Etching open-circuit

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]. 

When etching is carried out at open circuit potential, these holes must be supplied by an oxidizing agent of a rather positive standard potential, such as Ce [230],... 

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 ... 

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. 

This leads us to another important category of multi-electron photoprocesses involving the semiconductor itself. While photocorrosion is a nuisance from a device operation perspective, it is an important component of a device fabrication sequence in the microelectronics industry. Two types of wet etching of semiconductors can be envisioned [290]. Both occur at open-circuit but one involves the action of chemical agents that cause the simultaneous rupture and formation of bonds. This is exemplified by the action of H2O2 on GaAs [291] ... 

In the case of open-circuit etching processes, no electrical contacts are required at the back of the sample, which is more attractive from a technological point of view the semiconductor crystal is simply immersed in the etchant solution and in some cases illuminated with light of an appropriate wavelength. Depending on the mechanism, three types of open-circuit etching processes can be distinguished, i. e.,... 

It was probably Gerischer who, as early as in the 1960s, first recognized the value of semiconductor electrochemistry for investigating the mechanisms of these open-circuit etching processes [27, 38, 68-70]. 

The above cited factors influencing the hole injection kinetics have important consequences on the dark open-circuit etching behavior of GaP single crystals in alkaline Fe(CN) solutions. At the (lll)-face, the open-circuit etching rate is always found to be controlled by the rate of the charge transfer reaction (so-called kinetic control). At the (lll)-face, on the other hand, the etching rate is always found to be limited by ion diffusion towards the semiconductor surface, either of Fe(CN) (for Fe(CN) concentrations lower than 0.3 mol -1 or of OH (for Fe(CN) concentrations higher than 0.3 mol 1 ). This difference between the two polar... 

Fig. 12. The open-circuit etching rate r as a function of the square root of the flow rate u at p-GaP in aqueous 0.5 mol l Ee(CN) + KOH (pH = 13). O (lll)-face, (lll)-face. (From ref. (73), by permission of Pergamon Press).

Figure 13 shows schematically the current- and partial current-potential behavior of p-GaP ((a) and (b)) and n-GaP ((c) and (d)) in alkaline Fe(CN) solutions. In Fig. 13 (a) and (c), the partial current density at rest-potential or under open-circuit, and hence the etch rate, is limited by the cathodic partial reaction rate. This is the case for (111) GaP (for which the cathodic reaction is under kinetic control) and for (ITT) GaP at low Fe(CN) concentrations (for which the cathodic reaction is under diffusion control). In Fig. 13 (b) and (d), the partial current density at rest-potential or under open-circuit is limited by the anodic partial reactioi rate, which is limited by the OH diffusion rate (see Sec. 2.1) this is the case for (111) GaP at... 

As stated in Sec. 3.1, valuable information on the mechanism of chemical etching processes can similarly be obtained by studying the electrochemical behavior of the interface. In the particular case of GaP, the conclusion that open-circuit etching of GaP single crystals in acidic Br2 solutions proceeds via a chemical mechanism arises from two experimental observations. Firstly, current-potential measurements at p-GaP show that Br2 cannot inject holes into the valence band of GaP, so that elec-... 

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. 

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