Potential drop substrate

Because of the charged nature of many Langmuir films, fairly marked effects of changing the pH of the substrate phase are often observed. An obvious case is that of the fatty-acid monolayers these will be ionized on alkaline substrates, and as a result of the repulsion between the charged polar groups, the film reverts to a gaseous or liquid expanded state at a much lower temperature than does the acid form [121]. Also, the surface potential drops since, as illustrated in Fig. XV-13, the presence of nearby counterions introduces a dipole opposite in orientation to that previously present. A similar situation is found with long-chain amines on acid substrates [122]. 

In most systems the substrate electrodes are larger than the powered electrodes. This asymmetric configuration results in a negative dc self-bias voltage Vdc on the powered electrode. Without that, the difference in electrode areas would result in a net electron current per RF period [134, 169]. It has been shown that the ratio of the time-averaged potential drops for the sheaths at the grounded (V g) and the powered electrode (Vsp) are inversely proportional to a power of the ratio of the areas of the two electrodes (Ag, Ap) [134, 170-172] ... 

There are five possible physical phases in the current path in which the current conduction mechanisms are different as illustrated in Figure 19. They are substrate, space charge layer, Helmholtz layer, surface oxide film, and electrolyte. The overall change in the applied potential due to a change of current density in the current path is the sum of the potential drops in these phases ... 

Physically, the sensitivity of reactions to surface curvature can be associated with the space change layer or the resistance of the substrate. For moderately or highly doped materials, this sensitivity is only associated with the space change layer because the ohmic potential drop in the semiconductor substrate is very small. However, for lowly doped material a significant amount of potential can drop in the semiconductor to cause the current flow inside semiconductor to be also sensitive to the curvature of the surface. 

For a moderately or highly doped material the potential drop due to ohmic resistance in the substrate is very small. For example, for a substrate with a resistivity of 0.1 Qcm ( 1017/cm3) the potential drop in the substrate of 0.1 mm thick at a current density of 10... 

Figure 26. Potential drops along the current path in a pore AVsi = potential drop in silicon substrate, AVS = potential drop in the space change layer AV0I = potential drop in oxide AVh = potential drop in the Helmholtz layer, AVei = potential drop in electrolyte.

When the resistance of the substrate is high and a significant amount of potential is dropped in the substrate, the potential drop may not be uniform along a curved pore bottom due to the nonlinear potential distribution on the material surrounding the bottom. Formation of macro PS on lowly doped materials becomes possible under such a condition. 

The recorded current is caused not only by the heterogeneous electron transfer to the substrate (the Faradaic current ), but also by the current used to charge the electrical double layer, which acts as a capacitor. The measured potentials include the potential drop caused by the ohmic resistance in the solution, the iR drop. Both the charging current ic and the iR drop grows with the sweep rate it is always desirable to compensate for ic and iR drop, but it becomes imperative at higher sweep rates. There exist different ways to compensate electrically for these phenomena, and this makes it possible to operate up to about 103 V sec-1. It is assumed below that the data are obtained with proper compensation. 

Positive ions drift to the sheath edge where they encounter the strong field. The ions are then accelerated across the potential drop and strike the electrode or substrate surface. Because of the series capacitor or the dielectric coating of the electrodes, the negative potentials established on the two electrodes in a plasma system may not be the same. For instance, the ratio of the voltages on the electrodes depends upon the relative electrode areas... 

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