Anodic oxides current oscillation

As in the case of anodic processes, current oscillations occur in the potential region where there is a transition between the metal active and passive state and vice versa. The systems exhibiting oscillations are related to electrodissolution of iron and less to that of other metals such as cobalt, stainless steel, copper, and silver (see, e.g., [113-116] and references therein). In this case, the periodic process involves formation and dissolution of comparatively thick films of metal oxides, hydroxides, or other insoluble compounds. 

Oscillations have been observed in chemical as well as electrochemical systems [Frl, Fi3, Wol]. Such oscillatory phenomena usually originate from a multivariable system with extremely nonlinear kinetic relationships and complicated coupling mechanisms [Fr4], Current oscillations at silicon electrodes under potentio-static conditions in HF were already reported in one of the first electrochemical studies of silicon electrodes [Tul] and ascribed to the presence of a thin anodic silicon oxide film. In contrast to the case of anodic oxidation in HF-free electrolytes where the oscillations become damped after a few periods, the oscillations in aqueous HF can be stable over hours. Several groups have studied this phenomenon since this early work, and a common understanding of its basic origin has emerged, but details of the oscillation process are still controversial. 

During the oscillation the thickness of the oxide film on the silicon surface varies periodically, the frequency of which coincides with the associated current oscillation. Figure 5.55, for example, shows the variation of oxide thickness, about 2 nm in amplitude (or about 25% of the average thickness), during the photocurrent oscillation of n-type silicon at TVs e in a solution of 0.1 M [F and pH 4.4. ° The anodic current is... 

FIGURE 5.58. Voltage versus time curve (solid line) for an n -type silicon electrode anodized with a constant current density in NH4F. The thickness of the anodic oxide was measured by ellipsometry (open circles, dashed line fitted as a guide to the eye). The OCP dissolution time of the anodic oxide in the electrolyte was measured (values above arrows) at different points of the oscillation. The bar graph visualizes the proposed oscillation mechanism. After Lehmann. (Reproduced by permission of The Electrochemical Society, Inc.)... 

The complexity of the system implies that many phenomena are not directly explainable by the basic theories of semiconductor electrochemistry. The basic theories are developed for idealized situations, but the electrode behavior of a specific system is almost always deviated from the idealized situations in many different ways. Also, the complex details of each phenomenon are associated with all the processes at the silicon/electrolyte interface from a macro scale to the atomic scale such that the rich details are lost when simplifications are made in developing theories. Additionally, most theories are developed based on the data that are from a limited domain in the multidimensional space of numerous variables. As a result, in general such theories are valid only within this domain of the variable space but are inconsistent with the data outside this domain. In fact, the specific theories developed by different research groups on the various phenomena of silicon electrodes are often inconsistent with each other. In this respect, this book had the opportunity to have the space and scope to assemble the data and to review the discrete theories in a global perspective. In a number of cases, this exercise resulted in more complete physical schemes for the mechanisms of the electrode phenomena, such as current oscillation, growth of anodic oxide, anisotropic etching, and formation of porous silicon. 

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. 

Figure 2.83 Electrodeposition protocol for site-selective Pt deposition into the pores of an anodic oxide matrix prepared by current oscillations on a Si(100) surface.

Polarization curve of Cu in 0.1 M KOH with anodic and cathodic current peaks and the related reactions of formation and reduction of oxide layers, the indication of soluble products and the stability range of Cu metal and its oxides. The oscillations at peak CII are due to an oscillating photocurrent density ipj, caused by a chopped light beam. (From Strehblow, in... 

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