Semiconductors valence band processes

Charge transfer between energy states in semiconductor and in the redox system only occurs when there is a sufficient overlap of occupied and empty states. The actual currents across the interface can be derived for conduction or valence band processes by taking into account the equilibrium conditions, where and jy" = j,- = jy, and also 115 = 0 and p, = p . In the case... 

In the theory of non-equilibrium processes at solid state junction and also semiconductor-liquid interfaces, as developed in the previous section, frequently quasi-Fermi levels have been used for the description of minority carrier reactions [90, 91], A concept for a quantitative analysis for reactions at n- and p-type electrodes has been derived [92, 93], using the usual definition of a quasi-Fermi level (Eqs. (3a) and (3b)). Taking a valence band process as an example, the quasi-Fermi level concept can be illustrated as follows ... 

Figure 2.19 Theoretical current-potential curve for an n-type semiconductor in contact with a redox system, assuming a valence-band process.

In the present derivation we take a valence-band process as an example. According to the quasi-Fermi level concept, it is assumed that the same reaction with identical rates, i.e. equal currents, takes place at n- and p-type semiconductor electrodes of the same material if the density p of holes at the surface—or equivalently the quasi-Fermi levels Fp p —are equal at the surface of the two types of electrodes, as illustrated for an illuminated n-electrode and a p-electrode in the dark in Fig 2.21. This model is applicable if three conditions are fulfilled ... 

Figure 9.6 Charge transfer between a semiconductor electrode and a redox system. (A) Conduction band processes and (B) valence band processes.

As already discussed in Section 4.3, the reduction of a redox system having a large positive standard potential is expected to occur via the valence bands of many semiconductors. Experiments have shown, for instance, that the reduction of Ce" " ions (f/redox = +1.4 V) at GaP electrodes is already a valence band process. Since H2O2 has a standard potential (f/redox = +1.77 V) even larger than that of one also expects a valence band process,... 

In an intrinsic semiconductor, tlie conductivity is limited by tlie tlieniial excitation of electrons from a filled valence band (VB) into an empty conduction band (CB), across a forbidden energy gap of widtli E. The process... 

A number of electronic and photochemical processes occur following band gap excitation of a semiconductor. Figure 5 illustrates a sequence of photochemical and photophysical events and the possible redox reactions which might occur at the surface of the SC particle in contact with a solution. Absorption of light energy greater than or equal to the band gap of the semiconductor results in a shift of electrons from the valence band (VB) to... 

Charging of the surface accompanying adsorption process and resulting in the change of the energy profile of the bottom of the conductivity band and, naturally, the ceiling of the valence band in semiconductors... 

Electric current is conducted either by these excited electrons in the conduction band or by holes remaining in place of excited electrons in the original valence energy band. These holes have a positive effective charge. If an electron from a neighbouring atom jumps over into a free site (hole), then this process is equivalent to movement of the hole in the opposite direction. In the valence band, the electric current is thus conducted by these positive charge carriers. Semiconductors are divided into intrinsic semiconductors, where electrons are thermally excited to the conduction band, and semiconductors with intentionally introduced impurities, called doped semiconductors, where the traces of impurities account for most of the conductivity. 

Shallow acceptor levels lie close to the valence band and take up electrons from it to create holes in the valence band and produce p-type semiconductors. Interstitial nonmetal atoms often generate shallow acceptor levels because anion formation involves taking up extra electrons. Acceptor levels are said to be ionized when they take electrons from the valence band, creating holes in the process. The energy of a neutral acceptor atom is different to that of an ionized acceptor. The electrons on the ionized anions are often trapped and do not contribute to the conductivity. 

Upon excitation of a semiconductor, the electrons in the conduction band and the hole in the valence band are active species that can initiate redox processes at the semiconductor-electrolyte interface, including photocorrosion of the semiconductor, a change in its surface properties (photoinduced superhydrophilicity [13]), and various spontaneous and non-spontaneous reactions [14-19]. These phenomena are basically surface-mediated redox reactions. The processes are depicted in Fig. 16.1. Owing to the slow spontaneous kinetic of the reactions between the... 

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