Flat band potential measurement

Fig. 5.8 The energy levels of n-type M0S2 at the flat band potential relative to the positions of various redox couples in CH3CN/[n-Bu4N]C104 solution. The valence band edge of the semiconductor as revealed by accurate flat band potential measurement is at ca. +1.9 V vs. SCE implying that photooxrdations workable at Ti02 are thermodynamically possible at illuminated M0S2 as well. (Reproduced with permission from [137], Copyright 2010, American Chemical Society)...

Huang J, Wrighton MS (1994) Flat band potential measurements of naked and viologen-... 

Figure 3. Photoreduction on p-Si or redox couples with redox potentials lying above the conduction band edge as determined by dark flat-band potential measurements. (a) Photoreduction of 1,3 dimethoxy-4-nitrobenzene in methanol (E0 = —1.0V vs. SCE) (b) photoreduction of anthraquinone in acetonitrile (E0 = —0.95 V vs. SCE) Ec for p-Si in both cases in the dark is —0.85 V vs. SCE (------------------------------) dark (---) light (5).

Catalysts may be needed to minimize overpotential errors in flat band potential measurement Possible sample corrosion... 

Illumination of electrodes of PeZn in contact with a variety of redox electrolytes under open circuit conditions showed positive photovoltages as expected for a p-type electrode, when the rest potential in the dark is negative of its flat band potential. Measurements in KCl solutions containing various electroactive species... 

Gandia, J., Pujadas, M., Salvador, P. Electrolyte electroreflectance - easy and reliable flat-band potential measurements. J. Electroanal. Chem. 244, 69-79 (1988)... 

Current voltage curves for p-Si v/ith methanol and acetonitrile containing redox couples above the conduction band edge as determined from flat-band potential measurements. 

C/pB estimated by both electrical (Mott-Schottky) and optical (photocurrent voltammetry) methods in the media studied, for (11 l)-oriented ZnSe electrode surfaces. A different variation was observed for the (110) orientation at pH >6. At pH 0, for both (110) or (11 l)-oriented electrode surface, the flat band potential value was -1.65 V (SHE) and the measured potential stability range (no detected current) was -0.35 to +2.65 V (SHE). A comparison of band levels with the other II-VI compounds as well as decomposition levels of ZnSe is given in Fig. 5.6. 

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]. 

Vfb the flat-band potential and e the dielectric constant of the semiconductor. From Eq. (10.1) the flat band potential for an electrolytic contact can be calculated from measurements of Csc (V) if Nd is known or Vn, can be graphically determined by extrapolation of a Csc 2 versus V plot to zero capacitance, as shown in Fig. 10.2. The value of 0.75 V found for p-type Si, however, is unrealistically high. 

Equation (10.1) can be used to determine the doping density of a silicon substrate and its depth profile, even if the flat band potential is not known accurately. Diffusion doping, ion implantation or the growth of an epitaxial layer are common methods of producing doped regions in semiconductor substrates. The dopant concentration close to the surface can be measured by SRP or capacitance-... 

The flat band potential cem be estimated from the Mott-Schottl r plot of electrode capacity in the range of electrode potential where a depletion layer is formed as shown in Fig. 5-47 and in Fig. 5-49. The flat band potential can also be estimated by measuring the photopotential of semiconductor electrodes as shown in Fig. 5-62 the photopotential is zero at the flat band potential. 

TABLE 5- Tbe flat band potential , the iso-electric point pHi, the potential of the conduction band edge aiep) at pH q> for metal oxide semiconductor electrodes in aqueous solutions t, = band gap of metal oxides pH= solution pH at which the flat band potential is measured. [From Morrison, 1980.]... 

It is shown in Pig. i0-7 that the photopotential changes its sign at a certain potential which corresponds to the flat band potential of electrodes (See Fig. 10-6.). This provides a way of estunating the flat band potential by measuring the potential at which the photopotential changes its sign. 

The measurement of the differential capacity is now the most widely used method of determining the flat band potential

semiconductor materials the values of [Pg.267]

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