Pinning of Fermi level

In the absence of either surface states, which may pin the Fermi level at the interface between the dielectric and the electrode, the energy barriers, which in turn... 

Singh P, Singh R, Gale R, Rajeshwar K, DuBow J (1980) Surface charge and specific ion adsorption effects in photoelectrochemical devices. J Appl Phys 51 6286-6291 Bard AJ, Bocarsly AB, Pan ERF, Walton EG, Wrighton MS (1980) The concept of Fermi level pinning at semiconductor/liquid junctions. Consequences for energy conversion efficiency and selection of useful solution redox couples in solar devices. J Am Chem Soc 102 3671-3677... 

The fundamental quantity of interest, BE, is calculated from the KE (correcting for the work function 4>s). The sample is grounded to the spectrometer to pin the Fermi levels to a fixed value of the spectrometer (Fig. 1) so that the applicable work function is that of the spectrometer, sp [2], This instrumental parameter is a constant that can be measured. The BEs are then easily obtained from Eq. 2 ... 

Manipulating surface states of semiconductors for energy conversion applications is one problem area common to electronic devices as well. The problem of Fermi level pinning by surface states with GaAs, for example, raises difficulties in the development of field effect transistors that depend on the... 

Figure 5-41 illustrates the profile of electron level across the interfadal double layer of a semiconductor electrode (A) in the state of band edge level pinning and (B) in the state of Fermi level pinning. In Fig. 5-41 the cathodic polarization... 

In the state of Fermi level pinning, the Fermi level at the interface is at the surface state level both where the level density is high and where the electron level is in the state of degeneracy similar to an allowed band level for electrons in metals. The Fermi level pinning is thus regarded as quasi-metallization of the interface of semiconductor electrodes, making semiconductor electrodes behave like metal electrodes at which all the change of electrode potential occurs in the compact layer. 

The surface state capacity, Ch, is apparently zero in the range of potential where the Fermi level is located away from the surface state level (the state of band edge level pinning). As the Fermi level is pinned at the surface state, the capacity Ch increases to its maximum which is equivalent to the capacity Ch of the compact layer, because the surface state charging is equivalent to the compact layer charging in the state of Fermi level pinning. 

Fig. 8-24. Redox reaction currents via the conduction and the valence bands of semiconductor electrode as functions of electrode potential of semiconductor anodic polarization corresponds to Figs. 8-20, 8-21 and 8-22. i (i )= anodic (cathodic) current in (ip) = reaction crnrent via the conduction (valence) band BLP = band edge level pinning FLP = Fermi level pinning.

Fig. 8-27. Polarization curves for transfer of redox electrons at n-type and p-type semiconductor electrodes solid curve near Egaxa = reaction with the Fermi level of redox electrons dose to the valence band edge dashed curve near F redok = reaction with the Fermi level of redox electrons dose to the conduction band edge dot-dash curve (FLP)= reaction in the state of Fermi level pinning.

In the state of band edge level pinning where all the change in electrode potential occurs in the space diarge layer, Mec, the anodic polarization curve of the oxidative dissolution follows Eqn. 9-53. As anodic polarization increases, the electrode interface enters a state of Fermi level pinning, in which all the change in electrode potential occurs in the compact layer, A ir, and the concentration of surface cations in Eqns. 9-54 then decreases with increasrng anodic polarization. 

When the electrode interface is in the state of Fermi level pinning, however, the potential of the compact layer changes with the electrode potential hence, the equilibrium of the adsorption-desorption of protons on semiconductor electrodes depends on the electrode potential in the same way as that on metal electrodes. 

For the state of Fermi level pinning of the electrode interface, we may also derive the concentration of adsorbed hydrogen atoms in the same equation as Eqn. 9-64 for metal electrodes. 

In the active state, the dissolution of metals proceeds through the anodic transfer of metal ions across the compact electric double layer at the interface between the bare metal and the aqueous solution. In the passive state, the formation of a thin passive oxide film causes the interfadal structure to change from a simple metal/solution interface to a three-phase structure composed of the metal/fUm interface, a thin film layer, and the film/solution interface [Sato, 1976, 1990]. The rate of metal dissolution in the passive state, then, is controlled by the transfer rate of metal ions across the film/solution interface (the dissolution rate of a passive semiconductor oxide film) this rate is a function of the potential across the film/solution interface. Since the potential across the film/solution interface is constant in the stationary state of the passive oxide film (in the state of band edge level pinning), the rate of the film dissolution is independent of the electrode potential in the range of potential of the passive state. In the transpassive state, however, the potential across the film/solution interface becomes dependent on the electrode potential (in the state of Fermi level pinning), and the dissolution of the thin transpassive film depends on the electrode potential as described in Sec. 11.4.2. 

Fig. 11-11. Potential at a film/solution interface and potential dfp in a passive film as a fimction of anodic potential of a passive metal electrode in the stationary state the interface is in the state of band edge level pinning to the extent that the Fermi level e, is within the band gap, but the interface changes to the state of Fermi level pinning as e, coincides with the valence band edge Cy.

We have observed that p-GaP seems to be an effective hydrogen electrode because of the adsorbed aqueous species on the surface as reflected by the Nernstian dependence of on pH in Figure 3. Thus, it appears in many cases the chemisorption of ions at the interface is a necessary step for rapid charge transfer to occur. This clearly has a strong bearing on recent observations of Fermi level pinning in various PEC devices where strong chemisorption does not occur.(14,15)... 

The contribution of Fermi level pinning to the band alignment is one of the most important results of the performed studies. It is also very pronounced when CdS is replaced by In2S3. Corresponding results are presented in Sect. 4.5. 

Adding halides to the electrolyte, the shift of bands under illumination and thereby the photocurrent onset potential decreased in accordance with the change in the redoxpotential of the halide couple used. The results can only be understood in terms of Fermi level pinning by the redox couple about 0.5 to 0.8 V below... 

After transformation of the surface due to annealing a small photoeffect of 0.4 eV develops, which is related to the partial removal of Cu2+ and the formation of insulating InCl3 reducing the effect of Fermi level pinning. 

Such efficient minority-carrier injectors have been proposed as emitters for high-power transistors (Kroemer, 1957) and demonstrated using an n-type oxygen-rich polycrystalline Si emitter on a p-type c-Si base (Oh-uchi et al., 1979). The main problem expected to occur at heterojunctions between dissimilar materials is that associated with interfacial states that may either pin the Fermi level or act as generation-recombination centers. However, in the case of a-Si H the abundance of atomic hydrogen should help eliminate the interfacial states. 

---