Etching behavior alkaline solution

In contrast to acidic electrolytes, chemical dissolution of a silicon electrode proceeds already at OCP in alkaline electrolytes. For cathodic potentials chemical dissolution competes with cathodic reactions, this commonly leads to a reduced dissolution rate and the formation of a slush layer under certain conditions [Pa2]. For potentials slightly anodic of OCP, electrochemical dissolution accompanies the chemical one and the dissolution rate is thereby enhanced [Pa6]. For anodic potentials above the passivation potential (PP), the formation of an anodic oxide, as in the case of acidic electrolytes, is observed. Such oxides show a much lower dissolution rate in alkaline solutions than the silicon substrate. As a result the electrode surface becomes passivated and the current density decreases to small values that correspond to the oxide etch rate. That the current density peaks at PP in Fig. 3.4 are in fact connected with the growth of a passivating oxide is proved using in situ ellipsometry [Pa2]. Passivation is independent of the type of cation. Organic compounds like hydrazin [Sul], for example, show a behavior similar to inorganic ones, like KOH [Pa8]. Because of the presence of a passivating oxide the current peak at PP is not observed for a reverse potential scan. 

While the microscopic etch mechanism of ZnO single crystals in alkaline and acid solutions is well understood [109,117], a detailed understanding of the etching behavior of compact polycrystalline films is still not available. In the following we will discuss the relation between etching behavior of... 

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... 

In order to discuss the correlation between etching kinetics and etching morphology, let us first reconsider the current-potential characteristics of III-V semiconductors, as depicted in Fig. 1. For the sake of clarity, the behavior in alkaline solutions (Fig. 1 (b)) is treated first. As announced, the discussion is mainly based on experimental results obtained from GaP single crystals. 

H. Seidel, L. Csepregi, A. Heuberger, and H. Baumgartel, Anisotropic etching of crystalline silicon in alkaline solutions. I. Orientation dependence and behavior of passivation layer, J. Electrochem. Soc. 137, 3612, 1990. 

Silicon etching in KOH solutions have been extensively investigated, resulting in a body of information that shapes the current understanding of the etching behavior of silicon in alkaline solutions. The major characteristics and the principal reaction processes involved in all alkaline solutions appear to be similar to that in the KOH system although the detailed characteristics vary from system to system. Most notably,... 

Seidel H, Csepregi L, Heubager A, Baumgartel H (1990) An-isotrr c etching of crystalline silicon in alkaline solutions I Orientation dependence and behavior of passivation layers. J Electrochem Soc 137(11) 3612-3626... 

In general, polymers are much more resistant to attack by acidic and alkaline solutions than are metals. For example, hydrofluoric acid (HF) corrodes many metals as well as etch and dissolve glass, so it is stored in plastic bottles. A qualitative comparison of the behavior of various polymers in these solutions is also presented in Tables 17.4 and 17.5. Materials that exhibit outstanding resistance to attack by both solution types include polytetrafluoroethylene (and other fluorocarbons) and polyetheretherketone. 

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... 

---