Electroreflectance electrolyte

Duffy NW, Peter LM, Wang RL, Lane DW, Rogers KD (2000) Electrodeposition and characterisation of CdTe films for solar ceU applications. Electrochim Acta 45 3355-3365 Duffy NW, Peter LM, Wang RL (2002) Characterisation of CdS/CdTe heterojunctions by photocurrent spectroscopy and electrolyte electroreflectance/absorbance spectroscopy (EEA/EER). J Electroanal Chem 532 207-214 (see also references therein). 

Chaparro AM, Salvador P, Mir A (1996) The scanning microscope for semiconductor characterization (SMSC) Study of the influence of surface morphology on the photoelectrochemical behavior of an n-MoSe2 single crystal electrode by photocurrent and electrolyte electroreflectance imaging. J Electroanal Chem 418 175-183... 

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. 

We will illustrate the difficulties and the opportunities which are associated with two complementary measuring techniques Relaxation Spectrum Analysis and Electrolyte Electroreflectance. Both techniques provide information on the potential distribution at the junction of a "real" semiconductor. Due to the individual characteristics of each system, care must be taken before directly applying the results which were obtained on our samples to other, similarly prepared crystals. 

Electrolyte Electroreflectance (EER) is a sensitive optical technique in which an applied electric field at the surface of a semiconductor modulates the reflectivity, and the detected signals are analyzed using a lock-in amplifier. EER is a powerful method for studying the optical properties of semiconductors, and considerable experimental detail is available in the literature. ( H, J 2, H, 14 JL5) The EER spectrum is automatically normalized with respect to field-independent optical properties of surface films (for example, sulfides), electrolytes, and other experimental particulars. Significantly, the EER spectrum may contain features which are sensitive to both the AC and the DC applied electric fields, and can be used to monitor in situ the potential distribution at the liquid junction interface. (14, 15, 16, 17, 18)... 

Figure 6. Variation with potential of the electrolyte electroreflectance spectra of single-crystal n-CdSe in polysulfide solution (CdSe T = 300 K in NaOH/S"/S 1 1 1 ...

The adsorption of Cu2+ ions on the Ti02 forms electroactive surface states within the band gap of the oxide, whose energy position was determined by the electrolyte electroreflection method [285, 286]. These copper-induced surface states were established to be located ca. 1.1 eV below the conduction band edge. Information concerning the subbands of the surface states in the Ti02 electrodes modified with Ag, Pd, Pt and Au one can find in [286, 310, 311], as well as in Chapter 6 of this book. 

Hutton and Peter (1993) in a study of G2li xA xAs compared the bandgap values derived from photocurrent and photovoltage spectra with those derived by electrolyte electroreflectance spectroscopy (EER). They concluded that EER gives the most reliable results, provided that the doping density is not too high. 

Berlouis L. E. A., Peter L. M., Diskett D. J., Avery A. J., Astles M. G., Giess J. and Gordon N. T. (1990), Characterization of epitaxial Cd cHgi Te using electrolyte electroreflectance with in-situ electrochemical etching , J. Cryst. Growth 101, 153-156. 

Hutton R. S. and Peter L. M. (1993), Characterization of n-Ga xAlxAs (x < 0.3) epitaxial layers by photocurrent, photovoltage and electrolyte electroreflectance spectroscopies . Semiconductor Sci. Technol. 8, 1309-1316. 

Hutton R. S., Peter L. M., Batchelor R. A. and Hamnett A. (1994), The potential distribution across the semicondnctor-electrolyte interface—a stndy by electrolyte electroreflectance spectroscopy of GaAs and Gai AbAs nnder conditions of photodissolution , J. Electroanal. Chem. 375, 193-201. 

Salvador P., Tafalla D., Tributsch H. and Wetzel H. (1991), Reaction mechanisms at the n-FeSi/I interface—an electrolyte electroreflectance study , J. Electrochem. Soc. 138, 3361-3369. 

Tafalla D., Pujadas M. and Salvador P. (1989), Direct measurements of flat-band potential shifts under illumination of the semiconductor electrolyte interface by electrolyte electroreflectance , Surface Sci. 215, 190-200. 

The electrolyte electroreflectance (EER) method is successfully used [36, 37] to determine the flat band potential (t/fb). In this method, the optical reflectance at the semiconductor electrode surface is measured under modulation of the electrode potential by superimposition of a small AC voltage. The modulation of the electrode potential causes modulation of the density of majority carriers near the semiconductor sxrrface, which in turn, causes modulation in light reflectance. 

Several techniques can be used to determine the flatband potential of a semiconductor. The most straightforward method is to measure the photocurrent onset potential, ( onset- At potentials positive of (/>fb a depletion layer forms that enables the separation of photogenerated electrons and holes, so one would expect a photocurrent. However, the actual potential that needs to be applied before a photocurrent is observed is often several tenths of a volt more positive than ( fb- This can be due to recombination in the space charge layer [45], hole trapping at surface defects [46], or hole accumulation at the surface due to poor charge transfer kinetics [43]. A more reliable method for determining ( fb is electrolyte electroreflectance (EER), with which changes in the surface free electron concentration can be accurately detected [47]. The most often used method, however, is Mott- chottky analysis. Here, the 1/ Csc is plotted as a function of the applied potential and the value of the flatband... 

Gandia, J., Pujadas, M., Salvador, P. Electrolyte electroreflectance - easy and reliable flat-band potential measurements. J. Electroanal. Chem. 244, 69-79 (1988)... 

A cost effective experimental setup for optical modulettion experiments, recently built in our laboratory. Is shown in Fig. 8 (57). Similar setup was recently reported by Tian et al. (58). Experiments performed with this system include photoreflectance (PR), electrolyte electroreflectance (EER), surface photovoltage spectroscopy (SPV), 1st. and 2nd. harmonics photoinduced current-voltage characteristics, spectral response and d.c. current-voltage characteristics. One can switch electronically between experiments and perform any number of techniques without moving the cell or removing the electrode from the electrolyte. A variable neutral... 

Electroreflectance. The system s response here is the surface reflectivity. The technique is based on modulating the reflectivity of the semiconductor with low frequency bias voltage across the device. If the junction Is being formed with an electrolyte, the technique becomes known as Electrolyte Electroreflectance (EER). Excellent review articles cover the applications of this technique for analysis of optical properties of semiconductors and in the determination of the potential distribution across the interface of a solid or a liquid junction (59). 

Figure 10. Comparison between Photoreflectance (PR), Electrolyte-Electroreflectance (EER) and Photoreflectance in an electrolyte (EPR) of n-Si. The EPR and the EER were measured In the methanolic solution of oxidized and reduced dimethylferrocene in thefollowing concentrations 0,01 MFeCpj, SOiiMFeCPa andl MLiCIO supporting electrolyte under nitrogen atmosphere. The EPR was measured under open circuit conditions and the EER at a potential of 0.0 volt vs. Pt. The chopping frequency in all cases is 750 Hz (16b).

Figure 14. Contours of the variations in carrier concentrations across the surface of GaAs utilizing the electrolyte electroreflectance signal at 8370A. The numbers in the contours indicate percentage variations relative to the lowest concentration (106).

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