Silicon electrode potential

Figure 27. Minority charge carrier profiles near the semiconductor/electrolyte junction. calculated for a silicon interface at two different electrode potentials. Uf- -0.25 V and Uf= 5.0 V10 ((//= forward bias = t/ - Ufl>).

When relation (28) is properly fitted, B,C, and the flatband potential Up can be determined. For a silicon electrode in contact with 0.6 M nh4f... 

Equation (40) relates the lifetime of potential-dependent PMC transients to stationary PMC signals and thus interfacial rate constants [compare (18)]. In order to verify such a correlation and see whether the interfacial recombination rates can be controlled in the accumulation region via the applied electrode potentials, experiments with silicon/polymer junctions were performed.38 The selected polymer, poly(epichlorhydrine-co-ethylenoxide-co-allyl-glycylether, or technically (Hydrine-T), to which lithium perchlorate or potassium iodide were added as salt, should not chemically interact with silicon, but can provide a solid electrolyte contact able to polarize the silicon/electrode interface. 

Aluminum s low density, wide availability, and corrosion resistance make it ideal for construction and for the aerospace industry. Aluminum is a soft metal, and so it is usually alloyed with copper and silicon for greater strength. Its lightness and good electrical conductivity have also led to its use for overhead power lines, and its negative electrode potential has led to its use in fuel cells. Perhaps one day your automobile will not only be made of aluminum but fueled by it, too. 

The formation condition for PS can be best characterized by i-V curves. Figure 2 shows a typical i-V curve of silicon in a HF solution.56 At small anodic overpotentials the current increases exponentially with electrode potential. As the potential is increased, the current exhibits a peak and then remains at a relatively constant value. At potentials more positive than the current peak the surface is completely covered with an oxide film and the anodic reaction proceeds through the formation and dissolution of oxide, the rate of which depends strongly on HF concentration. Hydrogen evolution simultaneously occurs in the exponential region and its rate decreases with potential and almost ceases above the peak value. 

Let us now consider the charge state of the electrode. The emitter is positively biased. A p-type silicon electrode is therefore under forward conditions. If the logarithm of the current for a forward biased Schottky diode is plotted against the applied potential (Tafel plot) a linear dependency with 59 meV per current decade is observed for moderately doped Si. The same dependency of 1EB on VEB is observed at a silicon electrode in HF for current densities between OCP and the first current peak at JPS, as shown in Fig. 3.3 [Gal, Otl]. Note that the slope in Fig. 3.3 becomes less steep for highly doped substrates, which is also observed for highly doped Schottky diodes. This, and the fact that no electrons are detected at the collector, indicates that the emitter-base interface is under depletion. This interpretation is sup-... 

If VEB is increased, IEB increases and the current density at the electrode eventually becomes equal to JPS. It has been speculated that this first anodic current peak is associated with flat-band condition of the emitter-base junction. However, data of flat-band potential of a silicon electrode determined from Mott-Schottky plots show significant scatter, as shown in Fig. 10.3. However, from C-V measurement it can be concluded that all PS formation occurs under depletion conditions independent of type and density of doping of the Si electrode [Otl]. 

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. 

Fig. 3.4 The electrochemical l-V characteristic of n-type and p-type silicon electrodes of (100) and (111) orientation in 40% aqueous KOH solution at 60°C in the dark. The potential is scanned from cathodic to anodic at a sweep rate ofl mVs"1. Redrawn from results of [Sm6],...

As expected from the anisotropy of chemical etching of Si in alkaline solutions, the electrochemical dissolution reaction shows a strong dependence on crystal orientation. For all crystal orientations except (111) a sweep rate independent anodic steady-state current density is observed for potentials below PP. For (111) silicon electrodes the passivation peak becomes sweep rate dependent and corresponds to a constant charge of 2.4 0.5 mCcm-2 [Sm6]. OCP and PP show a slight shift to more anodic potentials for (111) silicon if compared to (100) substrates, as shown in Fig. 3.4. 

The charge states of the silicon electrode in alkaline electrolytes have not been investigated in detail. It can be assumed that the electrode represents an MIS structure above PP, while it behaves similarly to a Schottky junction for potentials below PP. 

It is shown that the rate-limiting step in the photoelectrochemical evolution of hydrogen in an HF electrolyte is linearly dependent on the excess electron concentration at the surface of the p-type silicon electrode. The rate of this step does not depend on the electrode potential and the H+ concentration in the solution, but is sensitive to the surface pretreatment [Sell]. The plateau in the I-V curve, slightly... 

At higher anodic potentials an anodic oxide is formed on silicon electrode surfaces. This leads to a tetravalent electrochemical dissolution scheme in HF and to passivation in alkaline electrolytes. The hydroxyl ion is assumed to be the active species in the oxidation reaction [Drl]. The applied potential enables OH to diffuse through the oxide film to the interface and to establish an Si-O-Si bridge under consumption of two holes, according to Fig. 4.4, steps 1 and 2. Details of anodic oxide formation processes are discussed in Chapter 5. This oxide film passivates the Si electrode in aqueous solutions that are free of HF. 

Fig. 4.11 Reverse current densities of moderately doped n-type (filled symbols) and p-type (open symbols) silicon electrodes in 3% HF and 1% HCI as a function of applied potential without illumination. The electrolyte was...

Fig. 4.12 Current-potential and etch rate-potential curves of p-type and n-type silicon electrodes in an electrolyte composed of 6 M HN03 and 6 M HF. Redrawn from [Kol4].

In alkaline electrolytes, in contrast, silicon is readily dissolved at OCP. Under cathodic conditions the dissolution becomes reduced, while under low anodic potentials the dissolution rate is enhanced compared to the OCP rate. If, however, the anodic potential becomes larger than the PP, the silicon electrode is passivated due to a thin anodic oxide film and the dissolution rate becomes negligible. The current density needed to keep the electrode in the passive state corresponds to the dissolution rate of the anodic oxide in the alkaline electrolyte used and is usually very low. 

More sophisticated than the use of an insulator to disconnect a part of the sample is the use of an electronic insulation realized by a p-n junction. Because the potential required to passivate a silicon electrode in alkaline solutions [Pa8] is smaller than the bias required to forward a junction, only the side of the junction that is connected will be passivated [Jal, Ge5], as shown in Fig. 4.16a. Note that a temporary... 

If a silicon electrode is anodically oxidized in an acidic electrolyte free of HF, the oxide thickness increases monotonically with anodization time. This is also true for alkaline electrolytes if the oxide formation rate exceeds the slow chemical dissolution of the anodic Si02. This monotonic behavior, however, is not necessarily associated with monotonic current-time or potential-time curves. 

Typical anodization curves of silicon electrodes in aqueous electrolytes are shown in Fig. 5.1 [Pa9]. The oxidation can be performed under potential control or under current control. For the potentiostatic case the current density in the first few seconds of anodization is only limited by the electrolyte conductivity [Ba2]. In this respect the oxide formation in this time interval is not truly under potentiostatic control, which may cause irreproducible results [Ba7]. In aqueous electrolytes of low resistivity the potentiostatic characteristic shows a sharp current peak when the potential is switched to a positive value at t=0. After this first current peak a second broader one is observed for potentials of 16 V and higher, as shown in Fig. 5.1a. The first sharp peak due to anodic oxidation is also observed in low concentrated HF, as shown in Fig. 4.14. In order to avoid the initial current peak, the oxidation can be performed under potentiodynamic conditions (V/f =const), as shown in Fig. 5.1b. In this case the current increases slowly near t=0, but shows a pronounced first maximum at a constant bias of about 19 V, independently of scan rate. The charge consumed between t=0 and this first maximum is in the order of 0.2 mAs cnT2. After this first maximum several other maxima at different bias are observed. 

The formation of pores during anodization of an initially flat silicon electrode in HF affects the I-V characteristics. While this effect is small for p-type and highly doped n-type samples, it becomes dramatic for moderate and low doped n-type substrates anodized in the dark. In the latter case a reproducible I-V curve in the common sense does not exist. If, for example, a constant potential is applied to the electrode the current density usually increases monotonically with anodization time (Thl, Th2]. Therefore the I-V characteristic, as shown in Fig. 8.9, is sensitive to scan speed. The reverse is true for application of a certain current density. In this case the potential jumps to values close to the breakdown bias for the flat electrode and decreases to much lower values for prolonged anodization. These transient effects are caused by formation of pores in the initially flat surface. The lowering of the breakdown bias at the pore tips leads to local breakdown either by tunneling or by avalanche multiplication. The prior case will be discussed in this section while the next section focuses on the latter. 

M NH4C1 are determined to be 0.14 V (SCE) and -0.54 V (SCE), respectively [Otl], A similar value of -0.35 V (SCE) is observed for n-type Si in 1 M HF by microwave reflectivity measurements [Na7]. Figure 10.3 summarizes values of Vh, obtained by different methods. Note that the scatter in these data is much larger for p-type silicon electrodes than for n-type. A similar scatter has been observed in the determination of the OPC potential of p-type electrodes, which is found to be more sensitive to parameters such as, for example, illumination intensity than that of n-type electrodes, as discussed in Section 3.2. 

Fig. 10.3 Graphical representation of flat band potentials of silicon electrodes as determined by different methods. The solid lines show a linear regression, the dotted lines...

Fig. 10-7. Photopotential as a function of electrode potential of n-type and p-type semiconductor silicon electrodes in an aqueous sulfate solution U = cell voltage before photoezcitation =...

However, silicon material in an aqueous solution is not a system in equilibrium. It is considered as a mixed system containing two redox couples with standard electrode potentials Ei and Ei separated by a wide interval. Then Eq. (13) must be modified to account for both components ... 

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