Germanium anodic dissolution

FIGURE 14.7 Polarization curves for the anodic dissolution of (1) /J-type and (2) -type germanium in 0.1 MHCl solution. 

A typical featnre of reactions involving the minority carriers are the limiting currents developing when the snrface concentration of these carriers has dropped to zero and they mnst be snpplied by slow dilfnsion from the bulk of the semiconductor. A reaction of this type, which has been stndied in detail, is the anodic dissolution of germanium. Holes are involved in the first step of this reaction Ge — Ge(II), and electrons in the second Ge(ll) —> Ge(IV). The overall reaction equation can be written as... 

Fig. 9-11. Polamation curves observed for anodic dissolution of n- pe and p-type semiconductor electrodes of germanium in 0.05 M NaOH solution = current of...

The direct electrochemical synthesis of metal alkoxides by the anodic dissolution of metals into alcohols containing conducting electrolytes was initially demonstrated by Szilard in 1906 for the methoxides of copper and lead.19 More recently the method has received some attention particularly in the patent literature.29-25 The preparation of the ethoxides of silicon, titanium, germanium, zirconium and tantalum by electrolysis of ethanolic solutions of NH Cl has been patented, although the production of the ethoxides was found to cease after several hours.24,25... 

Germanium — (Ge, atomic number 32) is a lustrous, hard, silver-white metalloid (m.p. 938 °C), chemically similar to tin. Ge is a low-band-gap - semiconductor that, in its pure state, is crystalline (with the same crystal structure as diamond), brittle, and retains its luster in air at room temperature. Anodic dissolution of the material occurs at potentials more positive than ca. -0.2 V vs SCE. Peaks in the voltammograms of germanium in acidic electrolyte are ascribed to a back-and-forth change between hydrogenated and hydroxy-lated surfaces [i]. Studies are often conducted at p-doped and n-doped Ge electrodes [ii] or at Ge alloys (e.g., GeSe) where photoelectrochemical properties have been of considerable interest [iii]. 

The expected Tafel slope of 60mV/decade is not always found. There are a number of reasons for this, aside from kinetic effects in the bulk of the semiconductor. The kinetic effects associated with faradaically active surface states is of considerable significance, as shown below, but another common problem is that part of the potential change may appear across the Helmholtz layer rather than across the depletion layer. A well-known case in point is germanium, for which the surface is slowly converted from "hydride to "hydroxylic forms as the potential is ramped anodically. This conversion gives rise to a change in the surface dipole and hence Aij/ AT. In fact, the anodic dissolution of p-germanium is found to follow a law [106]... 

Germanium is electrolytically dissolved in most electrolytes. Jirsa (15) studied the anodic dissolution of germanium... 

Assuming that the stable form of germanium in acid or neutral solutions is metagennanic acid, the over-all reaction for the anodic dissolution of germanium can be written as follows ... 

We consider, for instance, germanium in aqueous solution. The anodic dissolution of germanium occurs by either donating electrons into the conduction band or accepting holes out of the valence band of germanium as follows ... 

FIGURE 22.8 Polarization curves for anodic dissolution of n-type and p-type germanium electrodes in 0.05 kmol m 3 sodium hydroxide solution [12]. 

The general concept of the anodic dissolution process and reaction pathway was first explored with group-IV semiconductors (i.e. silicon and germanium) (1-3). 

Figure 8.1 Mechanism of the anodic dissolution of germanium (modified version after [6]).

Figure 8.2 Formation of intermediate states (surface states) during the anodic dissolution of germanium.

This is the anodic hole-emitting dissolution of germanium, and its Fermi level, F(Ge4+)> stands within the range of the band gap of the germanium electrode. In order for this reaction to occur, sF(Ge4+) will have to be higher than sF [23]. 

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