Electroreflectance effect

Electroreflectance effects arising from changing electron densities at the electrode surface as a result of the applied potential. 

The early treatments of the electroreflectance effect concentrated on the case of uniform electric fields and zero thermal broadening and was therefore suitable only for very lightly doped samples at very low temperatures. The optical properties of a solid are contained in the dielectric function. This function is complex, with the imaginary part only non-zero if the material actually absorbs light. The imaginary part of the dielectric function, e2, can be written for a single band-to-band transition as [186]... 

The electroreflectance effect described in the preceding section arises from a perturbation of the optical constants of the electrode by an applied electric field. 

Both the intensity and the frequency of the -OH stretch in water are critically dependent upon the extent of hydrogen bonding to the immediate environment [40] and thus is a sensitive probe of the various types of water existing at the electrode-electrolyte interface. However, there are additional physical effects that may be detected by the phase-sensitive IR-ATR technique such as those derived from the variation of the electric field at the surface these electroreflectance effects are primarily due to changes in the surface electron density. 

This intensity difference, AR, observed in an EMIRS experiment may result from several sources, e.g. electroreflectance effects arising from changing electron densities at the surface of an electrode under the influence of the applied potential, changes in the amount of adsorbed species, or re-organisation of the double layer, etc. Spectra arising from changes in the adsorbed layer may be derived from... 

The major areas of application of reflectance spectroscopy have been the elucidation of reaction mechanisms, double layer studies, investigations of underpotential deposition (UPD), and studies of the electroreflectance effect (ER). This range is too large for an in depth discussion to be given here. Instead, two examples of the type of information that can be obtained will be described (a third system, hydrogen adsorption on platinum, has been discussed in Chapter 7). Those readers interested in more details are referred to a recent review [1], and the literature cited therein. 

Cuesta A, Lopez N, Gutierrez C. 2003. Electrolyte electroreflectance study of carbon monoxide adsorption on polycrystalline silver and gold electrodes. Electrochim Acta 48 2949-2956. Date M, Hamta M. 2001. Moisture effect on CO oxidation over Au/Ti02 catalyst. J Catal 201 221-224. 

B. O. Seraphin, Electroreflectance R L. Aggarwal, Modulated Interband Magnetooptics D. F. Blossey and Paul Handler, Electroabsorption B. Batz, Thermal and Wavelength Modulation Spectroscopy 7. Balslev, Piezopptical Effects... 

Let us now briefly outline the structure of this review. The next section contains information concerning the fundamentals of the electrochemistry of semiconductors. Part III considers the theory of processes based on the effect of photoexcitation of the electron ensemble in a semiconductor, and Parts IV and V deal with the phenomena of photocorrosion and light-sensitive etching caused by those processes. Photoexcitation of reactants in a solution and the related photosensitization of semiconductors are the subjects of Part VI. Finally, Part VII considers in brief some important photoelectrochemical phenomena, such as photoelectron emission, electrogenerated luminescence, and electroreflection. Thus, our main objective is to reveal various photo-electrochemical effects occurring in semiconductors and to establish relationships among them. 

As an important example, let us consider the effect of electroreflection due to inhomogeneity of the distribution of free carriers in the space-charge region of a semiconductor (plasma electroreflection). The contribution of the electrons to the complex dielectric permittivity (an n-type semiconductor is considered for illustration and the contribution of the holes is neglected) is given by the expression (see, for example, Ziman, 1972)... 

It should be noted that dielectric and optical properties of the near-the-surface layer of a semiconductor, which vary in a certain manner under the action of electric field, depend also on the physicochemical conditions of the experiment and on the prehistory of the semiconductor sample. For example, Gavrilenko et al (1976) and Bondarenko et al. (1975) observed a strong effect of such surface treatment as ion bombardment and mechanical polishing on electroreflection spectra. The damaged layer, which arises in the electrode due to such treatments, has quite different electrooptic characteristics in comparison with the same semiconductor of a perfect crystalline structure (see also Tyagai and Snitko, 1980). 

While considering trends in further investigations, one has to pay special attention to the effect of electroreflection. So far, this effect has been used to obtain information on the structure of the near-the-surface region of a semiconductor, but the electroreflection method makes it possible, in principle, to study electrode reactions, adsorption, and the properties of thin surface layers. Let us note in this respect an important role of objects with semiconducting properties for electrochemistry and photoelectrochemistry as a whole. Here we mean oxide and other films, polylayers of adsorbed organic substances, and other materials on the surface of metallic electrodes. Anomalies in the electrochemical behavior of such systems are frequently explained by their semiconductor nature. Yet, there is a barrier between electrochemistry and photoelectrochemistry of crystalline semiconductors with electronic conductivity, on the one hand, and electrochemistry of oxide films, which usually are amorphous and have appreciable ionic conductivity, on the other hand. To overcome this barrier is the task of further investigations. 

Photoelectrochemistry (PEC) is emerging from the research laboratories with the promise of significant practical applications. One application of PEC systems is the conversion and storage of solar energy. Chapter 4 reviews the main principles of the theory of PEC processes at semiconductor electrodes and discusses the most important experimental results of interactions at an illuminated semiconductor-electrolyte interface. In addition to the fundamentals of electrochemistry and photoexcitation of semiconductors, the phenomena of photocorrosion and photoetching are discussed. Other PEC phenomena treated are photoelectron emission, electrogenerated luminescence, and electroreflection. Relationships among the various PEC effects are established. 

The optical properties of this new family of semiconductors are the subject of Volume 21, Part B. Phenomena discussed include the absorption edge, defect states, vibrational spectra, electroreflectance and electroabsorption, Raman scattering, luminescence, photoconductivity, photoemission, relaxation processes, and metastable effects. 

Fig. 109. Effect of (a) a thermal perturbation that causes a vertical displacement of the bands and (b) electroreflectance, causing a loss in translational symmetry with a concomitant relaxation of the requirement for vertical optical transitions in /e space. -------, Unperturbed ------,...

The basis of electroreflectance is more subtle than thermoreflectance Fig. 109(b) shows that the main effect arises from the fact that the presence of an electric field destroys the translational symmetry along one of the directions of the crystal. This loss of symmetry means that k need no longer be conserved along that axis and optical transitions need no longer be vertical in the Elk diagram. As for thermoreflectance, this effect will be most marked at the critical points, again allowing the spectroscopist to extract the data of real interest from the otherwise rather shapeless absorption spectrum of the solid. 

Bias potential effects on anisotropic electroreflectance of single-crystal silver. Journal of Electroanalytical Chemistry, 79, 1-17. 

The second reason for the lack of exploitation of electroreflectance, at least as a probe of electric field distribution, has been the rather limited development of the theory. Whilst the basic equations describing the effect have been known for some time [9], they are of considerable complexity and the simplifications that have been made, such as the Aspnes "third derivative modulation spectroscopy [10] and the extended lineshape theories of Raccah et al. [11] have regions of applicability that may not include all commonly found experimental conditions. There are two difficulties with these theories. The first is that the electric field strengths found in practice in the semiconductors commonly used in electrochemical research may be too high for simple lineshape theories to be applicable the essential requirement of such theories is that the lineshape should be independent of applied d.c. potential, a result not always found in practice, as discussed below. The... 

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