Germanium electrode capacity

Fig. 5-46. Differential capacity estimated for an electrode of intrinsic semiconductor of germanium by calculation as a function of electrode potential C = electrode capacity solid curve = capacity of a space charge broken curve = capacity of a series connection of a space charge layer and a compact layer. [From Goischer, 1961.)...

Bohnenkamp and Engell (3) measured the capacity of a germanium electrode in KOH as a function of potential with respect to a reversible half-cell using a counter electrode to bias the germanium surface. Theory predicted that this capacity should go through a minimum and also how this minimum should depend on equilibrium electron hole density in the germanium. They measured how the electrolytic potential for the capacity minimum depended on electron-hole ratio. In this case the capacity is a function of frequency so the interpretation is not as simple as in the case of Dewald s results. The conclusion was that this potential behaved within experimental error as expected. 

DIFFERENCES IN THE distribution of charges between a metal and a semiconductor electrode surface are considered. Using a simplified model the densities of the charge carriers in a semiconductor electrode can be calculated. Calculated results are in agreement with differential capacity measurements in germanium electrodes. The influence of current through the electrode surface and of the formation of surface traps is considered. 

The equivalent circuit for the calculation of the differential electrode capacity is shown in Fig. 2. It consists of a series resistance Rg, which represents the internal ohmic resistance of the germanium disk and of the metal contact. Since... 

This is exactly the same expression as for the diffuse part of the double layer in an electrolyte (1). Under the same assumptions, the capacity ofan n-type germanium electrode Is given by... 

Efimov and Erusalimchik (10) have criticized the results of Bohnenkamp and Engell (9), They state that the capacity values of the minimum of the capacity-potential curve reported by Bohnenkamp and Engell are too. small, compared with (heir own measurements, and assume that this is due to a poor contact at the reverse side of the germanium electrodes and an inadequate preparation of the surface. We tested the contact on the reverse side with the help of a sample contacted at both sides and found no capacitive component large enough to in-... 

Graetz J, Ahn CC, Yazami R, Fultz B (2004) Nanocrystalline and thin film Germanium electrodes with high lithium capacity and high rate capabilities. J Electrochem Soc 151 ... 

Figure 5-45 shows the differential capacity for an intrinsic semiconductor electrode of germanium estimated by calculation as a function of electrode potential. Here, the capacity is minimum at the flat band potential, Ea, where is zero. As the electrode potential shifts so far away from that the Fermi level at the interface may be dose to the band edge levels, Fermi level pinning is reaUzed both with A sc remaining constant and with Csc being constant and independent of the electrode potential. 

The results of the measurements with intrinsic, p-, and n-type germanium (N = 3.5 x 10 cm 3, Np = 7, 5 x l(r cm ) in IN KOH at 25° and 45°C are shown in Figs. 3-5. The circular frequency, w =2irf, used for the measurement is indicated at each curve. Ail measurements were made with and without illumination of the electrodes. The capacity-potential curve... 

The reversible cycling of up to 3.8 lithium atoms per germanium atom in the nanocrystalline electrode is considerably larger than the capacities measured in the analogous silicon system, which exhibited a reversible capacity of 1.1 lithium atoms per silicon atom. The enhanced lithium uptake is attributed to the higher diffiisivity of lithium in germanium at room temperature (Dce = 400 ZJsi). 

Electrodes of silicon and germanium amorphous films were prepared by depositing the material directly onto a copper substrate. The electrochemical cells were prepared and cycled under conditions previously described for the nanocrystalline materials. The thin films exhibited excellent electrochemical performance as demonstrated in Fig. 2.10a and c, with voltage profiles obtained from q cles 1, 25, and 50 for silicon and germanium amorphous films, respectively. Plots of the differential capacity are shown in Fig. 2.10b and d, respectively. 

These results broadly demonstrate the utility of nanoscale electrodes, which yield greater capacities and cycle life than their bulk counterparts. We showed that improved electrochemical performance in the alloy electrodes could be achieved by preparing the electrodes in an amorphous rather than nanocrystalline state. Finally, we demonstrated that silicon and germanium are viable lithium electrodes and, when prepared with the proper nanostructure and morphology, can be cycled 50 times with little capacity loss. 

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