Open-circuit etching, silicon

HF or H2O. A wide range of processes, including pore formation in n- and p-type silicon in HF solutions, pore formation in n-type silicon in HF solutions under illumination, and photoanodic dissolution of n-type silicon in NH4F solutions, can be explained by these models. In addition, they are consistent with the models developed for open-circuit etching of silicon in fluoride solutions, discussed in Sec. 2.2.2. 

Reactions (27) and (28) are similar to the proposed mechanism for the smoothening process that occurs during open-circuit etching of silicon in buffered HF solutions, given in reactions (2) and (3). In this case, electrochemical ligand exchange involves the transformation of Si-H - Si-OH followed by HF attack of the Si-Si backbonds. 

Aqueous electrolytes of high pH etch silicon even at open circuit potential (OCP) conditions. The etch rate can be enhanced or decreased by application of anodic or cathodic potentials respectively, as discussed in Section 4.5. The use of electrolytes of high pH in electrochemical applications is limited and mainly in the field of etch-stop techniques. At low pH silicon is quite inert because under anodic potentials a thin passivating oxide film is formed. This oxide film can only be dissolved if HF is present. The dissolution rate of bulk Si in HF at OCP, however, is negligible and an anodic bias is required for dissolution. These special properties of HF account for its prominent position among all electrolytes for silicon. Because most of the electrochemistry reported in the following chapters refers to HF electrolytes, they will be discussed in detail. 

In the positive branch of the i/V graph, anodic dissolution process will remove material from silicon crystals. The conditions for optimal etching of silicon have been extensively explored for micromachining or surface polishing in the fabrication of electronic devices. Most generally, the etch rate of silicon in HE solutions is isotropic among the various crystalKne orientations. The etch rate of silicon at room temperature at the open-circuit potential (OCP) is very low, on the order of 10 nm s , which is equivalent to 100 nA cm , in aqueous HE solutions. 

The influence of interface area on Voc is utilized in high efficiency crystalline silicon solar cells with the so-called point contact concept to increase the open circuit voltage by suppressing area related recombination [149], Recently, the point contact concept has been applied also in pc-Si H solar cells using photolithography or self-organized zinc oxide etch masks [150], However, so far Voc improvement could not be demonstrated. For more details of preparation and application of ZnO etch masks the reader is referred to the original work [118]. 

The conditions for the formation of stain etch films are similar to the case for chemical etching of silicon in HF or NH4F, except for the the addition of an oxidizing agent. As for any open-circuit process, the etching involes coupled oxidation and reduction reactions. For the FIF/HNO3 case, these are thought to be ... 

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. 

By opening this book, you have already decided that you need to know more about chemistry. Perhaps you want to learn how medicines are made, how fertilizers and pesticides work, how living organisms function, how new high-temperature ceramics are used in space vehicles, or how microelectronic circuits are etched onto silicon chips. How do you approach chemistry ... 

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