Electroreflectance theories

Reflectance spectroscopy in the infrared and visible ultraviolet regions provides information on electronic states in the interphase. The external reflectance spectroscopy of the pure metal electrode at a variable potential (in the region of the minimal faradaic current) is also termed electroreflectance . Its importance at present is decreased by the fact that no satisfactory theory has so far been developed. The application of reflectance spectroscopy in the ultraviolet and visible regions is based on a study of the electronic spectra of adsorbed substances and oxide films on electrodes. 

In order to explain the changing optical properties of AIROFs several models were proposed. The UPS investigations of the valence band of the emersed film support band theory models by Gottesfeld [94] and by Mozota and Conway [79, 88]. The assumption of nonstoichiometry and electron hopping in the model proposed by Burke et al. [87] is not necessary. Recent electroreflectance measurements on anodic iridium oxide films performed by Gutierrez et al. [95] showed a shift of optical absorption bands to lower photon energies with increasing anodic electrode potentials, which is probably due to a shift of the Fermi level with respect to the t2g band [67]. 

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. 

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. 

Abrantes L. M., Peat R., Peter L. M. and Hamnett A. (1987), Electroreflectance at the semiconductor-electrolyte interface—a comparison of theory and experiment for n-GaAs , Ber. Bunsenges. Phys. Chem. 91, 369-374. 

Hamnett A., Gilman J. and Batchelor R. A. (1992), Theory of electroreflectance and photoreflectance of semiconductors , Electrochim. Acta 37,949-956. 

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... 

The computational difficulties encountered in the theory above have led many workers to inquire whether or not simplifications may be found that would allow us to a simple physical insight into the spectra. Aspnes and later Raccah and co-workers were able to develop "low field theories of electroreflectance that have proved to be of great value, particularly in spectroscopic studies, and before considering the results of the more complete theory outlined above, we will turn to a consideration of these theories and discuss some applications. 

This approach has been used by Tomkiewicz et al. [17] to rationalise the electroreflectance spectrum of CdIn2Se4 unfortunately, the crystal studied in this report disintegrated before measurement of the donor intensity could be carried out to verify that the experimental conditions were such that the low-field theories could reasonably be expected to hold. Nevertheless, the intensity of the electroreflectance peak showed a marked dependence on potential, decreasing by a factor of ten over a 1V range as shown in Fig. 10(a). 

Fig. 9.1 Dr. Alexander Borisovich Ershler (1935-1989) with his group. From right Dr. chem. A.B. Ershler, Ph.D. phys. Eduard M. Podgaetskii, Engr. Tatyana S. Orekhova, Ph.D. chem. Ida M. Levinson another member of the group, postgraduate student Vladimir Kurmaz, was drafted at this time into the Soviet army. Areas of research adsorption theory of neutral organic compounds and its influence on kinetics of electrode reactions, electrode reactions of organomercury compounds, development of new electrochemical methods (high-speed pulse chronopotentiometry, electroreflection, and optical transitions at the metal-electrolyte interface, etc.). ELAN, 1975...

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