Barrier layer, electronic

Scanning electron microscopy (thickness topography porosity barrier layers fracture sections) Energy dispersive X-ray analysis (EDX)... 

Much of the difficulty in demonstrating the mechanism of breakaway in a particular case arises from the thinness of the reaction zone and its location at the metal-oxide interface. Workers must consider (a) whether the oxide is cracked or merely recrystallised (b) whether the oxide now results from direct molecular reaction, or whether a barrier layer remains (c) whether the inception of a side reaction (e.g. 2CO - COj + C)" caused failure or (d) whether a new transport process, chemical transport or volatilisation, has become possible. In developing these mechanisms both arguments and experimental technique require considerable sophistication. As a few examples one may cite the use of density and specific surface-area measurements as routine of porosimetry by a variety of methods of optical microscopy, electron microscopy and X-ray diffraction at reaction temperature of tracer, electric field and stress measurements. Excellent metallographic sectioning is taken for granted in this field of research. 

It would be preferable to incorporate both fluorescent and electron transport properties in the same material so as to dispense entirely with the need for electron-transport layers in LEDs. Raising the affinity of the polymer facilitates the use of metal electrodes other than calcium, thus avoiding the need to encapsulate the cathode. It has been shown computationally [76] that the presence of a cyano substituent on the aromatic ring or on the vinylene portion of PPV lowers both the HOMO and LUMO of the material. The barrier for electron injection in the material is lowered considerably as a result. However, the Wessling route is incompatible with strongly electron-withdrawing substituents, and an alternative synthetic route to this class of materials must be employed. The Knoevenagel condensation... 

Figure 11-17. Calculated current density as a function of bias (upper panel) and electron density as a function of position at 12 V bias (lower panel) for a two-layer electron-only (0.5 cV electron injection barrier) device with the energy level diagram for the two polymer layers shown in Fig. 11-13. The mobility of the left hand polymer is increased by a factor of ten in the enhanced mobility structure (dotted line).

To optimize the performance of single-layer Oocl-OPV5 devices, Ca instead of A1 was used as the cathode, which lowers the injection barrier for electrons with approximately 1.3 eV. The change of cathode resulted in a more than twofold re-... 

The characteristic feature of solid—solid reactions which controls, to some extent, the methods which can be applied to the investigation of their kinetics, is that the continuation of product formation requires the transportation of one or both reactants to a zone of interaction, perhaps through a coherent barrier layer of the product phase or as a monomolec-ular layer across surfaces. Since diffusion at phase boundaries may occur at temperatures appreciably below those required for bulk diffusion, the initial step in product formation may be rapidly completed on the attainment of reaction temperature. In such systems, there is no initial delay during nucleation and the initial processes, perhaps involving monomolec-ular films, are not readily identified. The subsequent growth of the product phase, the main reaction, is thereafter controlled by the diffusion of one or more species through the barrier layer. Microscopic observation is of little value where the phases present cannot be unambiguously identified and X-ray diffraction techniques are more fruitful. More recently, the considerable potential of electron microprobe analyses has been developed and exploited. 

Weisz, P. B. Electronic barrier layer fenomina in chemisorption and catalyse. 

To introduce a barrier layer when utilizing doped ceria electrolytes (SDC, GDC, or lanthanum-doped ceria, LDC) to prevent the reduction of Ce4+ to Ce3+. Reduction of cerium cations results in unwanted electronic conductivity that lowers fuel efficiency [34], and mechanical degradation that results from the volume expansion of cerium ions upon reduction [35],... 

Macroscopic n-type materials in contact with metals normally develop a Schottky barrier (depletion layer) at the junction of the two materials, which reduces the kinetics of electron injection from semiconductor conduction band to the metal. However, when nanoparticles are significantly smaller than the depletion layer, there is no significant barrier layer within the semiconductor nanoparticle to obstruct electron transfer [62]. An accumulation layer may in fact be created, with a consequent increase in the electron transfer from the nanoparticle to the metal island [63], It is not clear if and what type of electronic barrier exists between semiconductor nanoparticles and metal islands, as well as the role played by the properties of the metal. A direct correlation between the work function of the metal and the photocatalytic activity for the generation of NH3 from azide ions has been made for metallized Ti02 systems [64]. 

It is interesting to compare the biphenylamine substituted compounds with the corresponding carbazoles, phenoxazines, and phenothiazines. For the triaryla-mino-based structures, the carbazole 24 has the highest oxidation potential (0.69 V vs. Ag/0.01 Ag+) [102], followed by the phenoxazine 25a (0.46 V vs. Ag/0.01 Ag+) [166]. A similar observation was made for the corresponding derivatives of 36a the phenothiazine (0.27 V vs. Fc/Fc+) and the phenoxazine (0.29 V vs. Fc/Fc+) have higher oxidation potentials than the parent compound. The carbazole 37 has an even higher oxidation potential, but in this case the oxidation is not reversible [234]. The redox properties of carbazoles are not fully understood yet. In some devices, a carbazole such as CBP (10) was used as an interface layer on the cathode side, suggesting a lower barrier for electron injection [50]. 

Figure 2.25 Barrier layer cell. Radiation absorbed by the semiconductor, often selenium, causes electrons to be released and a small current flows which can be measured using a microammeter.

Diffusion Barriers. Diffusion barriers are used in the production of various components in the electronic industry. For example, electrochemically deposited nickel is used as a barrier layer between gold and copper in electronic connectors and solder interconnections. In these applications the product is a trilayer of composition Cu/Ni/Au. In another example, Ni and Co are considered as diffusion barriers and cladding materials in the production of integrated circuits and multichip electronic packaging. In this case the barrier metal (BM), Co or Ni, is the diffusion barrier between conductor and insulator (i.e., Cu and insulator), and the product trilayer is of composition Cu/BM/insulator. The common couple in these applications is the Cu/BM bilayer (BM, the diffusion barrier metal Co, Ni, or Ni-Co alloy). 

In electronic applications, where it is common to deposit copper and/or copper alloy and tin in sequence, with a nickel diffusion barrier layer, 0.5 fim thick, between the layers present, no failure occurs. Without the nickel layers between bronze/-copper/tin layers themselves, for instance, intermetaUic brittle layer(s) and Kirkendall voids are formed, leading eventually to separation of the coated system and substrate. 

---