Low defect concentration

EL Devices from Conjugated Polymers with a Low 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>... 

The problem at hand is the evaluation of the activity coefficient defined in Eq. (76). It will be assumed that only pairwise interactions between the defects need be considered at the low defect concentrations we have in mind. (The theory can be extended to include non-pairwise forces.23) Then the cluster function R(n) previously defined in Eq. (78) is the sum of all multiply connected diagrams, in which each bond represents an /-function, which can be drawn among the set of n vertices, the /-function being defined by Eqs. (66), (56), and (43). The Helmholtz free energy of interaction of two defects appearing in this definition can be written as... 

The importance of interactions amongst point defects, at even fairly low defect concentrations, was recognized several years ago. Although one has to take into account the actual defect structure and modifications of short-range order to be able to describe the properties of solids fully, it has been found useful to represent all the processes involved in the intrinsic defect equilibria in a crystal (with a low concentration of defects), as well as its equilibrium with its external environment, by a set of coupled quasichemical reactions. These equilibrium reactions are then handled by the law of mass action. The free energy and equilibrium constants for each process can be obtained if we know the enthalpies and entropies of the reactions from theory or... 

In solids the free positron lifetime r lies in the approximate range 100-500 ps and is dependent upon the electron density. Following implantation, the positrons are able to diffuse in the solid by an average distance L+ = (D+t)1//2, where D+ is the diffusion coefficient. This quantity is usually expressed in cm2 s-1 and is of order unity for defect-free metallic moderators at 300 K (Schultz and Lynn, 1988). The requirement of very low defect concentration arises because the value of D+ is otherwise dramatically reduced owing to positron trapping at such sites. 

Figure 17 Threshold field versus temperature for TTF-TCNQ at ambient pressure, showing the effect of low defect concentrations. (From Ref. 92.)...

As pointed out previously, one must assume a linear concentration gradient of defects in surface layers containing only one uniformly charged type of defects. It is now of interest to discuss the experimental verification of these assumptions. In view of the very low defect concentrations frequently... 

The transport properties of hydrogen in silicon are complicated by trapping processes and molecule formation. For single-crystal silicon with low defect concentration (<10 cm ), diffusion coefficients on the order of 10 ernes have been measured [41]. 

FIGURE 26.2 (a) The capacity versus cycle number showing the first 30 cycles for LiNiOj synthesized using the particulate sol-gel process, and (b) the first cycle voltage vs. lithium content for the low defect concentration of LiNi02-... 

An important aspect of any substrate that the reader should be familiar with is that it has a low defect concentration and good crystallinity. One measure of defects is the etch pit density or EPD, in which substrates are etched to highlight defects for counting. These defects subsequently often perpetuate themselves through the growing layer and become device failure loci. Crystallinity can be measured by X-ray diffracto-metry and is also a measure of material quality and hence defects. 

A similar argument applies to temperatures below 570 °C in atmospheres of low oxygen partial pressure low defect concentrations at the magnetite-iron and magnetite-haematite interfaces are fixed by the equilibria achieved there. 

Silica is also employed to prepare microporous inorganic membranes suitable for gas separation. De Vos et al. [163] reported the preparation of silica membranes with a very low defect concentration. They employed a sol-gel synthesis starting from tetraethylorhosilicate. These membranes consist of a microporous layer on top of a supported mesoporous y-Al203 membrane. The support layer provides mechanical strength to the selective silica top layer. The prepared membranes have a thick... 

Ef corresponds to the enthalpy of formation of a defect pair Frenkel or Schottky, cf. Chapter 3). In an alkali halide, E( is typically about 3 eV implying a very low defect concentration, of the order of 10 at room temperature. E is the enthalpy of defect migration, usually several eV in dense structures, and / is the correlation or Haven factor. Its value varies between 0 and 1 and takes into account unfruitful attempts at transport in a given direction, caused by random jumps of mobile species and the particular geometry of each site. In other words, this factor takes into account the correlation effects (/ = 1 when correlations are absent). Usually / is of the order of 0.5-0.8 and plays a role in determining the transport mechanism. 

Since the concentration of AB molecules can be regarded as constant at low defect concentration it can be included into the constant of the law of mass action, and we obtain ... 

The need for low defect concentrations in self-assembled systems... 

From diffusion experiments one learns that the temperature dependence of the diffusion coefficient can be written as = Z)f exp ( -QlRT), which is an Arrhenius-type relation. For example, in the case of carbon diffusion in iron or sodium diffusion in j5-alumina an Arr-henius-type relation is obeyed over many orders of magnitude of the diffusivities. According to eq. (5-19), Dj is the product of a point defect concentration and a diffusion coefficient of the point defects. Eq. (4-3) shows that the mole fraction of point defects is an exponential function of the reciprocal of the absolute temperature for low defect concentrations. Therefore, the individual jump frequency of a point defect, or of an atom or ion which is moved by the jump of a point defect, depends upon the temperature as exp - QjRT),... 

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