PS Formed at OCP

PS can also be formed chemically without the application of a current by simply immersing a silicon sample in a HF solution containing some oxidation agents such as nitric acid or chromium oxide. The oxidants can be the redox couples, which do not deposit onto the surface after the reduction, or metal ions which deposit onto the surface after the reduction. The PS formed under such a condition are often referred to as stain films or chemical PS. As a characteristic feature, the OCP of the silicon during formation of chemical PS in the presence of an oxidant is several hundred millivolts more positive than without addition of an oxidation agent. The formation of chemical PS can further be modified by illumination.  

Although the electrochemical nature of the processes involved in the formation of PS at open-circuit conditions (nonbiased) should be similar to that under anodic bias, there are several major differences in the formation conditions. The first is that at the OCP the driving force is provided by the oxidation agents, the reduction of which provides the anodic polarization of the electrode needed for silicon dissolution. Unlike the externally biased condition, the extent of polarization is limited by the oxidation power of the oxidation agents. The second is that the carrier supply at the open-circuit condition is localized and randomly oriented, while that at anodic potential is perpendicular to the surface. The anodic and cathodic sites in the chemical etching process must be in the vicinity of each other, and continuous alternations must occur between anodic and cathodic reactions on the surface at the pores tips. 

Thickness of the PS Films Formed under Open-Circuit Conditions  

Formation rate and thickness of PS at the OCP can be altered by depositing a metal film on part of the exposed surface to form a galvanic The dissolution 

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