Electron tunneling through the space charge layer

8 Electron tunneling through the space charge layer 

As anodic or cathodic polarization increases, the band level bending in a space charge layer (a depletion layer) becomes steeper, and the electron tunneling through the space charge layer is then ready to occur particularly in semiconductor electrodes of high concentrations of donors or acceptors where the space charge layer is thin. 

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Figure 8.7 Tunneling through the space-charge layer at equilibrium and for an anodic overpotential. Note that the band bending is stronger after the application of the overpotential. The arrows indicate electrons tunneling through the space-charge barrier.

Several years ago, two other methods for determining A were reported in the literature which are independent of knowledge of k. The first method is based on the fact that, at heavily doped semiconductors, electron transfer can also occur via tunneling through the space charge layer, as schematically... 

Electron transfer between redox systems within the electrolyte and the metal surface covered by a passivating oxide layer with band gap (E ), conduction band (CB), and valence band (VB) (1) Direct tunneling into free states of the CB, (2) tunneling through the space charge layer to the conduction band CB, (3) hopping via surface states to CB, (4) multiple hopping via interband states, (5) transfer via a subband SB, and (6) transfer into positive holes of valence band VB. (From Strehblow, in Passivity of Metals, R.C. Alkire, D.M. Kolb, eds., Wiley-VCH,... 

FIGURE 1.16. Circumstances in which electron transfer does not oecur between the energy levels at the band edge on the surface and those in the electrolyte, (a) Electron transfer via surface states (b) electron tunneling through the potential barrier in the space charge layer. 

When the semiconductor is highly doped, the space-charge region is thin, and electrons can tunnel through the barrier formed at a depletion layer. 

Fig. 40. Electron transfer from redox systems within the electrolyte across a semiconducting passive layer (n-type) (1) direct tunnelling to CB (2) tunnelling through space charge layer (3) transfer via surface states (4) hopping mechanism via interband states (5) transfer via sub-band and (6) transfer via valence band.

The latter process was responsible for the cathodic current. It must be emphasized that such cathodic currents were only observed with heavily doped Sn02 electrodes. Therefore it was concluded that reaction (73) occurred via tunneling of electrons through the thin space charge layer from the conduction band to Ru(III) molecules produced photochemically. ... 

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