Band structure, interface

While many of the standard electroanalytical techniques utilized with metal electrodes can be employed to characterize the semiconductor-electrolyte interface, one must be careful not to interpret the semiconductor response in terms of the standard diagnostics employed with metal electrodes. Fundamental to our understanding of the metal-electrolyte interface is the assumption that all potential applied to the back side of a metal electrode will appear at the metal electrode surface. That is, in the case of a metal electrode, a potential drop only appears on the solution side of the interface (i.e., via the electrode double layer and the bulk electrolyte resistance). This is not the case when a semiconductor is employed. If the semiconductor responds in an ideal manner, the potential applied to the back side of the electrode will be dropped across the internal electrode-electrolyte interface. This has two implications (1) the potential applied to a semiconducting electrode does not control the electrochemistry, and (2) in most cases there exists a built-in barrier to charge transfer at the semiconductor-electrolyte interface, so that, electrochemical reversible behavior can never exist. In order to understand the radically different response of a semiconductor to an applied external potential, one must explore the solid-state band structure of the semiconductor. This topic is treated at an introductory level in References 1 and 2. A more complete discussion can be found in References 3, 4, 5, and 6, along with a detailed review of the photoelectrochemical response of a wide variety of inorganic semiconducting materials. 

Electrode Band Structure and Interface States in Photoelectrochemical Cells... 

It is also interesting to consider charge-transfer models developed primarily for metal surfaces. There are clear parallels to the metal oxide case in that there is an interaction between discrete molecular orbitals on one side, and electronic bands on the other side of the interface. The Newns-Anderson model [118] qualitatively accounts for the interactions between adsorbed atoms and metal surfaces. The model is based on resonance of adatom levels with a substrate band. In particular, the model considers an energy shift in the adatom level, as well as a broadening of that level. The width of the level is taken as a measure of the interaction strength with the substrate bands [118]. Also femtosecond electron dynamics have been studied at electrode interfaces, see e.g. [119]. It needs to be established, however, to what extent metal surface models are valid also for organic adsorbates on metal oxides in view of the differences between the metal an the metal oxide band structures. The significance of the band gap, as well as of surface states in it, must in any case be considered [102]. 

Figure 1. Band Structure in a n-type Semiconductor A. Solid State. B. In contact with a liquid phase redox couple (0/R). IL=energy of the conduction band. Vertical line indicates solid-liquid interface. CB= conduction band VB = valence band.

Concerning the role of dimensionality we observed, in the three fully passivated cases, that the material is a semiconductor, as the band structure of the Si[i]-SiC>2 SL in Figure 39(a) shows, and that there is an opening of the gap as the thickness of the Si layer decreases. The band structure shows a gap which is slightly indirect for the presence of a state at the top of the valence band (mostly related to the Si atoms in the inner Si layer), that is partially due to the interaction between the interface Si and its double-bonded O atom. If we remove this extra O, leaving the two dangling bonds of the interface Si unsaturated, we find that the material is still a semiconductor with a new... 

The aim of this chapter is to present a simple but general band structure picture of the metal-semiconductor interface and compare that with the characteristics of the metal-conjugated polymer interface. The discussion is focused on the polymer light emitting diode (LED) for which the metal-polymer contacts play a central role in the performance of the device. The metal-polymer interface also applies to other polymer electronic devices that have been fabricated, e.g., the thin-film field-effect transistor3, but the role of the metal-polymer interface is much less cruical in this case and... 

Figure 4.31 shows ultraviolet photoelectron spectra recorded during the same interface experiment shown in Fig. 4.26. A clear transition from the Cu(In,Ga)Se2 valence band structure with a valence band maximum at 0.8eV binding energy to the ZnO valence band structure with a valence band maximum at 3eV is observed with increasing ZnO deposition. The well-resolved valence band features are enabled by the in situ sample preparation. Also very sharp secondary electron cutoffs are obtained, which allow for an accurate determination of work functions. The work functions of Cu(In,Ga)Se2 and ZnO are determined as 5.4 and 4.25 eV, respectively. These result in ionization potentials of 6.15 and 7.15 eV for Cu(In,Ga)Se2 and ZnO.

Fig. 4. Superposition of energy-band structures of a-Si H and c-Si along the interface (a) without bias and (b) with an applied reverse bias VR.

Fig. 18 The electronic band structures obtained for the a) Pd/c-ZrOa and b) CeCh interfaces. Adsorption site is 0 (Fig. 2). Energies are given in eV. Standard notations for the primitive cell in reciprocal space are used.

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