Energy levels polar semiconductors

Such an interfacial degeneracy of electron energy levels (quasi-metallization) at semiconductor electrodes also takes place when the Fermi level at the interface is polarized into either the conduction band or the valence band as shown in Fig. 5-42 (Refer to Sec. 2.7.3.) namely, quasi-metallization of the electrode interface results when semiconductor electrodes are polarized to a great extent in either the anodic or the cathodic direction. This quasi-metallization of electrode interfaces is important in dealing with semiconductor electrode kinetics, as is discussed in Chap. 8. It is worth noting that the interfacial quasi-metallization requires the electron transfer to be in the state of equilibrimn between the interface and the interior of semiconductors this may not be realized with wide band gap semiconductors. 

Fig. 10-14. Energy levels and polarization curves (current vs. potential) for anodic transfer ofphotoexdted holes in oxygen reaction (2 HgO. -t- 4h O24 4 H. ) on a metal electrode and on an n-type semiconductor electrode j = anodic reaction current ep(02 20)- Fermi level of oxygen electrode reaction dCpi, = gain of photoenergy q = potential for the onset of anodic photoexdted ox en reacti . 4 pi, (=-Ae.. le) = shift of potential for the onset of anodic oxygen reaction from equilibrium oxygen potential in the negative direction due to gain of photoenergy in an n-type electrode Eib = flat band potential of an n-type electrode.

Fig. 10-23. Energy levels and polarization curves for a redox reaction of anodic redox holes at a photoexdted n-type electrode and at a dark p-type electrode of the same semiconductor curve (1) = polarization curve of anodic transfer of photoexdted holes at an n-type electrode curve (2)= polarization curve of anodic transfer of holes at a p-type electrode in the dark (equivalent to a curve representing anodic current as a function of quasi-Fermi level of interfadal holes in a photoexdted n-type electrode) i = anodic transfer current of holes Eredox = equilibriiun potential of redox hole transfer N = anodic polarization at potential n (t) of a photoexdted n-type electrode P = anodic polarization at potential pE(i) of a dark p-type electrode.

In covalently bonded non-polar semiconductors the higher levels of the valence band are formed by electrons that are shared between neighbouring atoms and which have ground state energy levels similar to those in isolated atoms. In silicon, for instance, each silicon atom has four sp3 electrons which it shares with four similar atoms at the comers of a surrounding tetrahedron. As a result each silicon atom has, effectively, an outer shell of eight electrons. The... 

This mcclianism is not so efrcctivc in polar semiconductors. The conversion of empty hybrids to doubly occupied hybrids on a GaAs surface would require the double occupation of a gallium hybrid, which is unfavorable because of the polar energy. Indeed, recent experiments (Chye, Babalola, Sukegawa, and Spicer, 1975) indicate that the I crmi level is not pinned on surfaces of GaP at the vacuum. Nonetheless, Schottky barriers can arise at GaP- metal interfaces. Metal-induced surface states" have been proposed as a mechanism (discussed in Section 18-1 ) but the barriers could well arise simply from incorporation of metal atoms in the semiconductor or vice versa. 

The measured potential Vm, and thus jEf and K. can be varied through external polarization. Vm is the applied potential when the electrode is externally polarized and is the open-circuit potential without external polarization. When the semiconductor has no excess charge, there is no space charge region and the bands are not bent. The electrode potential under this condition is called the flatband potential Vn,. The flatband potential is an important quantity for a semiconductor electrode because it connects the energy levels of the carriers in the semiconductor to those of the redox couple in the electrolyte and it connects the paramete s that can be experimentally determined to those derived from solid-state physics and electrochemistry. It can generally be expressed as... 

For the ideally polarized semicondutor electrode, a space-charge region in the semiconductor forms when a potential is apphed across the semiconductor-solution interface so that the electrode potential is displaced from fb- Surface states are energy levels arising from orbitals localized on atoms of the lattice near a surface. It is easy to see, for example, that silicon atoms in a surface plane cannot be surrounded with the tetrahedral symmetry found in the bulk solid. Thus, the electronic properties of these atoms differ. Often surface states have energies in the band gap and have a big effect on the electronic properties of any junction made with the surface. 

Figure 635 Effect of anodic and cathodic polarization on energy levels in n-type semiconductor electrode [32].

Fig. 4 Current at films of PcZn (100 nm) vapor deposited on ITO (1 cm ) observed in contact to aqueous electrolytes during potentiostatic polarization. Illumination occurred either in the B band (3 X 10 photons cm s ) or in the Q band (7 x 10 photons cm s ). a In the presence of 0.1 M EDTA (+460 mV), b In the presence of 10 M O2 (-300 mV vs. SCE). c Schematic representation of frontier energy levels and observed photocurrents, (adapted and in part reprinted from Electrochim. Acta D. Schlettwein, E. Karmann, T. Oekermann, and H. Yanagi Wavelength- Dependent Switching of the Photocurrent Direction at the Surface of Molecular Semiconductor Electrodes Based on Orbital- Confined Excitation and Transfer of Charge Carriers from Higher Excited States , p. 4697 704, Copyright (2000), with permission from Elsevier Science)...

FIGURE 2.25. Correlation between electronic energy levels of an n-type semiconductor and a redox couple with electron exchange via the conduction band at different states of polarization (a) at equilibrium (slow electron exchange) (b) cathodic bias (fast electron transfer) (c) anodic bias (slow electron injection). 

Figure 6, Schematic showing energy correlations for photoassisted electrolysis of water using n-type TiOg as a photoanode and a metal cathode. Symbols as in Figures 3, 4, and 5, except Epis Fermi level for metal contact to TiO and E/ is higher Fermi level in metal cathode, polarized by an external source to a potential negative to the semiconductor anode. EF(Hi) and Ep(02) are abbreviated forms for Fermi energies for redox systems of Figure 3 (13j.

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