The I-V Characteristics of Silicon Electrodes in Acidic Electrolytes

In this section the I-V characteristic of a silicon electrode in acidic, and specially hydrofluoric electrolytes is discussed with emphasis on the different charge states of the semiconductor. The accompanying chemical reactions are briefly mentioned, but are discussed in detail in Chapter 4. 

A passivating oxide is formed under sufficiently anodic potentials in HF, too. However, there are decisive differences to the case of alkaline and fluoride-free acidic electrolytes. For the latter electrolyte the steady-state current density prior to passivation is zero and it is below 1 mA cnT2 for alkaline ones, while it ranges from mA cm-2 to A cm-2 in HF. Furthermore, in HF silicon oxide formation does not lead to passivation, because the anodic oxide is readily etched in HF. This gives rise to an anodic I-V curve specific to HF, it shows two current maxima and two minima and an oscillatory regime, as for example shown in Fig. 4.7. 

There are several methods to investigate the charge states of a semiconductor electrode, for example high-frequency resistometry (HFR) [Otl]. Below a transistor-like set-up, as shown in the inset of Fig. 3.2, will be discussed because it shows in an exemplary way the similarities and differences of solid-state junctions and liquid junctions. 

If minority carrier current (1BC, dotted line, symbols in Fig. 3.2) is detected at the collector, it can be concluded that the emitter is no sink for minorities. The absolute value of IEB depends not only on the charge state of the emitter-base junction and surface recombination velocity, but as well on bulk diffusion length and on sample thickness. However, the latter two parameters are constants for a given sample. 

Four different regimes of the I-V curve for moderately doped silicon electrodes in an HF electrolyte are shown in Fig. 3.2. These regimes will now be discussed in terms of the charge state of the electrode, the dependence on illumination conditions, the charge transfer, the mass transport, and accompanying chemical reactions. Transient effects are indicated in Fig. 3.2 by a symbol with an arrow. 

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