemical, and mechanical properties of the coating. 

Electrical Properties. Generally, deposited thin films have an electrical resistivity that is higher than that of the bulk material. This is often the result of the lower density and high surface-to-volume ratio in the film. In semiconductor films, the electron mobiHty and lifetime can be affected by the point defect concentration, which also affects electromigration. These effects are eliminated by depositing the film at low rates, high temperatures, and under very controUed conditions, such as are found in molecular beam epitaxy and vapor-phase epitaxy. 

Characterization. The proper characterization of coUoids depends on the purposes for which the information is sought because the total description would be an enormous task (27). The foUowiag physical traits are among those to be considered size, shape, and morphology of the primary particles surface area number and size distribution of pores degree of crystallinity and polycrystaUinity defect concentration nature of internal and surface stresses and state of agglomeration (27). Chemical and phase composition are needed for complete characterization, including data on the purity of the bulk phase and the nature and quaHty of adsorbed surface films or impurities. 

Quantum well interface roughness Carrier or doping density Electron temperature Rotational relaxation times Viscosity Relative quantity Molecular weight Polymer conformation Radiative efficiency Surface damage Excited state lifetime Impurity or defect concentration... 

Conventional physical descriptions of materials in the solid state are concerned with solids in which properties are controlled or substantially influenced by the crystal lattice. When defects are treated in typical solid state studies, they are considered to modify and cause local perturbations to bonding controlled by lattice properties. In these cases, defect concentrations are typically low and usually characterized as either point, linear, or higher-order defects, which are seldom encountered together. 

Sinee there are six unknowns and three equations, there are three independent variables. We ean associate these with any three elementary independent modes of point defect formation which conserve the numbers of atoms. These are like basis vectors for representing arbitrary point defect concentrations. Let us define them as follows ... 

Equivalent formulae can be produced in terms of the other point defect concentrations by substituting from (8). 

Formula for the chemical potentials have been derived in terms of the formation energy of the four point defects. In the process the conceptual basis for calculating point defect energies in ordered alloys and the dependence of point defect concentrations on them has been clarified. The statistical physics of point defects in ordered alloys has been well described before [13], but the present work represents a generalisation in the sense that it is not dependent on any particular model, such as the Bragg-Williams approach with nearest neighbour bond energies. It is hoped that the results will be of use to theoreticians as well as... 

Approximate formulae for the point defect concentrations close (but not too close) to the stoichimetric composition in AB alloys have been derived. They show that the prefactors in the Arrhenius formulae are sensitive functions of the stoichiometry, besides representing the usual formation entropy term. 

For all /7-type oxides, the defect concentration, and hence the electrical conductivity, increases with the oxygen pressure. 

EL Devices from Conjugated Polymers with a High Defect Concentration... 

In the following section an overview, of several models describing the charge carrier injection and transport of LEDs based on polymers and organic materials, is presented. The focus will be set on mctal/polymer (organic material)/nictal contacts based on a polymer with a low defect concentration will be discussed. A description of LEDs, based on iolymers with a high defect concentration e-m U>... 

In the case of a less disordered structure, the defect concentrations vary ac-... 

Here, we have expressed the concentration as the ratio of defects to the number of M- atom sites (this has certain advantages as we will see). We can than rewrite the defect equilibria equations of Table 3-3 and 3-4 in terms of numbers of intrinsic defect concentrations, shown as follows ... 

Equation 3.6.10. given above shows that intrinsic defect concentrations will increase with increasing temperature and that they will be low for high Enthalpy of defect formation. This arises because the entropy effect is a positive exponential while the enthalpy effect is a negative exponential. Consider the following examples of various types of compounds and the natural defects which may occur (depending upon how the compounds were originally formed) ... 

We now proceed as we did for the stoichiometric case, namely to develop defect- concentration equations for the non-stoichiometric case. Consider the effect of Anti-Frenkel defect production. From Table 2-1, we get Kaf with its associated equation, kAF In Table 2-2, we use Kxi for X-interstitial sites. Combining these, we get ... 

Herein is shown how 6 changes from negative to positive at the higher pressure ratios. For a hypotheticcd MXs compound (S = 1), which contains Anti-Freiikel defects, to obtain a 0.7% deviation from the stoichiometric composition requires a pressure increase of some 5000 fold, when the original intrinsic defect concentration is 10". However, if it is lO, only a two-fold increase in pressure is needed to cause the same effect on a deviation of 0.7%, i.e.- 8 = 0.007 in MXs 6. ... 

These are the eight equations (App. 2-12. to App. 2-18.) required to calculate the defect concentrations arising from the effects of the external factor, pBr2-... 

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