Double-sided electrolytic contact

Double-sided electrolytic contacts are favorable for this method of diffusion length measurement because they are transparent and the required SCRs are easily induced by application of a reverse bias. Therefore homogeneously doped wafers need no additional preparation, such as evaporation of metal contacts or diffusion doping, to produce a p-n junction. Furthermore, a record low value of surface recombination velocity has been measured for silicon surfaces in contact with an HF electrolyte at OCP [Yal], Note that this OCP value cannot be further decreased by a forward bias at the frontside, because any potential other than OCP has been found to increase the surface recombination velocity, as shown in Fig. 3.2. Note that contaminations in the HF electrolyte, such as Cu, may significantly increase the surface recombination velocity. This effect has been used to detect trace levels (20 ppt) of Cu in HF [Re5j. 

Figure 19.3 Measurement set-up of ion-specific electrodes. Schematic of a measuring cell with a ISE targeted for the fluoride ion using a double junction reference electrode. From the two sides of the membrane two equilibria are produced, LaF, - LaF -F F . To avoid osmosis of KCI into the solution, the reference electrode is surrounded by a separate chamber which contains an auxiliary non-interfering electrolyte. KNO, IM difficult to oxidize or to reduce, is often used for F , CP, P, CN , or Ag+. The measurement requires a high impedance mihivoltmeter (type pH-meter). Right, two examples of simple to construct coated wire electrodes . A wire forms an ohmic contact with the membrane (ISE for fluorine with a LaF, crystal and ISE for chloride with frit).

Finally, IRRAS of the electrode-electrolyte interface can be measured using the emersed electrode technique. This technique is based on the fact [445] that the compact DL remains intact upon removal of the electrode from elecfiolyte. To overcome the loss of the potential confiol upon emersion it was suggested [446] that an electrode that is in partial contact with electrolyte be rotated. In a more advanced double-cell technique [447], a cell similar to that shown in Fig. 4.45 is employed. The difference is that the thin internal tube holding the working electrode (Pt disk) serves also as a container for the second ceU. Both the main outer and internal auxiliary cells are filled with the same solution and have reference and counter electrodes inside. Since the Pt electrode is a good conductor its surfaces are equipotential. Therefore, any potential applied on the electrode back side is established at the outer surface that is pressed against the window for IR measurements. 

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