Current oscillation oxide properties

FIGURE 5.59. Schematic illustration of oxide properties and thickness during current oscillation, (a) Oxides at point A of the interface and point B at the surface have different properties, (b) Change of oxide density (shade) and thickness along with the change of current during current oscillation. 

When the conditions are such that the oxide formed at the Si/oxide interface, A, as it is moved to the surface, changes to the oxide at point B and has the etch rate of oxide B, the system is stable and no oscillation occurs. On the other hand, when the property of the oxide formed at A is not the same as that of the oxide by the time it reaches point B, the etch rate of the oxide film will vary and current oscillation may occur. When current oscillation occurs, as illustrated in Fig. 5.59b, the thickness and variation of properties in the thickness direction change with time. The structure and etch rate of the oxide formed at A with a thin film are different from that with a thick film because of the difference in aging time. 

The rate of oxide formation relative to dissolution of the oxide determines the surface coverage, thickness, and properties of oxide, occurrence of passivation and current oscillation as well as uniformity of anodic dissolution. 

Comparing Fig. 30 with Fig. 4, we can assume that in the potential region between 1.0 and 1.3 V, methanol oxidation could be an example for a negative differential resistance (NDR) system. In addition, from Fig. 30, it can be concluded that A-shaped systems with a negative differential resistance can oscillate at constant potential [current oscillations), but not at constant current. As we see below under Sect. 5.2.5.3 and Fig. 35, indeed, current oscillations are to be observed. The surface properties are modified by an electrochemical reaction alone, by the formation of platinum oxide. This Pt-0 formation is followed by a chemical reaction between the oxide and methanol in solution. CO2 is produced, and the current at the free surface sites increases to form new surface oxide, until the surface coverage approaches its maximum and the chemical reaction becomes stronger again. And the cycle starts anew. A characteristic of the (simple) NDR oscillators is that only one potential-dependent process is involved, that is, the electrochemical oxide formation. 

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